of many higher forms of vegetation, and this relationship is a very
fruitful field for study.

RELATION OF FUNGI TO SOIL PROCESSES.

The great cycle of changes occurring in the soil whereby organic matter
is gradually transformed and again made available as plant food is
entirely dependent upon micro-organisms. Until a decade ago it was
thought that bacteria were by far the most important group concerned
in the bringing about of these changes, but recent studies have shown
that, in at all events certain arcs of this great organic cycle,
the fungi have, perhaps, an equal part to play. The life of fungi
in the soil may, for our purposes, be considered from three points
of view--their part in the decomposition of carbon compounds, their
nitrogen relationships, and their work in the mineral transformations
of the soil.

CARBON RELATIONSHIPS.

Of primary importance in the carbon relationships of soil fungi is
the part played in the decomposition of the celluloses, which compose
almost all the structural remains of plant tissues. Our first real
knowledge of this subject was given by Van Iterson[28] in 1904 when he
showed the wide extent of cellulose destruction by fungi, and devised
methods whereby fifteen cellulose-decomposing forms, many of which have
since proved to be common soil fungi, were isolated. Three years later
Appel[1] published his account of the genus _Fusarium_, and showed
that many of the species could destroy filter paper. A difficulty was
introduced in 1908 by Schellenberg,[60] who, working with common soil
forms, found that only hemicelluloses and not pure cellulose were
destroyed. This has recently been supported by Otto,[48] but from the
practical point of view the discussion is academic for the amount of
pure cellulose in plants is insignificant.

In 1913 McBeth and Scales[43] showed that a considerable number
of common soil fungi were most active cellulose destroyers, pure
precipitated cellulose and cotton being readily attacked. This was
supported by McBeth in 1916,[42] whilst Scales[59] has found that
most species of _Penicillium_ and _Aspergillus_ decompose cellulose,
especially where ammonium sulphate is the source of nitrogen.
Waksman[65] tested twenty-two soil fungi and found that eleven
decomposed cellulose rapidly and four slowly, whilst Dascewska,[16]
Waksman,[66],[67] and others have concluded that soil fungi play
a more important part in the decomposition of cellulose and in
“humification” than soil bacteria. Schmitz[61] has recently shown that
cellulose-destroying bacteria play no important part in the decay of
wood under natural conditions.

In addition to the celluloses, practically all simple and complex
organic carbon compounds are attacked by soil fungi, and in many cases
the decomposition is very rapid.[26] Many _Actinomycetes_, _Aspergilli_
and _Penicillia_ are active starch splitters, and it is of interest to
note that some of the strongest cellulose decomposers (_Melanconium
sp._, _Trichoderma sp._, and _Fusaria_) secrete little diastase.[66]
The _Mucorales_ apparently do not attack cellulose, but can only
utilise pectin bodies, monosaccharides, and partly disaccharides.[26]
Dox and Neidig[19] have shown that various species of _Aspergillus_
and _Penicillium_ are able to attack the soil pentosans. Roussy,[58]
Kohshi,[24] Verkade and Söhngen,[64] and many other workers have found
that fats and fatty acids are readily used as food by soil fungi, and
Koch and Oelsner[33] have recently shown that tannins are readily
assimilated. Klöcker,[32] Ritter,[56] and others have shown that the
utilisation of many carbon compounds is to a large extent determined by
the source of nitrogen and its concentration in the pabulum.

There would seem, therefore, no doubt that the decomposition of
celluloses and other carbon compounds is of primary importance in the
life-activities of soil fungi.

NITROGEN RELATIONSHIPS.

In this section we shall consider the problems of nitrogen fixation and
nitrification, of ammonification, and of the utilisation of nitrogenous
compounds by soil fungi.

As soil fungi form so large a part of the soil population, the question
of whether they can make use of the free nitrogen of the air is of
primary importance. During the last two decades many investigators have
attempted to solve the problem, often studying allied or identical
species; but if one consults some thirty researches published during
this period, opinion is found to be about equally divided. Even,
however, in those studies where nitrogen fixation has been recorded the
amounts are very slight, usually being below 5 mgrms. per 50 c.c. of
solution, and often being obviously within the limits of experimental
error. Latham,[37] however, working on _Aspergillus niger_, recorded
variations ranging from a nitrogen loss of 42·5 mgrms. to a nitrogen
fixation of 205·1 mgrms. per 50 c.c. of medium. Ternetz[63] found
that different strains of _Phoma radicis_ may fix from 2·5 mgrms. of
nitrogen in the lowest case, to 15·7 mgrms. in the highest per 50 c.c.
of nutrient solution. Duggar and Davis[20] report that _Phoma betæ_
may fix nitrogen in quantities of 7·75 mgrms. per 50 c.c. of medium.
The latter authors, in a very able critique of the problem, indicate
certain possible sources of error in previous work, and if one examines
the studies in which nitrogen fixation has been recorded in the light
of these criticisms, it is difficult not to think that, with the
exception of the genus _Phoma_, good evidence for nitrogen fixation
by fungi is lacking. _Phoma betæ_ is a common pathogen attacking
beets, whilst _P. radicis_ is a mycorrhizal form inhabiting various
Ericales. Apart from these exact quantitative studies, which have
given a negative verdict, there is a considerable amount of positive
but indirect evidence for nitrogen fixation by mycorrhizal fungi,[55]
and it is very unfortunate that more of these forms have not been
investigated quantitatively. As the evidence stands to-day, one must
conclude that the fungus flora does not play any part in the direct
nitrogen enrichment of the soil.

Equally obscure is the question of nitrification and denitrification
by soil fungi, but this is the result of a lack of study rather than
of a plethora of indeterminate researches. Direct nitrification or
denitrification has not been established, but the work of Laurent[38]
and a few other workers appears to show that soil fungi can reduce
nitrates to nitrites.

The second primary nitrogen relationship that we have to consider is
the process of ammonification. The ammonifying power of soil fungi was
first demonstrated by Muntz and Coudon,[46] and by Marchal[40] in 1893,
the former showing that _Mucor racemosus_ and _Fusarium Muntzii_ gave a
larger accumulation of ammonia in soil than any of the bacteria tested;
and the latter that _Aspergillus terricola_, _Cephalothecium roseum_
and other soil fungi were active ammonifiers, especially in acid
soils. Shibata,[62] Perotti,[49] Hagem,[26] Kappen,[31] Löhnis,[39]
and others, have observed that urea, dicyanamide and cyanamide are
decomposed with the liberation of ammonia; and Hagem[26] has recorded
the same process for peptones, amino acids, and other organic nitrogen
compounds in plant and animal remains in the soil. The latter author
considers soil fungi more important ammonifying agents in the soil than
bacteria, a conclusion in which McLean and Wilson,[44] and perhaps most
later workers concur. McLean and Wilson[44] found large differences in
the ammonifying powers of various soil fungi, the _Moniliaceæ_ being
the strongest ammonifiers, the _Aspergillaceæ_ the weakest. Generic and
specific differences have been confirmed by Coleman,[15] Waksman,[67]
and other authors. Waksman and Cook[70] suggested that such variations
may be due, not to innate differences in the metabolic activities of
the several organisms, but to differences in reproductive times, and
that there might be some relationship between sporogeny and the ability
to accumulate nitrogen. Kopeloff[35] has carried out experiments on
the inoculation of sterilised soil with known quantities of spores
and found that, although the amount of ammonia accumulated increased
with the number of spores the proportion was not direct but modified
by the food supply. After the first five days’ growth, the rate of
ammonia production varied markedly in a two-day rhythm which seemed
to be due to the metabolism of the fungus rather than to recurrent
stages of spore formation and germination in the life history. The
amount of ammonia liberated has been shown by recent work[66] to depend
upon the available sources of carbon and nitrogen. In the absence of
a carbohydrate supply the protein is attacked both for carbon and
nitrogen, and since more of the former is required much ammonia is
liberated. In addition, however, to the carbon and nitrogen control,
the process of ammonification by soil fungi is intimately related to
physical conditions. Working with pure cultures, McLean and Wilson,[44]
Coleman,[15] Kopeloff,[35] Waksman and Cook,[70] and other students,
have shown that the amount of ammonia accumulated depends upon such
factors as the presence of phosphates, the period of incubation of the
fungi, aeration, the moisture in the soil, the temperature, the degree
of soil acidity, the type of soil, and so forth.

That fungi take a very important place as ammonifying agents in the
soil can no longer be doubted, but the question yet remains to be
considered of the balance of profit or loss resulting from their
activities. It has usually been considered that a part of the ammonia
freed is used by the fungi themselves, but that the greater part
is liberated, and so rendered available to nitrifying organisms.
Both Neller[47] and Potter and Snyder[51] found that typical soil
fungi inoculated into sterile soil grew with a vigour approximately
equal to the growth induced by an inoculation of the entire soil
flora. This is largely to be accounted for by the fact that when
soils are sterilised by heat or by certain chemicals, breaking-down
changes occur, and substances are liberated which are peculiarly
favourable to fungus growth. This fact must be borne in mind when
interpreting ammonification and other studies where the method is that
of inoculation of fungi into sterilised soil. In many cases it tends
to nullify any application of the results to normal soils, whilst in
others the conclusions must be accepted with some reserve. In all
cases Potter and Snyder[51] found that fungi caused a diminution in
the amount of nitrates, that the ammonia was not much changed in
amount, and that there was a decrease in the quantities of soluble
non-protein nitrogen. The range of organic and inorganic nitrogenous
compounds utilisable by soil fungi is very great. Ritter[56] has shown
that certain forms can use the nitrogen of “free” nitric acid in the
medium; Ritter,[56] Hagem,[26] and others, that soil fungi can use
ammonia nitrogen equally with nitrate nitrogen, and Ehrenberg[21]
concluded that soil fungi play a more important part in the building
of albuminoids from ammonia than bacteria do. Ehrlich[22] has shown
that various heterocyclic nitrogen compounds and alkaloids can serve as
sources of nitrogen to soil fungi, whilst Ehrlich and Jacobsen[23] have
found that soil fungi can form oxy-acids from amino-acids. Hagem,[26]
Povah,[52] Bokorny,[6],[8] and others, state that for many soil forms
organic nitrogen sources are better than inorganic sources, and that
peptones, amino-acids, urea, and uric acids, etc., are very quickly
utilised by species of _Mucor_, yeasts, and so forth. Butkevitch,[12]
and Dox[18] have recently found that it depends on circumstances which
compounds of protein molecule can be utilised by particular fungi, and
that soil fungi can utilise both amino and amido complexes for the
formation of ammonia. In 1919 Boas[4] showed for _Aspergillus niger_
that if a number of nitrogenous compounds are available the fungus
absorbs the most highly dissociated.

In the welter of scattered observations on the utilisation of
nitrogenous compounds, it is difficult to trace any clear issue. That
proteins, amino-acids, and other complex organic compounds are readily
broken down to ammonia by soil fungi is clear, and, on the other hand,
it is also clear that soil fungi utilise extensively ammonia and
nitrates as sources of nitrogen. On which side the balance lies it is
yet impossible to say.

MINERAL RELATIONSHIPS.

Heinze[27] and Hagem[26] have stated that soil fungi make the insoluble
calcium, phosphorus, and magnesium compounds in soil soluble and
available for plant food; and Butkevitch[12] has used _Aspergillus
niger_ in determining the availability of the mineral constituents,
but practically no work has yet been carried out on these problems.
A further matter on which sound evidence is greatly to be desired is
the part played by soil fungi in the oxidation processes of iron and
sulphur.

A point which may be mentioned here, as it is of some considerable
practical importance, is the large quantity of oxalic, citric, and
other acids formed by certain common soil fungi. Acid formation
is partly dependent upon the species of fungus--even more the
physiological race within the species--and partly upon the substratum,
particularly the source of carbon.[5],[54] It is interesting that as
a group _Actinomycetes_ do not form acids from the carbon source but
alkaline substances from the nitrogen sources.[69]

CONTROL OF SOIL FUNGI.

In the preceding sections an attempt has been made to sketch rapidly
the chief outlines of the widespread relationships of soil fungi and
of the fundamental part that they play in the biochemical changes
occurring in the soil. It will be evident, even from this survey,
that their occurrence is of the utmost agricultural importance, both
when helpful as in mycorrhizal relationships or as agents in making
complex organic materials available as plant food, or when harmful
as when causal agents of disease in plants. It is clear that could
the soil fungi be controlled to human ends by the encouragement of
the useful forms and the elimination of the harmful, a valuable power
would be placed in the hands of the grower of plants. Certain aspects
of this control, the cruder and more destructive perhaps, are already
practicable, whilst the finer and more constructive aspects remain
possibilities of to-morrow.

Theoretically, the technique of control is selective in that it aims
to determine one or more particular fungi, leaving the remaining flora
untouched. Its highest expression is seen, perhaps, in the utilisation
of pure cultures of mycorrhizal fungi for horticultural purposes, such
as orchid cultivation, but there is no reason why this should not be
done for other purposes on a field scale similar to the way in which
cultures of special strains of the root nodule organisms of legumes
are employed. A second aspect is the direct encouragement of special
components of the fungus flora for particular purposes by selective
feeding. Thus, in a laboratory experiment, McBeth and Scales[43] record
an increase of 2000 times in cellulose-destroying and other soil fungi
by this method. It has been pointed out that soil fungus activities
such as ammonification, proteolysis and carbohydrate decomposition are
controlled by factorial equilibria, and for special purposes it would
seem feasible to weight the balance so that particular activities may
be favoured. A further step in this direction is the controlling of
particular physical conditions so that the activities of certain fungi
may be restricted. Professor L. R. Jones[30] and his colleagues at
Madison have shown the primary importance of the control of the soil
temperature in certain parasitic relationships; the work of Gillespie
and Hurst[25] and later workers has demonstrated that the parasitism
of certain species and strains of _Actinomyces_ upon the potato
is conditioned by definite ranges of soil acidity; and many other
relationships of similar nature are known. Data along such lines are
rapidly accumulating, and in certain cases are already susceptible of
practical application. In other cases, particular soil fungi are less
open to persuasive influences, and more drastic treatment needs to be
adopted. Certain chemicals mixed intimately with the soil increase or
diminish the numbers of particular fungi or groups of fungi; whilst
these organisms may be totally eliminated from the soil by wet or
dry heat for definite periods or by treatment with potent fungicides
such as formaldehyde. Although soil sterilisation and crude treatment
in other ways has been practised for decades, the possibility of a
more delicate control of soil fungi is only now being realised. Its
concrete expression will depend upon the progress that is made in exact
knowledge of the activities of soil fungi under natural and controlled
conditions, of the balance of factors in the environment which controls
any particular function and of the genetic nature of the soil fungi
which occur. Each of these aspects is a fruitful field of study.

