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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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.