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.