RELATION TO SOIL FERTILITY.

From a general survey of the researches that have been carried out on
soil fungi during the past two decades certain issues emerge. It would
seem clear that fungi occupy, perhaps, a primary place as factors in
the decomposition of celluloses, and thus may be the chief agents in
the transformation of plant remains to humus and to soluble compounds
which can be used as food by the nitrogen-fixing bacteria. Furthermore,
soil fungi are very important ammonifiers, but whether the balance
of ammonia freed is utilised by the fungi themselves, or whether it
is made available to nitrifying bacteria is not yet clear. If the
latter is the case, soil fungi play a valuable indirect rôle in the
accumulation of available plant food in the soil. On the other hand, by
utilising nitrates as sources of nitrogen, fungi may play an important
part in the depletion of the nitrogenous food in the soil available to
crop plants. Thirdly, soil fungi apparently take no part in the direct
nitrogen enrichment of the soil. Thus, soil fungi would seem to be the
most important factor in the first half of that great cycle whereby
organic remains become again available as organic food.

The impression left on one’s mind by the study of the life of fungi
in the soil is of an infinitely complex series of moving equilibria,
the living activities being determined by both biological and
physico-chemical conditions. All these factors play an integral part in
the life of the soil fungi and must be considered if a true picture is
to be drawn. The principal factors may be classified into the following
groups: Most evident, perhaps, are the natures and specificities of
the fungi and the relative composition of the fungus flora. Equally
important, however, are the quantity and quality of the foods available
and the non-biological environment which results from the complex
series of physical and chemical changes occurring in the soil causally
independent of the organisms present, which interacts with the equally
vast series of changes resulting from fungus activities. Finally, one
must consider the interacting biological environment of surface animals
and plants and the microscopic fauna and flora. The complexities are
such that only the application of Baconian principles can unravel
them. A beginning has been made in the study of pure cultures of soil
fungi on synthetic media, and much valuable data have accrued, but
it is obviously not possible to apply directly to soil the results
obtained in such work. They remain possibilities; in certain cases
probabilities, but nothing more. A further step, one already taken
and of great promise, is the investigation of the changes occurring
in sterilised soils inoculated with known quantities of one or more
pure cultures of particular soil fungi. Such intensive study of single
factors in a standardised natural or artificial soil, to which has been
added a pedigreed fungus, is, perhaps, the most fruitful avenue of
progress. In all such work, however, one must bear acutely in mind the
fact that a sterilised soil and, still more, an artificial soil, is a
very different complex from a normal soil, and that results obtained
from the inoculation of such soils are not applicable directly in the
elucidation of ordinary soil processes. At present there is no method
known of completely sterilising a soil which does not destroy the
original physico-chemical balance. It is evident that the complexities
are such that chemist, physicist, and biologist must all co-operate
if the significance of the processes is to be understood, and a solid
foundation laid for future progress and for practical application.

  [1] Appel, O., “Untersuchungen über die Gattung _Fusarium_,” Mitt.
  Biol. Reichanst. Land- u. Forstw., 1907, 4.

  [2] Bernard, N., “L’évolution dans la symbiose. Les Orchidées et
  leurs Champignons commensaux,” Ann. Sci. Nat. (Bot.), Ser. 9, 1909, 9.

  [3] Bewley, W. F., “Anthracnose of the cucumber under glass,” Journ.
  Min. Agric., 1922, xxix.

  [4] Boas, F., “Die Bildung löslicher Stärke im elektiven
  Stickstoff-Stoffwechsel,” Ber. deut. bot. Ges., 1919, 37.

  [5] Boas, F., und Leberle, H., “Untersuchungen über Säurenbildung bei
  Pilzen und Hefen II.,” Biochem. Ztschr., 1918, 92.

  [6] Bokorny, T., “Benzene derivatives as sources of nourishment,”
  Zentr. Physiol., 1917, 32.

  [7] Bokorny, T., “Sugar fermentation and assimilation,” Allg. Brau.
  Hopfen Zeit., 1917, 57.

  [8] Bokorny, T., “Verhaltung einiger organischer Verbindungen in der
  lebenden Zelle,” Pflügers Archiv., 1917, 168.

  [9] Brown, P. E., “Mould action in soils,” Science, 1917, 46.

  [10] Burgeff, H., “Die Wurzelpilze der Orchideen,” Jena. 1909.

  [11] Bussey, W., Peters, L., and Ulrich, P., “Ueber das Vorkommen
  von Wurzelbranderregern im Boden,” Arb. Kais. Biol. Anst. Land- u.
  Forstw., 1911, 8.

  [12] Butkevitch, V. S., “Ammonia as a product of protein
  transformations caused by mould fungi, and the conditions of its
  formation,” Recueil d’articles dedié au Prof. C. Timiriazeff, 1916.

  [13] Butler, E. J., “An account of the genus _Pythium_ and some
  _Chytridiaceæ_,” Mem. Dept. Agr. India, 1907, Bot. Ser. 5, 1.

  [14] Christoph, H., “Untersuchungen über die mykotrophen Verhältnisse
  der Ericales und die Keimung von Pirolaceen,” Beihefte Bot. Centr.,
  1921, 28.

  [15] Coleman, D. A., “Environmental factors influencing the activity
  of soil fungi,” Soil Sci., 1916, 2.

  [16] Dascewska, W., “Étude sur la désagrégation de la cellulose dans
  la terre de bruyère et la tourbe,” Univ. Genève, Inst. Bot., 1913, S.
  8.

  [17] De Bruyn, H. L. G., “The saprophytic life of _Phytophthora_ in
  the soil,” Meded. v. d. Landbouwhoogeschool Wageningen, 1922, xxiv.

  [18] Dox, A. W., “Amino acids and micro-organisms,” Proc. Iowa Acad.
  Sci., 1917, 24.

  [19] Dox, A. W., and Neidig, R. E., “Pentosans in lower fungi,”
  Journ. Biol. Chem., 1911, 9.

  [20] Duggar, B. M., and Davis, A. R., “Studies in the physiology of
  the fungi. (I.) Nitrogen fixation,” Ann. Mo. Bot. Gard., 1916, 3.

  [21] Ehrenberg, P., “Die Bewegung des Ammoniakstickstoffs in der
  Natur,” Mitt. Landw. Inst., Breslau, 1907, 4.

  [22] Ehrlich, F., “Yeasts, moulds, and heterocyclic nitrogen
  compounds and alkaloids,” Biochem. Ztschr., 1917, 79.

  [23] Ehrlich, F., and Jacobsen, K. A., “Über die Umwandlung von
  Aminosäuren in Oxysäuren durch Schimmelpilze,” Ber. Deut. Chem.
  Gesell., 1911, 44.

  [24] Frank, B., “Ueber die auf Wurzelsymbiose beruhende Ernährung
  gewisser Bäume durch unterirdische Pilze,” Ber. d. Deut. Bot.
  Gesell., 1885, 3.

  [25] Gillespie, L. J., and Hurst, L. A., “Hydrogen-ion
  concentration--soil type--common potato scab,” Soil Sci., 1918, 6.

  [26] Hagem, O., “Untersuchungen über Norwegische Mucorineen,”
  Vidensk. Selsk. I., Math. Naturw. Klasse, 1910, 7.

  [27] Heinze, B. H., “Sind Pilze imstande den elementaren Stickstoff
  der Luft zu verarbeiten und den Boden an Gesamtstickstoff
  anzureichen,” Ann. Mycol., 1906, 4.

  [28] Van Iterson, C., “Die Zersetzung von Cellulose durch Aërobe
  Mikroorganismen,” Centr. f. Bakt., 1904, ii, 11.

  [29] Jensen, C. N., “Fungous flora of the soil,” Agric. Expt. Sta.
  Cornell, Bull. 1912, 315.

  [30] Jones, L. R., “Experimental work on the relation of soil
  temperature to disease in plants,” Trans. Wisc. Acad. Sci., 1922, 20.

  [31] Kappen, H., “Ueber die Zersetzung des Cyanamids durch Pilze,”
  Centr. f. Bakt., 1910, ii, 26.

  [32] Klöcker, A., “Contribution à la connaissance de la faculté
  assimilatrice de douze espèces de levure vis-à-vis de quatre Sucres,”
  Compt. Rend. Trav. Lab., Carlsberg, 1919, 14.

  [33] Koch, A., und Oelsner, A., “Einfluss von Fichtenharz und Tannin
  auf den Stickstoffhaushalt des Bodens und seiner physikalischen
  Eigenschaften,” Centr. f. Bakt., 1916, ii, 45.

  [34] Kohshi, O., “Ueber die fettzehrenden Wirkungen der Schimmelpilze
  nebst dem Verhalten des Organfettes gegen Fäulnis,” Biochem. Ztschr.,
  1911, 31.

  [35] Kopeloff, N., “The inoculation and incubation of soil fungi,”
  Soil Sci., 1916, 1.

  [36] Kusano, S., “_Gastrodia elata_ and its symbiotic association
  with _Armillaria mellea_,” Journ. Coll. Agric., Imp. Univ., Tokyo,
  1911, iv.

  [37] Latham, M. E., “Nitrogen assimilation of _Sterigmatocystis
  niger_ and the effect of chemical stimulation,” Torrey Bot. Club,
  Bull. 1909, 36.

  [38] Laurent, “Les reduction des nitrates en nitrites par les graines
  et les tubercles,” Bull. Acad. Roy. Sci. Belg., 1890, 20.

  [39] Löhnis, F., “Ammonification of cyanamid,” Ztschr. f.
  Gärungsphysiol., 1914, v.

  [40] Marchal, E., “Sur la production de l’ammoniaque dans le sol par
  les microbes,” Bull. Acad. Roy. Sci. Belg., 1893, 25.

  [41] Mazé, P., Vila et Lemoigne, “Transformation de la cyanamide en
  urée par les microbes du sol,” Compt. Rend. Acad. Sci., Paris, 1919,
  169.

  [42] McBeth, I. G., “Studies on the decomposition of cellulose in
  soils,” Soil Sci., 1916, I.

  [43] McBeth, I. G., and Scales, F. M., “The destruction of cellulose
  by bacteria and filamentous fungi,” U.S. Dept. Agric, Bur. Pl. Ind.,
  1913, Bull. 266.

  [44] McLean, H. C, and Wilson, G. W., “Ammonification studies with
  soil fungi,” New Jersey Agric. Expt. Sta., 1914, Bull. 270.

  [45] Melin, E., “Ueber die mykorrhizenpilze von _Pinus silvestris_
  (L.) und _Picea abies_ (L.), Karst.” Svensk. Botan. Tidskr., 1921, xv.

  [46] Muntz, A., and Coudon, H., “La fermentation ammoniaque de la
  terre,” Compt. Rend. Acad. Sci., Paris, 1893, 116.

  [47] Neller, J. R., “Studies on the Correlation between the
  production of carbon dioxide and the accumulation of ammonia by soil
  organisms,” Soil Sci., 1918, 5.

  [48] Otto, H, “Untersuchungen über die Auflösung von Zellulosen und
  Zellwänden durch Pilze,” Dissert., Berlin, 1916.

  [49] Perotti, B., “Uber das physiologische Verhalten des Dicyanamides
  mit Rücksicht auf seinen Wert als Düngemittel,” Centr. f. Bakt.,
  1907, ii, 18.

  [50] Peyronel, B., “Nuovi casa di rapporti micorizici tra
  Basidiomiceti e Fanerogame arboree,” Bull. Soc. Bot. Ital., 1922.

  [51] Potter, R. S., and Snyder, R. S., “The production of carbon
  dioxide by moulds inoculated into sterile soil,” Soil Sci., 1918, 5.

  [52] Povah, A. H. W., “A critical study of certain species of
  _Mucor_,” Bull. Torrey Bot. Club, 1917, 44.

  [53] Pratt, O. A., “Soil fungi in relation to diseases of the Irish
  potato in Southern Idaho,” Journ. Agric. Res., 1918, 13.

  [54] Raistrick, H., and Clark, A. B., “On the mechanism of oxalic
  acid formation by _Aspergillus niger_,” Biochem. Journ., 1919, 13.

  [55] Rayner, M. C., “Nitrogen fixation in Ericaceae,” Bot. Gaz.,
  1922, 73.

  [56] Ritter, G. E., “Contributions to the physiology of mould fungi,”
  Voronege, 1916.

  [57] Rosseels, E., “L’influence des microorganismes sur la croissance
  des végétaux supérieurs,” Bull. Soc. Centrale Forest. Belg., 1916, 23.

  [58] Roussy, A., “Sur la vie des champignons en milieux Gras,” Compt.
  Rend. Acad. Sci., Paris, 1909, 149.

  [59] Scales, F. M., “The Enzymes of _Aspergillus terricola_,” Journ.
  Biol. Chem., 1914, 19.

