THE LIFE OF FUNGI IN THE SOIL.
In the last chapter fungi were considered as so many specific but
functionless units in the soil. Unless, however, they are regarded
merely as inert spore contaminations from the air, a view which is
now no longer tenable, their very presence implies the existence
of innumerable vital relationships between the organisms and their
environment. From this point of view the studies treated in the
previous chapter are but the necessary first steps to an understanding
of the relation of soil fungi to living plants and of the part played
by them in the soil economy.
RELATION OF SOIL FUNGI TO LIVING PLANTS.
Older classifications of fungi frequently divided these organisms
into four categories--parasites, saprophytes, facultative parasites,
and facultative saprophytes, but the further mycological studies are
carried the more clearly it is seen that these groups are entirely
artificial. There are probably few fungi that cannot, under particular
conditions, invade living tissues, and it only seems a question of
time before at all events the vast majority of fungi will be grown on
synthetic media in the laboratory. From our present point of view the
importance of this lies in the fact that fungi living saprophytically
in the soil may, given the right conditions or the presence of some
particular host plant, become parasites or symbionts, and conversely
well-known pathogens may live a saprophytic existence. Thus Cucumber
Leaf Spot is caused by _Colletotrichum oligochætum_, and Bewley[3] has
repeatedly isolated this fungus from glasshouse manure and refuse of
various kinds. In his early studies, Butler[13] isolated many parasitic
species of _Pythium_ from Indian soils, and the presence of _P. de
Baryanum_ as a soil saprophyte has been confirmed by Bussey, Peters,
and Ulrich.[11] De Bruyn[17] has recently found that most species of
_Phytophthora_, including _P. erythroseptica_ and _P. infestans_ may
live as saprophytes in the soil, whilst Pratt[53] has isolated from
virgin lands and desert soils various fungi, which cause disease in
potatoes. In 1912 Jensen[29] gave a list of twenty-three “facultative
parasites” isolated from soil, and these are but a moiety of those
which could be listed to-day.
Furthermore, it was shown by Frank[24] many decades ago that forest
humus is not merely a mass of the remains of animals and plants,
but that a considerable part of its organic substance is made up of
fungus hyphæ, which ramify and penetrate in all directions. Evidence
is rapidly accumulating that this is also true of most other soils
containing organic matter. It is well known that many of the higher
plants live in symbiotic or commensal relationship with these humus
fungi, which are present in the host tissues as mycorrhiza, and further
studies only serve to show the widespread and fundamental nature
of this relationship. Thus many _Basidiomycetes_[50] (species of
_Tricholoma_, _Russula_, _Cortinarius_, _Boletus_, _Elaphomyces_, etc.)
possess a mycorrhizal relationship with various broad leaved trees,
such as beech, hazel, and birch[57] and with various conifers and
certain Ericales. Other Ericales show this relationship with species of
the genus _Phoma_,[62] many orchids, with species of _Rhizoctonia_[2]
(or _Orcheomyces_[10]), whilst _Gastrodia elata_ contains _Armillaria
mellea_.[36] Certain species of _Pteridophyta_ and _Bryophyta_ are
also known to certain mycorrhizal fungi. Of the numerous fungi taking
part in these mycorrhizal relationships, only a small number have yet
been identified, but there is little doubt that perhaps the majority
of these organisms must be regarded as true soil forms.[14],[45] The
mycological flora of the soil thus plays an important part in the life
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.
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