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.