  [60] Schellenberg, H. C., “Untersuchungen über das Verhalten einiger
  Pilze gegen Hemizellulosen,” Flora, 1908, 98.

  [61] Schmitz, H., “The relation of bacteria to cellulose fermentation
  induced by fungi with special reference to the decay of wood,” Ann.
  Mo. Bot. Gard., 1919, vi.

  [62] Shibata, K., “Uber das Vorkommen vom Amide spaltenden Enzymen
  bei Pilzen,” Beitr. Chem. Physiol. u. Path., 1904, 5.

  [63] Ternetz, C., “Über die Assimilation des atmosphärischen
  Stickstoffs durch Pilze,” Jahrb. f. wiss. Bot., 1907, 44.

  [64] Verkade, P. E., and Söhngen, N. L., “Attackability of cis- and
  trans-isomeric unsaturated acids by moulds,” Centr. f. Bakt., 1920,
  ii, 50.

  [65] Waksman, S. A., “Soil fungi and their activities,” Soil Sci.,
  1916, 2.

  [66] Waksman, S. A., “The influence of available carbohydrate upon
  ammonia accumulation by micro-organisms,” Journ. Amer. Chem. Soc.,
  1917, 39.

  [67] Waksman, S. A., “Proteolytic enzymes of soil fungi and
  _Actinomycetes_,” Journ. Bact., 1918, 3.

  [68] Waksman, S. A., “On the metabolism of _Actinomycetes_,” Proc.
  Soc. Amer. Bact. Abstract Bact., 1919, 3.

  [69] Waksman, S. A., “The influence of soil reaction upon the growth
  of _Actinomycetes_ causing potato scab,” Soil Sci., 1922, xiv.

  [70] Waksman, S. A., and Cook, R. C., “Incubation studies with soil
  fungi,” Soil Sci., 1916, 1.

CHAPTER IX.

THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA).

The micro-organisms of the soil have been fully discussed in the
preceding chapters of this volume. There now remains to be considered
the fauna of invertebrate animals, other than protozoa, which inhabit
that same medium. In the first place, it is necessary to define what
groups of invertebrate animals are to be regarded as coming under the
category of soil organisms. The latter expression has rather a wide
application and, for the present purpose, is held to mean any organism
of its kind which, in some stage or stages of its life-cycle, lives
on or below the surface of the soil. It will be obvious that, with so
comprehensive a definition, the intimacy of the association of these
animals with the soil will vary within very wide limits. Some animals
pass their whole life-cycle in the soil; others are only present during
a limited phase, and not necessarily in a trophic condition, but since
their occurrence is constant, they cannot be entirely omitted from
consideration.

Unlike the groups of organisms which have been dealt with in the
foregoing pages, the invertebrates of the soil do not admit, as a rule,
of investigation in culture media. It is, in consequence, much more
difficult to achieve in the laboratory the same control over their
environmental conditions. This fact in itself largely explains why
the interpretations of field observations in animal ecology have not
usually been subjected to the test of laboratory experimentation. The
study of animal ecology, in so far as the denizens of the soil are
concerned, is of very recent birth. It has not, as yet, passed the
preliminary stage of cataloguing empirical data, and much spade work
will be necessary before the various factors controlling the phenomena
actually observed are understood.

Owing to the paucity of information available, this chapter is
essentially based upon observations conducted at Rothamsted. Its object
is not so much to attempt to evaluate the invertebrate fauna of the
soil, as to suggest a line of ecological work demanding investigation
on land of many different types.

METHOD OF INVESTIGATING THE SOIL FAUNA.

The method adopted at Rothamsted consists in taking weekly soil samples
from a given area for a period of twelve months. Each sample is a cube
of soil, with a side dimension of nine inches, and a total content
of 729 cubic inches. The samples are taken by means of an apparatus
consisting of four iron plates, which are driven into the ground down
to the required depth so as to form a kind of box, which encloses a
cube of soil (_vide_ Morris, 1922 A). The latter is then removed in
layers, each layer being transferred to a separate bag for the purpose.
When the complete sample has been extracted, there are five bags
containing layers of soil taken from the surface to a depth of 1″, from
1″ to 3″, from 3″ to 5″, from 5″ to 7″, and 7″ to 9″ respectively.
Below a depth of 9″ no samples have been taken.

The sample obtained in this manner may be gradually worked into small
fragments by hand, and examined whenever necessary under a binocular
microscope for the smaller organisms present. This procedure, however,
is very tedious and has been replaced by the use of an apparatus
consisting of a series of three sieves, with meshes of decreasing size
(_vide_ Morris, 1922). The soil is washed through these sieves by means
of a stream of water, and the meshes of the final strainer are small
enough to retain all except the most minute organisms present, while at
the same time they allow the finest soil particles to be carried away.
When desirable, the effluent can be passed through a bag or sieve of
bolting silk, in order to collect such organisms that may have passed
through the third sieve.

In addition to the actual taking and examination of the samples, a
botanical survey of the area under investigation is made; chemical
and mechanical analyses of the soil are also required. It is further
necessary to take soil temperature readings, to determine the moisture
content of the samples taken, and the amount of organic matter which
they contain.

GROUPS OF INVERTEBRATA REPRESENTED IN THE SOIL.

The various groups of invertebrates represented in the soil may be
briefly referred to in zoological order.

_Nematoda._--The Nematoda or thread-worms are chiefly animal parasites,
nevertheless they usually lead an independent existence in the soil
in certain stages of their development. The numerous small species
belonging to the family _Anguillididæ_, or eel-worms, form a definite
constituent of the soil fauna; they are generally free-living and
non-parasitic. Certain members of this family, however, are enemies of
cultivated plants.

_Annelida._--Terrestrial Annelida are almost entirely confined to the
order _Oligochæta_, the majority of which are earthworms (_Terricolæ_),
whose whole life-cycle is passed within the confines of the soil.
The small white worms of the family _Enchytræidæ_ belong to the
aquatic section (_Limicolæ_) of the order, but they have various
representatives which are abundant in damp soil containing organic
matter.

_Mollusca._--The terrestrial Mollusca are included in the sub-order
_Pulmonata_ of the _Gastropoda_. These organisms, which include the
snails (_Helicidæ_) and slugs (_Limacidæ_), regularly deposit their
eggs in moist earth. Slugs adopt the soil as a frequent habitat, only
leaving it for feeding purposes in the presence of sufficient moisture.
They are frequent consumers of vegetation, with the exception of
_Testacella_, which is carnivorous.

_Crustacea._--The few species of Crustacea inhabiting the soil belong
to the order _Isopoda_, family _Oniscidæ_, which are popularly referred
to as “woodlice,” “slaters,” etc.

_Myriapoda._--The _Diplopoda_ or millipedes include enemies of
various crops and are common denizens of the soil. The _Chilopoda_ or
centipedes are usually less abundant and are carnivorous. The minute
_Symphyla_ are often evident but are of minor importance.

_Insecta._--Insects form the dominant element in the invertebrate
fauna. Phytophagous species devour the subterranean parts of plants,
and notable examples are afforded by the larvæ of _Melolontha_,
_Agriotes_ and _Tipula_. Saprophagous forms are abundantly represented
by the _Collembola_, and by numerous larval _Diptera_ and _Coleoptera_.
Predaceous species preying upon other members of the soil fauna
are exemplified by the _Carabidæ_ and many larval _Diptera_.
Parasitic species pass their larval stages on or within the bodies
of other organisms. The groups of _Hymenoptera_, and the dipterous
family _Tachinidæ_, which exhibit this habit, constitute, along
with predaceous forms, one of the most important natural agencies
controlling the multiplication of insect life. There are also insects
(ants, and other of the aculeate _Hymenoptera_) which utilize the
soil as a suitable medium wherein to construct their habitations or
brood chambers, without necessarily deriving their food from the soil.
Lastly, there are many insects, notably _Lepidoptera_, which only
resort to the soil for the purpose of undergoing pupation. The insect
fauna is, therefore, a closely inter-connected biological complex; for
a discussion and an enumeration of its representatives reference may be
made to papers by Cameron (1913, 1917), and Morris (1921, 1922 a).

_Arachnida._--The two principal classes represented in the soil are the
_Areinida_, or spiders, and the _Acarina_, or mites, and ticks. The
_Areinida_, which are well-known to be carnivorous, are an unimportant
constituent of the fauna. _Acarina_, on the other hand, are abundant,
and exhibit a wide range of feeding habits; most of the soil forms are
probably carnivorous, and either free-living or parasitic.

NUMBER OF ORGANISMS PRESENT AND THEIR DISTRIBUTION IN DEPTH.

In computing the number of invertebrates normally present in a given
type of soil, the method adopted consists of making individual counts
of all such organisms as occur in each sample of a series taken over
a period of twelve months. This method considerably reduces errors
due to season and to the possible deviation of one or more samples
from the average. If the total number of these organisms is known for
the samples taken, it becomes a simple procedure to arrive at their
approximate numbers per acre.

TABLE XIV.

(Based on Morris, 1922 A.)

  +------------------------------+---------------+-------------+
  |                              |Unmanured Plot.|Manured Plot.|
  +------------------------------+---------------+-------------+
  |Insects                       |   2,474,700   |  7,727,300  |
  +------------------------------+---------------+-------------+
  |Larger Nematoda and Oligochæta|               |             |
  |  Limicolæ                    |     794,600   |  3,600,400  |
  +------------------------------+---------------+-------------+
  |Myriapoda--                   |               |             |
  |  Diplopoda                   |     596,000   |  1,367,000  |
  |  Chilopoda                   |     215,400   |    208,700  |
  |  Symphyla                    |      64,000   |    215,500  |
  |                              +---------------+-------------+
  |    Total                     |     875,400   |  1,791,200  |
  +------------------------------+---------------+-------------+
  |Oligochæta (Terricolæ)        |     457,900   |  1,010,100  |
  +------------------------------+---------------+-------------+
  |Arachnida--                   |               |             |
  |  Acarina                     |     215,400   |    531,900  |
  |  Areinida                    |      20,200   |     20,200  |
  |                              +---------------+-------------+
  |    Total                     |     235,600   |    552,100  |
  +------------------------------+---------------+-------------+
  |Crustacea (Isopoda)           |      33,700   |     80,800  |
  +------------------------------+---------------+-------------+
  |Mollusca (Pulmonata)          |      13,500   |     33,700  |
  +------------------------------+---------------+-------------+
  |    Total Invertebrata        |   4,885,400   | 14,795,600  |
  +------------------------------+---------------+-------------+

[Illustration: FIG. 20.--Distribution in depth of the more important
groups of soil invertebrates in the manured and unmanured (or control)
plots at Rothamsted. (From Morris, “Annals of Applied Biology,” vol.
ix., nos. 3 and 4, Cambridge University Press.)]

Table XIV. represents a numerical estimate of the invertebrate fauna of
two plots of arable land at Rothamsted. The soil is clay with flints
overlying chalk, and the land in question has been devoted for eighty
years to continuous cropping with wheat; one plot (No. 3) receives an
annual dressing of farmyard manure at the rate of 14 tons per acre, and
the other plot (No. 2) receives no natural or artificial fertilizer.
The significant feature in a comparison of the fauna of the two plots
is the great numerical increase in organisms due to the addition of
manure. From the point of view of distribution in depth, Fig. 20
clearly demonstrates that the bulk of the fauna is concentrated in the
first three inches of the soil. With the exception of the _Acarina_ it
is evident that the limits of vertical distribution extend below the
depth of nine inches investigated, although the numbers of organisms
likely to be present are inconsiderable. The _Oligochæta_, or true
earthworms, occur in Rothamsted soil in numbers very much in excess of
the figures given by Darwin, who quoted observations by Hensen. The
latter authority calculated that there were 53,767 earthworms in an
acre of garden soil, and estimated that about half that number would
be present in an acre of corn field. In the Rothamsted investigations
their numbers exceeded Hensen’s estimate over 16 times in unmanured
land, and over 36 times in manured land.

In an area of pasture-land in Cheshire few insects occurred below
a depth of 2 inches, and they reached the limit of their vertical
distribution at or near 6 inches. Their number (3,586,000 per acre)
is considerably in excess of that present in unmanured arable land at
Rothamsted.

  1, _Collembola_; 2, _Thysanura_; 3, _Orthoptera_; 4, _Thysanoptera_;
  5, _Hemiptera_; 6, _Lepidoptera_; 7, _Coleoptera_; 8, _Diptera_; 9,
  _Hymenoptera_.

FIG. 21.--Number of individuals in the different orders of insects in
manured and unmanured arable land at Rothamsted. (From Morris, “Annals
of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University
Press.)]

DOMINANCE OF CERTAIN SPECIES AND GROUPS.

In Fig. 21 a numerical analysis is given of the different orders
of insects represented in Rothamsted soil. The ascendency of the
_Hymenoptera_ and _Collembola_ is almost entirely due to the occurrence
of three species in large numbers, viz., the ant _Myrmica lævinodis_
and the _Collembola_, _Onychiurus ambulans_ and _O. fimetarius_. In
the unmanured plot these two _Collembola_ constituted 13 per cent. of
the insects and the species of ant accounted for nearly 28 per cent.
In the manured plot they amounted respectively to 27 per cent. and 36
per cent. of the insects present. Next in order of numerical ascendency
are larval _Diptera_, mainly belonging to the families _Cecidomyidæ_,
_Chironomidæ_, and _Mycetophilidæ_. The _Diptera_ are followed by
the _Coleoptera_, whose most abundant representatives are larval
_Elateridæ_ (wireworms).

  1, _Collembola_; 2, _Thysanura_; 3, _Orthoptera_; 4, _Thysanoptera_;
  5, _Hemiptera_; 6, _Lepidoptera_; 7, _Coleoptera_; 8, _Diptera_; 9,
  _Hymenoptera_; 10, _Diplopoda_; 11, _Chilopoda_; 12, _Areinida_; 13,
  _Acarina_.

