[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.