FIG. 22.--Number of species of different orders of invertebrates
present in the manured and unmanured (or control) plots at Rothamsted.
(From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4,
Cambridge University Press.)]

In point of view of number of species present (Fig. 22), _Coleoptera_
take the front rank; in the unmanured plot they are very closely
approached by _Collembola_ and _Diptera_.

Passing from the insects, the next assemblage of organisms in point of
number of individuals are the smaller worms. The difficulties attending
the specific identification of these organisms are great, and, in the
present survey, the _Nematodes_ and all the smaller _Oligochætes_ have
not been separated.

The abundance of the _Myriapoda_ is mainly due to the prevalence of
_Diplopoda_, which are represented by four species. The _Chilopoda_
almost entirely consist of a single species _Geophilus longicornis_.

The dominant group of the _Arachnida_ is the _Acarine_ family
_Gamascidæ_, which are represented by about a dozen species.

  +---------+-------------+-------------+------------+--------------+
  |         |Phytophagous.|Saprophagous.|Carnivorous.|Heterophagous.|
  +---------+-------------+-------------+------------+--------------+
  |Unmanured|             |             |            |              |
  |plot     |     14      |     48      |     13     |      20      |
  |Manured  |             |             |            |              |
  |plot     |     13      |     58      |      9     |      20      |
  +---------+-------------+-------------+------------+--------------+

CLASSIFICATION OF SOIL INVERTEBRATES ACCORDING TO FEEDING HABITS.

From the point of view of the fauna as a whole, the zoological
classification of the soil invertebrates is only significant when the
various groups are analysed according to the feeding habits of their
members. All animals are directly or indirectly dependent upon plant
life for their nutrition. For the present purpose they are divided
into four categories, and the position of each class of animals in the
scheme is based upon the habits of its chief representatives in the
soil. Definite information on this subject, however, is not always
forthcoming, and it is only possible to achieve approximate estimates.
In the table above the percentages in number of individuals present in
the two plots investigated at Rothamsted are given under each type of
feeding habit.

It must be borne in mind that these estimates only apply to average
conditions; the outbreak of a plant pest in any one year must naturally
materially alter the proportions given. The phytophagous organisms are
represented by a certain number of the _Insecta_ together with the
pulmonate _Mollusca_. Carnivorous forms which are mainly beneficial
from the agricultural standpoint, include _Insecta_, together with
the _Chilopoda_, many _Acarina_ and the _Areinida_. Saprophagous
forms constitute the dominant element of the soil fauna. More than
30 per cent. of the _Insecta_ exhibit this habit, which is also the
dominant one in the _Oligochæta_, _Symphyla_, and in many of the soil
_Nematodes_. Heterophagous species include all those of somewhat
plastic habits; for the most part they are saprophagous, but, on the
other hand, a considerable proportion of the species attack growing
plants or exhibit both habits. Under this category are included a
certain number of the _Insecta_, the _Diplopoda_, _Isopoda_, and some
_Acarina_.

THE INFLUENCE OF ENVIRONMENTAL FACTORS UPON THE INVERTEBRATES OF THE
SOIL.

Since animals are endowed with powers of independent locomotion: they
are not necessarily tied to their environment to the same extent
that plants are. The investigation of the influence of environmental
factors sooner or later involves a study of the tropisms of the
animals concerned. Until these are adequately understood it is
scarcely possible to arrive at any exact conclusions relative to their
behaviour in the soil. Insects, for example, respond to the stimuli of
various, and often apparently insignificant forces, acting upon their
sensory organs. Such responses are known as chemotropism, phototropism,
hydrotropism, thermotropism, and so forth according to the nature of
the stimuli. Tropisms are automatic and, so far as they distinguish
sensations, are independent of any choice, and consequently of psychic
phenomena. Animal automatism, however, does not present the rigidity
of mechanical automatism. Differential sensibility, vital rhythms, or
periodicity, etc., are other important aspects of animal behaviour.

The environmental factors, affecting more especially the insect
population of the soil, have been discussed by Cameron (1917) and
Hamilton (1917), and certain broader aspects of animal ecology by
Adams (1915) and Shelford (1912). These factors are so numerous and
so inter-connected, that it is only possible to refer to them briefly
in the space available. As might be expected, soils that are of a
light and open texture are the ones most frequented by soil insects,
nutritional and other factors being equal. Furthermore, it has already
been shown that in arable land insects and other animals penetrate to
a greater depth than in pastures. This fact is primarily due to the
greater looseness of the soil occasioned by agricultural operations,
which ensure at the same time better drainage aeration, and greater
facilities for penetration. Hamilton found that soil insect larvæ are
very sensitive to evaporation, and especially so if the temperature
is 20° C. or over. In their natural habitat the relative humidity of
the air, in moist or wet soil, is not far below saturation, and the
temperature of the soil rarely goes above 20°-23°C., and then only in
exposed, dry, hard soil in which these larvæ do not occur.

The significance of the rate of evaporation as an environmental
factor was first emphasised by Shelford. According to him the best
and more accurate index of the varying physical conditions affecting
land animals, wholly or in part exposed to the atmosphere, is the
evaporating power of air. By means of a porous cup-atmometer, as
devised by Livingston, Shelford has carried out an important series
of experiments on the reactions of various animals to atmospheres of
different evaporation capacities, and reference should be made to his
text-book.

The importance of the organic matter present in the soil is well
illustrated in the table on p. 152. The great increase in the number of
insects and other animals is partly due to their direct introduction
along with the manure, and partly to their entry into the soil in
response to chemotropic stimuli exerted by fermentation. Organic matter
influences the fauna in other ways also; it increases the moisture
content of the soil, and it provides many species with an abundance of
food material. Also, the amount of carbon dioxide present in the soil
is partly dependent upon decaying organic matter. Hamilton conducted
experiments on the behaviour of certain soil insects in relation to
varying amounts of carbon dioxide. Although his work is of too limited
a nature to be accepted without reserve, it lends support to the
conclusions of Adams who says: “The animals which thrive in the soil
are likely to be those which tolerate a large amount of carbon dioxide,
and are able to use a relatively small amount of oxygen, at least for
considerable intervals, as when the soil is wet during prolonged rains.
The optimum soil habitat is therefore determined, to a very important
degree, by the proper ratio or balance between the amount of available
oxygen and the amount of carbon dioxide which can be endured without
injury.”

Little is known concerning the occurrence of ammonia in the soil
atmosphere, but its presence in minute quantities is probably an
important chemotropic factor in relation to saprophagous organisms
which are the largest constituent of the fauna. A great increase in
Dipterous larvæ occurs on the addition of farmyard manure, and this
is noteworthy in the light of Richardson’s experiments (1916), which
indicate that ammonia exercises a marked attraction for _Diptera_,
which spend some part of their existence in animal excrement in some
form or another.

The nature of the vegetation supported by the soil is of paramount
importance in relation to phytophagous organisms, and examples need
scarcely be instanced of certain species of soil insects being
dependent upon the presence of their specific food plants.

THE RELATION OF SOIL INVERTEBRATES TO AGRICULTURE.

The relation of these organisms to agriculture may be considered from
three points of view: (_a_) their influence upon the soil itself; (_b_)
their relation to the nitrogen cycle; and (_c_), their direct influence
upon economic plants.

(_a_) The behaviour of earthworms as a factor inducing soil fertility
is discussed by Darwin in his well-known work on the subject, and their
action may be briefly summarised as follows. In feeding habits they
are very largely saprophagous, and consume decaying vegetable matter
including humus, which they swallow, together with large quantities of
soil. Earthworms come to the surface to discharge their fæces (“worm
casts”), and in this process they are continually bringing up some of
the deeper soil to the air. Darwin estimated that earthworms annually
brought to the surface of the soil in their “casts” sufficient earth
to form a layer ·2 inch in depth, or 10 tons per acre. Their action,
along with the atmosphere, are the chief agencies which produce the
uniformity and looseness of texture of the surface soil. By means of
their burrows earthworms facilitate the penetration of air and water
into the soil, while their habit of dragging leaves and other vegetable
material into these burrows increases the organic matter present
below the surface. These facts are generally agreed upon, but it is a
disputed point whether earthworms, by devouring organic matter, aid the
conversion of the latter into plant food more rapidly than takes place
solely through the activities of micro-organisms.

Soil insects and other arthropods, by their burrowing activities,
are also instrumental in loosening the soil texture and thereby
facilitating soil aeration and the percolation of water. The action of
termites in warmer countries is discussed by Drummond in his “Tropical
Africa,” who compares the rôle of subterranean termites to that of
earthworms. The great abundance of ants renders them also significant
in this same respect, and very few species are direct enemies of the
agriculturist.

[Illustration: FIG. 23.--Diagram showing the Relation of the Soil
Invertebrata (other than Protozoa) to the Nitrogen Cycle.]

(_b_) In their relation to the nitrogen cycle (_vide_ p. 174), the
activities of the soil invertebrates may be expressed diagrammatically,
as a side-chain in the process (Fig. 23). The proteins, elaborated by
plants, are utilised as nitrogenous food by the phytophagous animals
present. The waste products of the latter, which contain the nitrogen
not used for growth or the replacement of loss by wear and tear, are
returned to the soil. Here they disintegrate, and are ultimately
converted into ammonium salts, mainly by bacterial action. The dead
bodies of these animals are also broken down by various means, becoming
eventually chemically dissociated and available as plant food. Animal
(and plant) residues serve, however, as food for the large number of
saprophagous invertebrates present in the soil. In this event the
nitrogen contained in such residues becomes “locked up,” as it were,
for the time being in their bodies. Both saprophagous and phytophagous
animals are preyed upon by carnivorous species, but ultimately the
nitrogen is returned to the soil upon the death of those organisms.
The amount present in the bodies of the whole invertebrate fauna has
been calculated by Morris (1922) upon analyses furnished by chemists
at Rothamsted. It is estimated that the fauna of manured land contains
about 7349 grm., or 16·2 lb. of nitrogen per acre, and that of
untreated land, 3490 grm., or 7·5 lb. per acre. These amounts are equal
respectively to the nitrogen content of 103·6 lb. and 48 lb. of nitrate
of soda.

The primary question affecting agriculture is, whether any notable
loss of nitrogen is occasioned by the presence of these organisms in
the soil. It has been mentioned that their nitrogenous waste material,
and their dead bodies, ultimately undergo disintegration; any loss,
if any, takes place during the latter process. With the more complex
compounds it probably consists in the production of amino-acids and
their subsequent hydrolysis or oxidation. During this process an
appreciable loss of nitrogen in the gaseous form occurs. This loss,
which is discussed on p. 173 would represent the net deficit occasioned
by the incidence of invertebrates in the soil. Against this loss must
be placed the beneficial action of such organisms as earthworms, which,
in all probability, more than counterbalances it.

(_c_) Many soil insects, on account of their phytophagous habits, are
well-known to be some of the most serious enemies of agriculture.
Certain of these, and also other classes of invertebrates, which are
likewise directly injurious, have been instanced in the earlier pages
of this chapter. Detailed information on this subject will be found in
textbooks of economic zoology, notably the volume by Reh (1913).

LITERATURE REFERRED TO.

  ADAMS, C. C., “An Ecological Study of Prairie and Forest
  Invertebrates,” Bull. Illin. St. Lab. Nat. Hist., 1915, xi.

  CAMERON, A. E., “General Survey of the Insect Fauna of the Soil,”
  Journ. Econ. Biol., 1913, viii. “Insect Association of a Local
  Environmental Complex in the District of Holmes Chapel, Cheshire,”
  Trans. Roy. Soc. Edin., 1917, lii.

  DARWIN, C., “Vegetable Mould and Earthworms,” London, 1881.

  HAMILTON, C. C., “The Behaviour of some Soil Insects in Gradients of
  Evaporating Power of Air, etc.,” Biol. Bull., 1917, xxxii.

  MORRIS, H. M., “Observations on the Insect Fauna of Permanent Pasture
  in Cheshire,” Ann. App. Biol., 1921, vii. “On a Method of Separating
  Insects and other Arthropods from Soil,” Bull. Entom. Res., 1922,
  xiii. “The Insect and Other Invertebrate Fauna of Arable Land at
  Rothamsted,” Ann. App. Biol., 1922 A, ix.

  REH, L., In Sorauer’s “Pflanzenkrankheiten,” 1913, iii.

  RICHARDSON, C. H., “The Attraction of Diptera to Ammonia,” Ann. Ent.
  Soc. Amer., 1916, ix.

  RUSSELL, E. J., “The Effect of Earthworms on Soil Productiveness,”
  Journ. Agric. Sci., 1910, iii.

  SHELFORD, V. E., “Animal Communities in Temperate America,” Chicago.
  1914, “The Importance of the Measure of Evaporation in Economic
  Studies of Insects,” Journ. Econ. Entom., 1912, vii.

CHAPTER X.

THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR RELATION TO
THE GROWING PLANT.

In the preceding chapters it is shown that the soil is normally
inhabited by a very mixed population of organisms, varying in size from
the smallest bacteria up to nematodes and others just visible to the
unaided eye, on to larger animals, and finally earthworms, which can
be readily seen and handled. These organisms all live in the soil, and
therefore must find in it the conditions necessary for their growth.
We have dealt in the first chapter with the supplies of water, air,
and heat, without which life is clearly impossible. Equally necessary
is the source of energy, for the organism requires energy material as
surely as the motor engine requires petrol, and it ceases to function
unless an adequate supply is forthcoming.

All the energy comes in the first instance from the sun, if we exclude
the unknown but probably small fraction coming from radio-active
elements. But this radiant energy is not utilisable by the soil
population, excepting surface algæ; it has to be transformed into
another kind. So far, chlorophyll is the only known transformer;
it fixes the energy of sunlight and stores it up in bodies like
hemicellulose, sugar, starch, protein, etc. The transformation is
imperfect; even the heaviest yielding crops grown under glass, in
conditions made as favourable as our knowledge permits, utilise only
about 4 per cent. of the total energy available during their period of
growth; in natural conditions not more than 0·4 per cent. is utilised.
Such as it is, however, the energy fixed in the plant represents all,
indeed more than all, that the soil organisms can obtain.

In the state of Nature, vegetation dies and is left on the soil. Two
things may then happen. It may become drawn into the soil by earthworms
and other agents; the energy supply is thus distributed in the soil
to serve the needs of the varied soil population. This is the normal
case, associated with the normal soil population and the normal flora.
If, however, the mingling agents are absent, the dead vegetation lies
like a mat on the surface of the soil, only partially decomposing,
unsuitable for the growth of most seedlings, and effectually preventing
most of the vegetation below from pushing a way through: thus there
comes to be no vegetation at all, or only a very restricted and special
flora. The soil population becomes also specialised. Peats and acid
grassland afford examples.

On the neutral grass plots at Rothamsted, the dead vegetation does not
accumulate on the surface but is rapidly decomposed or drawn into the
soil, leaving the surface of the earth bare and free for the growth
of seedlings. On the acid plots dead vegetation remains long on the
surface, blotting out all new growth excepting two or three grasses
which form underground runners capable of penetrating the mat, and
sorrel, the seedling roots of which seem to have the power of boring
through a fibrous layer of this sort. It is possible to remove the
mat entirely by bacterial action alone, if sufficient lime be added
periodically to make the reaction neutral, but failing these repeated
additions the mat persists.

We shall confine ourselves to the normal case where earthworms bring
the source of energy into the soil.

Directly the energy is available, it begins to be utilised. Two laws
govern the change. The first is well-known to biologists: it states
that the total energy of the system remains constant and can neither
be increased nor diminished except from outside; in other words, that
energy can be neither created nor destroyed. The second law is less
familiar: it is that energy once transformed to heat by one organism
cannot be used again by another. It is not destroyed; it remains
intact, but is useless to the organism. One cannot have an indefinite
chain of organisms living on each other’s excretory products; there was
a certain quantity of energy in the food eaten by the first, and no
more than this quantity can be got out whether one organism obtains the
whole or whether others share it.

The outside value for the amount of energy fixed in the soil is
obtainable by combustion of the soil in a calorimeter, but much of
this is not available to the soil organisms. The normal sedimentary
soils of England still contain decomposition products of the débris
of plants and animals originally deposited with them, but in the long
course of ages much of the extractable energy has been utilised. The
soil population is thus dependent on recently grown vegetation, and it
is therefore largely confined to the layer, usually in this country
about 6 inches thick, through which the recently dead vegetation is
distributed. Below this level there may be sufficient air, water,
temperature, etc., but there is insufficient source of energy for any
large population.

Unfortunately there is no ready means for distinguishing between the
total and the actually available quantity of energy in the soil. But
it is not difficult, by adopting the Rothamsted analytical method, to
ascertain the approximate amount of energy that has been transformed in
a given period. The Rothamsted plots are periodically analysed and a
balance sheet is drawn up showing how much of each constituent has been
added to and removed from the soil in the intervening period. For two
of the Broadbalk plots the results are shown in Tables XV., XVI.

The dunged plot receives 14 tons farmyard manure per annum, a quantity
in excess of what would usually be given; the unmanured plot, on the
other hand, has received no manure for many years and is abnormally
poor. Normal soils lie somewhere between these limits, but tending
rather to the value for the dunged than for the unmanured plot. It will
be seen that each acre of the dunged land loses on an average 41,000
calories per day, while each acre of the unmanured land loses on an
average 2700 calories per day.

TABLE XV.--MATERIAL BALANCE SHEET: BROADBALK SOIL, ROTHAMSTED.

(LB. PER ACRE PER ANNUM.)

  +-------------------------+-------------+---------------+
  |                         |  Farmyard   |  No Manure    |
  |                         |Manure Added.|    Added.     |
  |                         +------+------+-------+-------+
  |                         |  C.  |  N.  |   C.  |   N.  |
  +-------------------------+------+------+-------+-------+
  |Added  in farmyard manure| 3600 |  200 |  nil  | nil   |
  |Added in stubble         |  300 |    3 |  100  |   1   |
  +-------------------------+------+------+-------+-------+
  |     Total added         | 3900 |  203 |  100  |   1   |
  |Taken from soil          |  nil |  nil |  200  | nil   |
  |Stored in soil           |  200 |   30 |  nil  | nil   |
  +-------------------------+------+------+-------+-------+
  |     Lost from soil      | 3700 |  170 |  300  | nil[H]|
  |     Per cent.           |   95 |   84 |  100  | nil   |
  +-------------------------+------+------+-------+-------+

  Initial C : N ratio in farmyard manure, 18 : 1

  Final C : N ratio in soil, 10 : 1.

  [H] Gain of 6 lb. See p. 173.

TABLE XVI.--ANNUAL ENERGY CHANGES IN SOIL: BROADBALK. APPROXIMATE
VALUES ONLY.

MILLIONS OF KILO CALORIES PER ACRE PER ANNUM.

  +---------------------------------+-------------+---------+
  |                                 |  Farmyard   |No Manure|
  |                                 |Manure Added.|  Added. |
  +---------------------------------+-------------+---------+
  |Added  in manure                 |     14      |   nil   |
  |Added in stubble                 |      2      |   0·3   |
  |                                 +-------------+---------+
  |     Total added                 |     16      |   0·3   |
  |Taken from soil                  |     nil     |   0·5-1 |
  |Stored in soil                   |     0·5-1   |   nil   |
  |                                 +-------------+---------+
  |     Dissipated per annum        |     15      |    1    |
  +---------------------------------+-------------+---------+
  |Per day: calories                |    41,000   |   2700  |
  |Equivalent to                    |    12 men.  |  ¾ man. |
  |The human food grown provides for|     2 men.  |  ½ man. |
  +---------------------------------+-------------+---------+

These numbers are interesting when we reflect that the human food
produced on the dunged land yields only 7000 calories per day, from
which it is clear that our agricultural efforts so far provide more
energy for the soil population, for which it was not intended, than for
ourselves.

The account is not complete; we have omitted all reference to the
oxidation of ammonia and of elements other than carbon. Nature
seems to be in an unexpectedly economical mood in the soil, and all
compounds which can be oxidised with liberation of energy seem to
have corresponding organisms capable of utilising them. Even phenol,
benzene, hydrogen, and marsh gas can all be oxidised and utilised as
energy sources by some of the soil population.

Even with this remarkable power the soil population has insufficient
energy to satisfy all its possibilities; our present knowledge
indicates that energy supply is, in this country at any rate, the
factor limiting the numbers of the population. Increases in the water
supply or the temperature of the soil produce no consistent effect on
the population, but directly the energy supply is increased the numbers
at once rise.

MATERIAL CHANGES.

These transformations of energy involve transformations of matter.
The original plant residues may be divided roughly into substances
forming the structure of the plant, such as the hemicelluloses, the
pentosans, gums, and the contents of the cell--the protoplasm and the
storage products, protein; in addition, there are smaller quantities of
fats and waxes and other constituents. Some of the easily-decomposable
carbohydrates never reach the soil at all, being broken down by
intracellular respiration or attack of micro-organisms. But much of the
structure material--hemicelluloses, pentosans, etc.--remains.

Once the plant residues pass through the earthworm bodies they become
completely disintegrated and lose all signs of structure.

The only visible product so far known is humus, the black sticky
substance characteristic of soil and of manure. Two modes of formation
have been suggested. Carbohydrates, sugars, pentosans, etc., are
known to yield furfuraldehyde or hydroxymethylfurfuraldehyde on
decomposition, and it has been shown at Rothamsted that this readily
condenses to form a humus-like body, if not humus itself. In the
laboratory the reaction is effected in presence of acid, but even
amino-acids suffice. All the necessary conditions occur in the soil,
and humus formation may proceed in this way.

Some of the structure material--the lignin--contains aromatic ring
groupings. Fischer and Schrader have shown that in alkaline conditions
these ring substances absorb oxygen and form something very like humus.
It is quite possible that humus formation also proceeds in the soil
in this way. Whether the two products are chemically identical is not
known.

The scheme can be represented thus:--

                      Cell structure material
                 +----------------+----------------+
                 |                                 |
             Aliphatic                         Aromatic
          (Hemicelluloses,                   (Lignin, etc.,
          Pentosans, etc.)                   in presence of
                 |                          oxygen and under
         +-------+------------+            aerobic conditions)
         |                    |                    |
    Fatty acids         Furfuraldehyde             |
         |                    or                   |
         |        Hydroxymethylfurfuraldehyde      |
         |           (in presence of acid)         |
         |                     ·                   |
         |                       ·                 |
         |                         ·               |
         |                           ·             |
         |                             ·           |
         |                               ·         |
         |                                 ·       |
         |                                   ·     |
         |                                     ·   |
         ↓                                      ↘  ↓
  Calcium carbonate.                            Humus.

The disintegration of the cell and the first stages in the
decomposition of the structure material are almost certainly brought
about by micro-organisms. Whether they complete the process is not
known: purely chemical agencies could easily account for part.

The decomposition of protein in the soil has not been studied in any
detail. From what is known of the acid hydrolysis and the putrefactive
decompositions, however, it is not difficult to draw up a scheme which,
at any rate, accords with the facts at present known. It is probable
that the protein gives rise to amino-acids, which then break down by
one of the known general reactions.

Two types of non-nitrogenous products may be expected: The aliphatic
amino-acids give rise to ammonia and fatty acids; these form
calcium salts which break down to calcium carbonate. The aromatic
amino-acids--tyrosin, phenylalanine, etc.--which would account for
about 6 per cent. of the nitrogen of vegetable proteins, would be
expected to give ammonia and phenolic substances. Now phenols are
poisonous to plants and if no method existed for their removal the
accumulation would ultimately render the soil sterile. Matters would be
even worse on cultivated soils, since cows’ urine, which enters into
the composition of farmyard manure and is the chief constituent of
liquid manure, contains, according to Mooser, no less than 0·25 to 0·77
grams of _p_-cresol per litre,[I] a quantity three to ten times that
present in human urine. Fortunately this contingency never arises, for
the soil contains a remarkable set of organisms capable of decomposing
the phenols and leaving the soil entirely suitable for plant growth.
This affords an interesting case of an organism--in this case the
plant--growing well in a medium in spite of some adverse condition, not
because it is specially adapted to meet this condition, but because
some wholly different agent removes it.

  [I] Mooser, Zeitschrift physiol. Chem., 1909, lxiii., 176. No phenol
  was found. It is possible that the _p_-cresol is not entirely derived
  from the protein, but that some comes from the glucosides in the
  animals’ food.

Other ring compounds, e.g. pyrrol, arise in smaller quantity in the
decomposition of protein, but their fate in the soil is not known.

We may summarise the probable changes of the protein as follows:--

                                 Protein.
                +-------------------+------------------+
                |                   |                  |
            Aliphatic           Aromatic             Other
           amino-acids         amino-acids         compounds
                |                   |           (Pyrrol, etc.)
       +--------+--------+ +--------+--------+
       |                 | |                 |
  Fatty acids and      Ammonia           Phenolic
  hydroxy acids           |              compounds
       |               Nitrite               |
       |                  |                  |
       |                  |                  |
       ↓                  ↓                  ↓
    Calcium            Nitrate              CO₂
   carbonate

It must be admitted that the evidence is indirect. The rate of
oxidation of ammonia by bacteria in the soil is more rapid than the
rate of formation, so that ammonia is practically never found in the
soil in more than minimal amounts (1 or 2 parts per 1,000,000); indeed,
the only evidence of its formation was for a long time the fact that
no compound other than ammonia could be oxidised by the nitrifying
organism. It has, however, since been shown at Rothamsted that ammonia
accumulates in soils in which the nitrifying organism has been killed.

Nothing is known of the mechanism of the oxidation of ammonia beyond
the fact that it is biological; the reaction is not easily effected
chemically at ordinary temperatures. Possibly the organism assimilates
ammonia at one end of a chain of metabolic processes and excretes
nitrates at the other. Or, the reaction may be simply a straight
oxidation for energy purposes, the ammonia changing to hydroxylamine
and then to nitrous and nitric acids.

The nitrate does not remain long in the soil. Some is taken up by the
plant and some is washed out from the soil. Part, however, either of
the nitrate itself or of one of its precursors is converted into an
insoluble form: probably it is changed into protein by the action of
micro-organisms; it then goes through the whole process once more.

These are the general outlines; they present no particular chemical
difficulties. When we come to details, however, there is much that
cannot be understood.

First of all, there is the slow rate at which complex nitrogen
compounds disappear from the soil in comparison with the rate of
oxidation of the carbon. Thus, in the original plant residues, there
is some forty times as much carbon as nitrogen: before they have been
long in the soil there is only ten times as much carbon as nitrogen;
this seems to be the stable position. What is the reason for this
preferential oxidation of the carbon? No explanation can yet be given.

[Illustration: FIG. 24.

  X-axis: 1887-8 1890-1 1900-1 1910-11

  Y-axis: ℔ per acre]

An equally difficult problem arises in connection with the length of
time the process will continue. Decomposition of the nitrogen compounds
never seems to be complete in the soil; it dribbles on interminably.
In the year 1870 Lawes and Gilbert cut off a block of soil from its
surroundings and undermined it so that the drainage water could be
collected and analysed. The soil has been kept free from vegetation
or addition of nitrogen compounds from that time till now; yet it has
never failed to yield nitrates, and the annual yield falls off only
very slowly (Fig. 24). This same peculiarity is seen in the yield of
crops on unmanured land: it decreases, but very gradually; even after
eighty years the process is far from complete, and there is no sign
that it will ever come to an end.

TABLE XVII.--APPROXIMATE LOSS OF NITROGEN FROM CULTIVATED SOILS:
BROADBALK WHEAT FIELD, ROTHAMSTED, FORTY-NINE YEARS (1865-1914.)

  +------------------------+---------------------+---------------------+
  |                        | Rich Soil: Plot 2.  | Poor Soil: Plot 3.  |
  |                        |   Lb. per Acre.     |     Lb. per Acre.   |
  +------------------------+---------------------+---------------------+
  |Nitrogen in soil in 1865|·175 per cent. = 4340|·105 per cent. = 2720|
  |Nitrogen added in       |                     |                     |
  |manure, rain (5 lb. per |                     |                     |
  |annum), and seed (2 lb. |                     |                     |
  |per annum)              |       10,140        |        340          |
  +------------------------+---------------------+---------------------+
  |Nitrogen expected in    |                     |                     |
  |1914                    |       14,480        |       3060          |
  |Nitrogen found in 1914  |·259 per cent. = 5950|·095 per cent. = 2590|
  +------------------------+---------------------+---------------------+
  |Loss from soil          |         8530        |        470          |
  |Nitrogen accounted for  |                     |                     |
  |in crops                |         2500        |        750          |
  +------------------------+---------------------+---------------------+
  |Balance, being dead loss|         6030        |       -280[J]       |
  |Annual dead loss        |          123        |       -  6[J]       |
  +------------------------+---------------------+---------------------+

  [J] Gains. Possibly the result of bacterial action.

A further remarkable fact connected with the decomposition of the
nitrogen compounds is that it seems invariably to be accompanied by
an evolution of gaseous nitrogen. Apparently there are two cases.
Under anaerobic conditions many of the soil organisms have the power
of obtaining their necessary oxygen from nitrates, thereby causing
a change in the molecule which leads in some cases to liberation of
gaseous nitrogen; but the same result seems to be attained in aerobic
conditions, especially when carbon is being rapidly oxidised.

It is possible that the reaction is the same, and that in spite of the
general aerobic conditions there is locally an anaerobic atmosphere.
But it is also possible that some direct oxidation of protein or
amino-acids may yield gaseous nitrogen. However it is brought about
it affects a considerable proportion of the entire stock of nitrogen,
and it becomes more serious as cultivation is intensified. Thus, on
the Broadbalk plot receiving farmyard manure the loss is particularly
heavy; on the unmanured plot it cannot be detected. The nitrogen
balance-sheet is shown in Table XVII.

The oxidation of carbonaceous matter, however, is not invariably
accompanied by a net loss of nitrogen; in other circumstances there is
a net gain. In natural conditions there seems always to have been some
leguminous vegetation growing; the gain may, therefore, be ascribed
to the activity of the nodule organism. In pot experiments, however,
it has been found possible, by adding sugar to the soil, to obtain
gains of nitrogen where there is no leguminous vegetation, and this is
attributed to the activity of Azotobacter.

The nitrogen cycle as observed in the soil is as follows:--

            +--------------→ Protein ←----------------+
            |                 ·   ·                   |
            |               ·       ·                 |
            |             ·           ·               |
  By certain|           ·               ·             |
  organisms |           |               |             |By Azotobacter,
  and by    |           ↓               |             |Clostridium,
  growing   |        Ammonia            | Mechanism   |nodule organisms,
  plants    |           |               | uncertain   |etc.
            |           ↓               |             |
            |        Nitrite            |             |
            |           |               ↓             |
            |           ↓            Gaseous          |
            +------- Nitrate ------→ Nitrogen --------+

                     By denitrifying organisms

There has been but little study of the process of decomposition of the
other compounds in plants. Part, if not all, of the sulphur is known
to appear as sulphate, and some of the phosphorus as phosphate. It is
certain that the plant constituents decompose, for there is no sign of
their accumulation in the soil. They may exert transitory effects, but
there is nothing to show permanent continuance. The toxic conditions
which cause trouble in working with pure cultures of organisms in
specific cultures media do not, so far as is known, arise in the soil.
All attempts to find bacterio-toxins or plant toxins in normal soils
have failed. The product toxic to one organism seems to be a useful
nutrient to another, and so the mixed population keeps the soil healthy
for all its members.

There is little precise knowledge as to the part played by the
different members of the soil population in bringing about these
changes.

We know in a general way that earthworms effect the distribution of
the plant residues in the soil, and serve to disintegrate them; there
is no evidence, however, that they play any indispensable part in the
decomposition. Many root and other fragments do not go through this
process; observation shows that fungi can force a way in, and they may
be followed by nematodes which continue the disintegration. Possibly
some of the flagellates help, and certainly the bacteria do. After that
nothing is certain. We cannot, with certainty, assign any particular
reaction in the decomposition to any specific organism, with the
exception of the oxidation of the phenolic substances, the conversion
of ammonia to nitrite and nitrate, and the fixation of nitrogen. With
these exceptions many organisms seem capable of bringing about the
reactions, and indeed some of the reactions may be purely chemical and
independent of biological agencies.

The relationships between the soil population and soil fertility are
readily stated in general outline, but they are by no means clear cut
when one comes to details; fertility is a complex property, and some
of its factors are independent of soil micro-organisms.

The general relationship between plants and soil organisms is one of
complete mutual interdependence. The growing plant fixes the sun’s
energy and converts it into a form utilisable by the soil organisms;
without the plant they could not exist. The plant is equally dependent
on the soil organisms in at least two directions: their scavenging
action removes the dead vegetation which would, if accumulated on the
surface of the soil, effectively prevent most plants from growing.
Further, the plant is dependent on the soil population for supplies
of nitrates. Nothing is known about the relative efficiencies of the
various soil organisms as scavengers. Numerous fungi and bacteria are
effective producers of ammonia, the precursor of nitrates; it is not
known, however, whether flagellates and such higher forms as nematodes
act in this way.

This widespread power of producing ammonia makes it impossible in our
present knowledge to regard any particular group of organisms as _par
excellence_ promoters of fertility. Indeed, it is safest not to attempt
to do so. The primary purpose of the activities of a soil organism
is to obtain energy and cell material for itself; any benefit to the
plant is purely incidental. For cell material it must have nitrogen
and phosphorus; here it competes with the plant. If it produces more
ammonia than it utilises--in other words, if it is driven to nitrogen
compounds for its energy, then the plant benefits. If, on the other
hand, it absorbs more ammonia than it produces, as happens when it
derives its energy from non-nitrogenous substances, the plant suffers.
Thus, addition of peptone to the soil or an increase in bacterial
numbers effected without addition of external energy (e.g. by partial
sterilisation) leads to increased ammonia supply, and, therefore,
to increased fertility. But addition of sugar to the soil causes so
great an increase of numbers of bacteria and other organisms that
considerable absorption of ammonia and nitrate occurs, and fertility
is for a time depressed.

Both actions proceed in soils partially sterilised by organic
substances, such as phenol, which are utilised by some of the soil
organisms; there is first a great rise in numbers of these particular
organisms with a depression of ammonia and nitrate, then a drop to the
new level, higher than the old one, and an increased production of
ammonia and nitrate resulting from the partial sterilisation effects.

We must then regard the soil population as concerned entirely to
maintain itself, and only incidently benefiting the plant, sometimes,
indeed, injuring it; always essential, yet always taking its toll, and
sometimes a heavy toll, of the plant nutrients it produces.

This effect makes it difficult to deduce simple quantitative
relationships between bacterial activity and soil fertility, and the
difficulty is increased by the fact that bacteria and plants may both
be injured or benefited by the same causes, so that high bacterial
numbers in a fertile soil would not necessarily be the cause, but might
be simply the result of fertility.

The circumstance that certain soil organisms--bacteria, algæ, and
fungi--themselves assimilate ammonia and nitrate may account for
the remarkable slowness of nitrate accumulation, to which reference
has already been made. The protein formed from the assimilated
nitrogen remains in the bodies of the organisms, living or dead, till
decomposition sets in. It is not difficult to picture a cycle of events
in which much of the nitrate formed is at once reabsorbed by other
organisms, and only little is actually thrown off into the soil. Such a
process might continue almost interminably so long as any carbonaceous
material remained.

Finally, we come to the very interesting problem--is it possible to
control the population of the soil?

The problem may seem superfluous in view of the difficulties just
mentioned. Some aspects of it, however, are fairly clearly defined.

In the first instance, some organisms appear to be wholly harmful to
the plant; among them are parasitic eelworms and fungi, and bacteria
causing disease.

Control of these organisms can be brought about by partial
sterilisation, and of all methods heat is the most effective, but it
is costly, and attempts are now being made to replace it by chemical
treatment. The results are promising, but the investigation is
laborious; the organisms show specific relationships, and in finding
a sufficiently potent and convenient poison it is necessary in each
case to make an investigation into the relationship between chemical
constitution and toxicity to the particular organism concerned.
Formaldehyde is usually potent against fungi, and the cresols, and
particularly their chlor- and chloronitro-derivatives, are potent
against animals (eelworms, etc.).

One group of organisms is wholly beneficial, those associated with
leguminous plants. Attempts have been made to increase their activities
by inoculating the soil with more vigorous strains. The practical
difficulties still remain very considerable, but there is hope that
they may be overcome.

It is also possible to shift the balance of the soil population in
certain directions. Special groups of soil organisms can be caused
to multiply temporarily, if not permanently, by satisfying their
particular requirements. Thus, when a soil has been heated above 100°
C. it becomes specially suited to the growth of fungi, and quite
unsuited to certain bacteria such as the nitrifying organisms and
others; if this heated soil is infected with a normal soil population
the fungi develop to a remarkable extent. The nodule organisms appear
to be stimulated by addition of farmyard manure and of phosphates, and
the phenol-destroying organisms by successive small additions of phenol.

Finally, quite apart from the control of disease organisms, it is
possible to alter the soil population considerably by partial
sterilisation, using a temperature of only about 60° C., or a poison
like toluene that favours few of the soil organisms. This problem has
already been discussed in Chapter I.

The control of the soil population is still only in its infancy, but
it already promises useful developments. It cannot, however, be too
strongly insisted that the only sure basis of control is knowledge, and
we cannot hope to push control further till we have learned much more
about the soil population than we know at present.

AUTHOR INDEX.

  ADAMETZ, 118.

  Adams, 158.

  Aiyer, 113.

  Appel, 133.

  Artari, 107.

  Ashby, 42.

  BARTHEL, 24.

  Beijerinck, 6, 37, 41, 42, 46, 107.

  Berthelot, 5, 6, 41.

  Bewley, 47, 51, 132.

  Bezssonoff, 69.

  Boas, 138.

  Bokorny, 138.

  Bonazzi, 45.

  Boresch, 107.

  Boussingault, 3.

  Bredemann, 24.

  Bristol, 106.

  Brizi, 113.

  Brown and Halversen, 127.

  Burgess, 41.

  Burrill, 48.

  Bussey, Peters and Ulrich, 132.

  Butkevitch, 138, 139.

  Butler, 132.

  CAMERON, 150, 158.

  Chodat, 107.

  Christensen, 46.

  Clayton, 28, 43.

  Coleman, 127, 136.

  Conn, 23, 54, 61, 123.

  Cramer, 39.

  Crump, 57, 79, 80.

  Cunningham, 69.

  Cutler, 57, 58, 78, 80.

  DALE, 118, 121.

  Darwin, 153.

  Dascewska, 134.

  De Bruyn, 132.

  van Delden, 42.

  Delf, 87.

  Doryland, 33, 40.

  Dox, 138.

  Dox and Neidig, 134.

  Drummond, 160.

  Duggar and Davis, 135.

  Duvaine, 20.

  EHRENBERG, 138.

  Ehrlich and Jacobsen, 138.

  Esmarch, 102, 103.

  FABRICIUS, 61.

  Feilitzen, 61.

  Fischer, 118, 126.

  Forte, 101.

  Frank, 132.

  Fritsch, 112.

  GAINEY, 46.

  Gillespie and Hurst, 140.

  Goddard, 118, 120, 121.

  Golding, 49.

  Goodey, 68, 73, 79, 105.

  Greaves, 42, 61.

  Green, 37.

  Grintzesco, 107.

  Groenewege, 42.

  HAGEM, 118, 121, 136, 138, 139.

  Hamilton, 158, 159.

  Hansen, 48.

  Hanzawa, 43.

  Harrison, 113.

  Heinze, 139.

  Hellriegel, 5, 6, 46.

  Hensen, 100.

  Hesselmann, 36.

  Hill, 94.

  Hiltner, 23.

  Hopkins, 36.

  Hutchinson, C. M., 42.

  Hutchinson, H. B., 27, 43, 47, 51, 57, 105.

  VAN ITERSON, 133.

  JENSEN, 118, 132.

  Jewson, 123.

  Joffe, 37.

  Jones, D. H. and Murdock, 127.

  Jones, L. R., 140.

  KAPPEN, 136.

  Karrer, 105.

  Kaserer, 27.

  Klöcker, 134.

  Koch, A., 44, 134.

  Koch, R., 20, 53.

  Kofoid, 88.

  Kohshi, 134.

  Kopeloff, 118, 136.

  Kossowitsch, 111.

  Krainskii, 44.

  Krzeminiewski, 43.

  Kufferath, 107.

  LATHAM, 135.

  Laurent, 136.

  Lawes and Gilbert, 5.

  Lebedeff, 27.

  Leeuwenhoeck, 20.

  Lendner, 118.

  Lipman, C. B., 41, 42, 44, 54.

  Lipman, J. G., Blair, Owen, and McLean, 94.

  Löhnis, 22, 43, 69, 136.

  MAGNUS, 107.

  Malpighi, 46.

  Marchal, 34, 136.

  Martin, 73.

  Martin and Lewin, 69.

  McBeth, 28, 134.

  McBeth and Scales, 118, 140.

  McLean and Wilson, 118, 136.

  Mockeridge, 43.

  Moore, G. T., 105.

  Morris, 150, 151, 162.

  Muntz and Coudon, 118, 136.

  NABOKICH, 27.

  Nagaoka, 38.

  Nakano, 107.

  Nasir, 94, 95.

  Neller, 137.

  OMELIANSKI, 27, 42.

  Orla-Jensen, 26, 35.

  Otto, 133.

  Oudemans and Koning, 118.

  PASTEUR, 3, 20.

  Perey, 94.

  Perotti, 136.

  Petersen, 104.

  Pillai, 43.

  Potter and Snyder, 137, 138.

  Povah, 138.

  Pratt, 132.

  Prescott, 61.

  Pringsheim, 107.

  RAMANN, 118.

  Rathbun, 120.

  Reh, 162.

  Remy, 118.

  Richards, 112.

  Richardson, 159.

  Ritter, 138.

  Robbins, W. J., 109.

  Robbins, W. W., 105.

  Roussy, 134.

  Russell, 112.

  Russell and Hutchinson, 57, 66, 94.

  SALUNSKOV, 42.

  Sandon, 57, 75.

  Scales, 28, 134.

  Schellenberg, 133.

  Schindler, 107.

  Schloesing, 3, 4, 34.

  Schmitz, 134.

  Schramm, 111.

  Servettaz, 109.

  Seydel, 44.

  Sherman, 69.

  Shibata, 136.

  Söhngen, 26, 27, 134.

  TAKAHASHI, 118.

  Taylor, 120.

  Ternetz, 135.

  Treub, 112.

  Truffaut, 69.

  VERKADE and Söhngen, 134.

  Von Ubisch, 109.

  WAKSMAN, 37, 118, 120, 121, 123, 125, 126, 134, 136.

  Waksman and Cook, 136.

  Wann, 111.

  Warington, 4, 34.

  Waynick, 42.

  Welwitsch, 112.

  Werkenthin, 120, 121.

  West, 88, 105.

  Whiting, 36.

  Wilfarth, 46.

  Winogradsky, 4, 6, 34, 41, 44.

SUBJECT INDEX.

  _Absidia_, 121.

  _Acarina_, 150, 151, 157.

  Acid formation by Fungi, 139.

  Acidity of soil, 17; effect on Actinomyces, 140; relation to
  nitrification, 36.

  _Actinomycetes_, 119, 134, 139.

  Aeration of soil, effect on bacteria of, 61.

  _Agriotes_, 150.

  Air supply in soil, 17.

  Algæ, agents causing disappearance of nitrate from soil, 12;
  associations of, in soil, 105, 106; blue green, 102 _sqq._ (see also
  Cyanophyceæ and Myxophyceæ); colonisation of new ground by, 112;
  conditions of growth for, 101, 104, 107, 108; distribution of, 102,
  104, 106, 109; economic significance of, 100, 102; filamentous,
  106; flora of soil, 101, 112; formation of humus substances, 112;
  fragmentation of filaments, 107, 110; frequency of occurrence, 102
  _sqq._; glucose, effect of, on growth, 108, 109; green, 104 _sqq._
  (see also Chlorophyceæ); importance in cultivation of rice, 113;
  numbers in soil of, 109, 110; nutrition of, 107, 108, 110; producers,
  of organic substance, 100; pure cultures of, 107, 111; relation to
  gaseous interchange in soil, 113; relation to soil moisture, 112;
  seasonal changes in numbers of, 88; subterranean, 105.

  Alkaloids, as source of nitrogen for fungi, 138.

  _Alternaria_, 119.

  Amino-acids, formation of, by algæ, 108.

  Amino-compounds, decomposition of, by fungi, 136, 138.

  Ammonia, assimilation of, by bacteria, 33, 40, 45; effect of partial
  sterilisation on soil content of, 66; formation in soil, 170;
  formation in soil by bacteria, 32 _sqq._; formation in soil by fungi,
  135 _sqq._, 141; influence of physical conditions on formation of,
  137; property of attracting Diptera, 159; utilisation by higher
  plants, 36.

  Ammonium sulphate, effect on fungi, 121, 126, 127.

  _Anabæna_, 102, 112.

  _Annelida_, 149.

  Antagonism of salts in soil, 60.

  Ants, 153.

  _Arachnida_, 150, 151.

  Arctic soil, bacterial flora of, 24.

  _Areinida_, 150, 151, 157.

  _Armillaria_, 132.

  _Ascomycetes_, 119.

  _Aspergillaceæ_, 136.

  _Aspergillus_, 119, 120, 135, 136, 138, 139.

  Azotobacter, 6, 41, 95, 96; assimilation of nitrates by, 45;
  decreasing efficiency in liquid culture, 44; indicator of soil
  acidity, 44.

  BACILLARIACEÆ, 100 (see also Diatom).

  _Bacillus amylobacter_, distribution of, 24.

  _Bacillus radicicola_, 24, 46 _sqq._; inoculation of soil with, 50;
  life cycle of, 47.

  Bacteria, association with algæ in nitrogen fixation, 111; anærobic
  respiration of, 37; effect of arsenic on, 61; cellulose destroying,
  134; changes in morphology in culture, 22, 47; classification of main
  groups, 23, 25; composition of cells of, 39; inverse relationship
  with protozoa, 10, 79, 82 _sqq._; isolation from soil, 21; methods
  of describing, 21; method of estimating numbers of, 53 _sqq._, 80;
  nitrogen fixation by, 110, 111; numbers in relation to algæ, 110;
  numbers in soil, 52 _sqq._; oxidation of hydrogen by, 27, 37; effect
  of partial sterilisation on, 8, 9, 66, 67; part played in soil
  fertility by, 7; pure cultures, isolation by plating, 20; seasonal
  changes in numbers of, 59, 87 _sqq._; effect of salts on, 60; short
  time changes in numbers of, 11, 57, 58; effect of temperature on, 67;
  uneven distribution of, 57.

  _Basidiomycetes_, 119, 123, 132.

  Beets, attacked by _Phoma betæ_, 135.

  _Boletus_, 132.

  _Botrytis_, 122.

  Bryophyta, 100, 132.

  _Bumilleria_, 105.

  CALCIUM compounds in soil and fungi, 139.

  _Carabidæ_, 150.

  Carbohydrates, decomposition by bacteria, 26 _sqq._; decomposition by
  fungi, 140; decomposition in soil, 168; effect on ammonia production
  in soil, 33; presence in algal sheath and bacteria, 111.

  Carbon, changes in amount in soil, 167; relationships of bacteria,
  27; relationships of fungi, 133; source of, for soil bacteria, 39;
  sources of, for soil fungi, 139.

  Carbon dioxide, assimilation by algæ, 99, 107, 108; assimilation by
  soil bacteria, 35, 36, 40.

  Carotin, in algæ, 100; formed by _Spirochæta cytophaga_, 29.

  _Cecidomyidæ_, 155.

  Cellulose, decomposition by bacteria, 27, _sqq._; decomposition by
  fungi, 133, 134, 141; relation of nitrogen supply to decomposition
  of, 30; decomposition in soil, 168; as source of energy for nitrogen
  fixation, 43.

  Centipedes, see _Chilopoda_.

  _Cephalosporium_, 120.

  _Cephalothecium_, 136.

  _Chilopoda_, 157.

  _Chironomidæ_, 155.

  _Chlorella_, 108.

  _Chlorococcum_, 105.

  _Chlorophyceæ_, 100.

  Chlorophyll, loss of, from algæ, 108.

  Ciliates, classification of, 72; cyst wall of, 73.

  Citric acid, formation of, by fungi, 139.

  _Cladosporium_, 119.

  Clamp connections in fungi, 119.

  Classification, of algæ, 100; of bacteria, 23, 25; of fungi, 131; of
  protozoa, 69 _sqq._

  Climate, effect of, on algæ, 101.

  _Clostridium_, 41, 44; as fixer of nitrogen, 6.

  _Coccomyxa_, 104.

  _Coleoptera_, 150, 154, 155.

  _Collembola_, 150, 153, 154.

  _Colletotrichum_, 131.

  Commensals, 132.

  _Conjugatæ_, 100.

  _Cortinarius_, 132.

  Cotton, destroyed by fungi, 134.

  Counting, of algæ, 109; of bacteria, 53 _sqq._; of fungi, 122; of
  protozoa, 77, 79, 80.

  Cresol, decomposition of, by bacteria, 22, 24, 31.

  Criteria, physiological, of fungi, 128.

  Crop growth, effect on fungi, 122.

  _Cryptomonadineæ_, 100.

  Cucumber leaf spot, 131.

  Cyanamide, decomposition of, by fungi, 136.

  _Cyanophyceæ_, 103 (see also _Myxophyceæ_ and blue-green algæ).

  _Cylindrospermum_, 102.

  Cysts, 68, 73, 74.

  DENITRIFICATION, by bacteria, 37; by fungi, 136.

  Desiccation, resistance to, by algæ, 106.

  Dew, relation to algæ, 101, 113.

  Diatoms, 104 _sqq._ (see also _Bacillariaceæ_).

  Dicyanamide, decomposition of, by fungi, 136.

  Dipeptides, formation of, by algæ, 108.

  _Diplopoda_, 157.

  _Diptera_, 150, 154, 155, 159.

  Disaccharides and fungi, 134.

  EARTHWORMS, abundance of, in soil, 153; effect of, in soil, 13, 160,
  175.

  Eel-worms, 149 (see also _Nematoda_).

  _Elaphomyces_, 132.

  _Enchytræidæ_, 149.

  Energy, laws of, 165; relationships of soils, 166; requirements of
  soil organisms, 15, 16.

  Energy supply, relation of bacterial activities to, 25 _sqq._, 40,
  44; sources of, for soil bacteria, 26 _sqq._, 40, 43; supplies of,
  for soil organisms, 111, 164, 167, 168.

  Environmental conditions in soil, 16.

  Eremacausis, 2.

  Ericales, 132, 135.

  _Euglena_, 99.

  _Euglenaceæ_, 100.

  Experimental error, in bacterial counts, 54; in fungal counts, 124.

  FARMYARD manure, see Manure.

  Fats, used by fungi, 134.

  Fatty acids used by fungi, 134.

  Fertility of soil, views on, 2; effect of decomposition of plant
  residues on, 1, 165; effect of organisms on, 175.

  Filter paper, destruction of, by fungi, 133; destruction of, by
  _Spirochæta cytophaga_, 28.

  Fixation of nitrogen, discovery of, by Berthelot, 5; by bacteria, 40
  _sqq._; by algæ, 110, 111; by mixtures of bacteria and algæ, 111; by
  fungi, 135 _sqq._ (see also Nitrogen Fixation).

  _Flagellatæ_, 100.

  Flax sickness and fungi, 122.

  Formaldehyde, as agent for destroying fungi, 141.

  Fungi, control of, in soil, 139 _sqq._; counting of, 122;
  distribution of, in soil, 119 _sqq._, 127; fertilisers, effect of, on
  numbers of in soil, 126; as facultative parasites, 131, 132; fruiting
  bodies of, 123; destruction of hemicelluloses by, 133; individual,
  122, 123; action on monosaccharides of, 134; mineral relationships
  of, 139; mycorrhizal, 132, 135, 139, 140; heterocyclic nitrogen
  compounds and, 138; occurrence in soil, 118; qualitative study of,
  118; selective feeding of, 140; specific determination of, 119.

  _Fungi imperfecti_, 119.

  _Fusaria_, 134.

  _Fusarium_, 119, 120, 122, 128, 133, 136.

  _Gamascidæ_, 156.

  Gases of swamp water (Paddy soils), 113.

  _Gastrodia_, 132.

  Gelatinous envelope of algæ, 109, 111.

  Geographical distribution of azotobacter, 41; of soil bacteria, 24;
  of protozoa, 75, 76; of soil fungi, 119, 125.

  Germination, of algal spores, 107.

  Glucose, use of, by algae, 108, 109, 111; use of, by moss protonema,
  109.

  Glycocoll, formation of, by algæ, 108.

  _Granulobacter_, 42.

  Greenland, bacteria in soil from, 24.

  “Grunlandmoor,” fungi in, 126.

  _Hantzschia_, 105.

  _Hemiptera_, 154.

  _Heterokontæ_, 100.

  “Hochmoor,” fungi in, 126.

  _Hormidium_, 104.

  Humus, the food of plants, 1; formation of, by fungi, 134, 141;
  formation of, in soil, 168; forest, 132; fungal hyphæ as constituent
  of forest humus, 132.

  Hydrogen ion concentration, in soil, 17; effect on fungi of, 124.

  _Hymenoptera_, 150, 154.

  _Insecta_, 150, 157.

  Insects, numbers present in soil, 154.

  Invertebrata, definition of, 147; method of investigating, 148;
  groups represented, 149; distribution in the soil, 151; dominant
  species and groups, 153; environmental factors of, 157; feeding
  habits, 156; relation to agriculture, 160; relation to nitrogen
  cycle, 161.

  Iron compounds, oxidation by fungi, 139.

  _Isopoda_, 150, 151.

  _Leguminosæ_, association with bacteria, 46 _sqq._; enrichment of
  ground by, 5.

  _Lepidoptera_, 150, 154.

  Life cycles, of bacteria, 22, 47; of protozoa, 72 _sqq._

  Lime, effect on fungi in soil, 121, 126.

  _Lyngbya_, 112.

  MAGNESIUM compounds, effect on fungi, 139.

  Manganese compounds, effect on bacteria, 61.

  Manure, farmyard, effect on algæ, 109, 110; effect on numbers of
  bacteria, 60; effect on numbers of fungi, 126; effect on numbers of
  insects, 154, 155.

  Manure, Artificial, effect on fungi, 127.

  Manure, town stable, occurrence of disease organisms in, 132.

  _Mastigophora_, classification of, 71; species of, 71.

  Media, containing nitrates, chemical analysis of, 111; for counting
  soil bacteria, 54; for counting protozoa, 79; for counting fungi,
  119, 123.

  _Melanconium_, 134.

  _Melolontha_, 150.

  Methane, oxidation of, by bacteria, 26, 27.

  Millipedes, see _Diplopoda_.

  Mites, see _Acarina_.

  _Mollusca_, 149, 157.

  _Moniliaceæ_, 136.

  _Mucor_, 120, 121, 136, 138.

  _Mucorales_, 121, 134.

  _Mucorineæ_, 118.

  _Mycetophilidæ_, 155.

  Mycorrhiza, 132, 135, 139, 140.

  _Myriapoda_, 150, 156.

  _Myxophyceæ_, 100 (see also _Cyanophyceæ_ and blue-green algæ).

  NAPHTHALENE, decomposition of, by bacteria, 31.

  _Naviculoideæ_, 100.

  _Nematoda_, 149, 151, 157.

  Nitrate, assimilation by algæ, 105, 108, 111; assimilation by
  bacteria, 33, 40, 44, 51; assimilation by fungi, 136, 138; removal
  from soil, 12, 112, 171; variations in amount in soil, 11.

  Nitre-beds, 1.

  Nitrification, and bacteria, 34; chemical changes in, 171; and fungi,
  136; energy supply in, 35; mechanism of, 1, 3; and soil fertility, 1,
  3.

  Nitrites and fungi, 136; formation by bacteria, 34.

  _Nitrobacter_, 35.

  Nitrogen, changes in amount in soil, 167; cycle in soil, 161;
  fixation by bacteria, 6, 40 _sqq._; fixation by fungi, 135, 136, 141;
  fixation of, in clover plant, 5; increase by protozoa of fixation
  of, 94, 95 (fig.); fixation sources of energy for, 43, 49; gain of,
  in soil, 174; in invertebrates, 162; loss of, by leaching, 112;
  loss of, from cultivated soils, 173; relationships of fungi, 135;
  relationships of algæ, 110-112; relationships of bacteria, 32 _sqq._,
  40 _sqq._; relationships of insects, 162.

  _Nitrosococcus_, 35.

  _Nitrosomonas_, 35.

  Nodule Organism of the Leguminosæ, 6, 46 _sqq._

  _Nostocaceæ_, 100, 101, 102, 107.

  _Oligochæta_, 149, 151, 153, 157.

  _Oospora_, 120.

  _Orcheomyces_, 132.

  Orchid cultivation and fungi, 132, 140.

  _Orthoptera_, 154.

  _Oscillatoriaceæ_, 100, 102.

  Osmotic pressure, influencing effect of salts on bacteria, 50.

  Oxalic acid, formation of, by fungi, 139.

  Oxidations effected by soil organisms; by bacteria, 26 _et seq._; by
  fungi, 139.

  Oxygen, absorption by soils, 4.

  PARTIAL sterilisation of soil, 8, 66 _sqq._, 96, 178; influence of
  organic antiseptics, 177; limiting factor in, 67, 68.

  Pectin, effect of, on fungi, 134.

  _Pedras negras_, 112.

  _Penicillia_, 134.

  Pentosans, effect of, on fungi, 134.

  Peptones, decomposition of, by fungi, 136, 138; source of nitrogen
  for algæ, 108.

  Periodicity, of protozoa in soil, 90 _sqq._ (fig.), 92 (fig.), 93.

  Phenol, decomposition of, by bacteria, 24, 25, 31.

  Phenylalanine, formation of, by algæ, 108.

  _Phoma_, 132.

  _Phormidium_, 106.

  Phosphates, availability of, influenced by bacteria, 52; by fungi,
  139; effect on bacteria, 46, 51, 60.

  Photosynthesis, 99, 100, 107, 110, 113.

  Phycocyanin, 100.

  Physical conditions in soil, 16.

  Physiological criteria, of bacteria, 22; of fungi, 128.

  _Phycomycetes_, 119.

  _Phytophthora_, 132.

  Plant disease, and fungi, 139.

  Plant residues, decomposition of, in soil, 168; influence of soil
  reaction on, 165; relation to soil fertility, 1, 165.

  Plasticity of fungi, 119.

  _Plectonema_, 106.

  Potassium salts, effect on bacteria, 60; influence of bacteria on the
  availability of, 52.

  Protein, decomposition of, in soil, 169; decomposition by bacteria,
  32; decomposition by fungi, 138, 140.

  _Protococcales_, 100.

  _Protoderma viride_, 105.

  Protonema of mosses, 100, 105, 106, 109.

  Protophyta, chlorophyll-bearing, 100.

  Protozoa, inoculation into soil of, 85 _sqq._; isolation from soil,
  69; classification of, 69 _sqq._; life histories of, 72 _sqq._;
  species of, in soil, 70 _sqq._; distribution of, in soil, 74 _sqq._;
  retention of, by soil, 78 (fig.); size of, 90; reproductive rates,
  93; inverse relation with bacteria, 79 _sqq._; presence of trophic
  forms in soil, 9; numbers of, in soil, 90, 96, 97; fluctuations in
  numbers of, 10, 81 (fig.), 82; external conditions, effect on, 82;
  seasonal changes, effect on, 87 _sqq._; weight of, 90.

  _Pteridophyta_, 132.

  _Pythium_, 132.

  REACTION of soil, 17.

  Reaction of soil, effect on bacteria, 36, 37, 46, 48, 61; effect on
  protozoa, 93, 94 (see also hydrogen ion concentration).

  Relationships of Fungi, commensal, 132; mycorrhizal, 132; symbiotic,
  132.

  _Rhizopoda_; classification of, 70, 71; species of, 70, 71.

  Rhythm, supposed in ammonification by fungi, 137.

  _Rhizoctonia_, 132.

  _Rhizopus_, 119, 120.

  Rice plant, aeration of roots, 113; physiological disease of, 113.

  Rock Phosphate as base for nitrifying organisms, 36.

  Rothamsted, Broadbalk plot 2 (Farmyard Manure) algæ, 109; fungi, 125,
  127; Insects, 152.

  Rothamsted, Broadbalk plot 3 (Unmanured) algæ, 109; fungi, 120, 122,
  127; Insects, 152.

  Rothamsted, Broadbalk Plots 10, 11, and 13; 122, 127.

  Rothamsted, Barnfield Plot 1-0 (Farmyard Manure), Protozoa, 80.

  Rothamsted, unmanured grass plot, 120.

  _Russula_, 132.

  Rusts, 119.

  _Saccharomyces_, 120.

  Saprophytes, facultative, 131.

  Saprophytism and algæ, 108, 110.

  _Scenedesmus_, 108.

  Seasonal fluctuations in numbers of soil organisms, 12, 87 _et seq._,
  125.

  Selective media, use of, in isolation of soil bacteria, 21.

  Serological tests, separation of varieties of _B. radicicola_ by, 48.

  Slugs, see _Mollusca_.

  Smuts, 119.

  Snails, see _Mollusca_.

  Soil; comparison of, by volume, 17; effect of depth below surface
  on algæ, 101, 104, 109, 110, 113; effect of depth below surface on
  insects, 151; effect of depth below surface on fungi, 121, 126, 127;
  effect of various treatments on fungi, 126, 127, 132; environmental
  factors in, 16; inoculation of, for leguminous plants, 50; moisture
  (see Water supply); population, control of, 177 _sqq._; population,
  methods of investigation, 10, 15; sterilisation and fungi, 137, 138,
  141 (see Partial Sterilisation); stored, survival of algæ in, 107;
  type and fungi, 121, 126, 127.

  Soil conditions, effect on bacteria, 33, 36, 37, 40, 46, 48, 50, 59
  _sqq._; effect on protozoa, 82.

  Soil fertility, see Fertility of soil.

  _Spicaria_, 120.

  Spiders, see _Areinida_.

  _Spirochæta cytophaga_, 28, 43.

  Spore forming bacteria in soil, 23, 34.

  Spore, fungus, inhibition of formation, 123; presence in air of, 118.

  Standardisation of cultural methods for soil bacteria, 54 _sqq._

  Starch, decomposition of, by fungi, 134.

  _Stichococcus_, 108.

  Straw; effect on nitrate production in soil, 33; manure, 29; rotting
  of, 30.

  Sulphur oxidation, by bacteria, 37; by fungi, 139.

  Symbiosis, of Azotobacter with other organisms, 42, 43, see also
  Mycorrhiza and Nodule organism.

  _Symphyla_, 150, 151, 157.

  _Symploca_, 112.

  _Tachinidæ_, 150.

  Tannins, used by fungi, 134.

  Temperature of soil and fungi, 127, 140.

  Termites, 160.

  _Testacella_, 149.

  _Thiospirillum_, 37.

  _Thysanura_, 154.

  _Thysanoptera_, 154.

  _Tipula_, 150.

  Toluene, decomposition by soil bacteria, 31.

  _Tolypothrix_, 112.

  _Trichoderma_, 119, 120, 122, 134.

  _Trochiscia_, 105.

  Tropisms, 157.

  _Ulothrix_, 105.

  _Ulotrichales_, 100.

  Urea, by fungi, 136, 138.

  Uric acid, utilisation of, by fungi, 138.

  _Vaucheria_, 104, 106.

  Vitality, retention of, by algæ and moss protonema, 105, 107.

  WATER; supply in soil, 17; and algæ, 112; bacteria, 50, 61, 82;
  fungi, 127; protozoa, 82.

  Wireworms, 155.

  Wood, decay of, 134.

  Woodlice, 150; (see also _Isopoda_).

  YEASTS, 138.

  _Zygnema_, 104.

  _Zygorrhynchus mœlleri_, 119, 120, 121.

PRINTED IN GREAT BRITAIN BY THE UNIVERSITY PRESS, ABERDEEN

  Transcriber’s Notes

  Inconsistent and archaic or unusual spelling, capitalisation,
  italicisation, hyphenation, etc. have been retained, unless mentioned
  below. The names and classifications of the organisms as used in the
  book do not always conform to modern names and classifications; these
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  Depending on the hard- and software used and their settings, not all
  elements may display as intended.

  Page 14, table, lower right hand cell: the data given add up to 8,
  not to 9.

  Page 47, ·9 × ·18 in size: the source document does not include the
  units; presumably the sizes are in microns.

  Page 118, endnote 8c (2×): this note does not exist.

  Subject Index, entry Zygorrhynchus mœlleri: also refers to
  Zygorrhynchus vuilleminii.

  Changes made:

  Footnotes, tables and illustrations have been moved out of text
  paragraphs; some tables have been split or re-arranged.

  Several minor obvious typographical, punctuation and spelling errors
  (including accents) have been corrected silently. In several cases
  spelling differences (mainly of proper names) between the text and
  the index and endnotes have been standardised. In the indexes and in
  tables some ditto marks have been replaced with the dittoed
  text. Some page references have been corrected to indicate the
  correct page number.

  Indented text under illustrations is not present as such in the
  source document, but has been transcribed from the illustration for
  legibility and ease of reference. Some tables have been re-arranged
  or split to fit the available width.

  Page 28, 29: MacBeth changed to McBeth as elsewhere (in the Author
  Index the entries MacBeth and McBeth have been merged).

  Page 32, formula: 30 changed to 3O.

  Page 58: From Barnfeild, ... changed to From Barnfield, ....

  Page 85: closing bracket deleted after ... Table VII. and Fig. 13.

  Page 90, Table VIII, column 5: 350·000 and 150·000 changed to 350,000
  and 150,000.

  Page 97: No creature lies or dies to itself, ... changed to No
  creature lives or dies to itself, ...

  Page 104: Danske Aerofile Alghe changed to Danske Aërofile Alger.

  Page 114: Recherche sulla Malattia del Riso ... changed to Ricerche
  sulla Malattia del Riso ....

  Page 115: ... sur de polymorphisme ... changed to ... sur le
  polymorphisme ....

  Page 116: literature notes 38 (Robbins) and 48 (Schindler) changed to
  33 and 34 respectively.

  Page 120: Zygorrhynchus vuillemini changed to Zygorrhynchus
  vuilleminii as elsewhere.

  Page 126: references to Waksman[24] and [24_e_] changed to [25] and
  [25_e_].

  Page 129: ... preparée de la pres de Russum ... changed to ...
  préparée de la terre humeuse du Spanderswoud, près de Bussum ....

  Page 134: reference to Kohshi[24] changed to [34].

  Page 143: Sämenbildung changed to Säurenbildung (entry 5);
  Wurzelbranderregern im Baden changed to Wurzelbranderregern im Boden
  (entry 11).

  Page 144: ... Umwandlung von Aminosamen in Oxysämen ... changed to
  ... Umwandlung von Aminosäuren in Oxysäuren ....; ... Wirkungen der
  Schimmelze ... changed to ... Wirkungen der Schimmelpilze ....;
  Hydrogen-iron concentration changed to Hydrogen-ion concentration.

  Page 145: Ztschr. f. Garungs. Physiol. changed to Ztschr. f.
  Gärungsphysiol.

  Page 146: einige Pilze gegen Hemizellulosen changed to einiger Pilze
  gegen Hemicellulosen.

  Page 157: Such responses are known chemotropism ... changed to Such
  responses are known as chemotropism ....

  Page 170: ... alphatic amino-acids ... changed to ... aliphatic
  amino-acids ....