The Relation of Soil Invertebrates to Agriculture.
The relation of these organisms to agriculture may be considered from three points of view: (a) their influence upon the soil itself; (b) their relation to the nitrogen cycle; and (c), their direct influence upon economic plants.
(a) The behaviour of earthworms as a factor inducing soil fertility is discussed by Darwin in his well-known work on the subject, and their action may be briefly summarised as follows. In feeding habits they are very largely saprophagous, and consume decaying vegetable matter including humus, which they swallow, together with large quantities of soil. Earthworms come to the surface to discharge their fæces (“worm casts”), and in this process they are continually bringing up some of the deeper soil to the air. Darwin estimated that earthworms annually brought to the surface of the soil in their “casts” sufficient earth to form a layer ·2 inch in depth, or 10 tons per acre. Their action, along with the atmosphere, are the chief agencies which produce the uniformity and looseness of texture of the surface soil. By means of their burrows earthworms facilitate the penetration of air and water into the soil, while their habit of dragging leaves and other vegetable material into these burrows increases the organic matter present below the surface. These facts are generally agreed upon, but it is a disputed point whether earthworms, by devouring organic matter, aid the conversion of the latter into plant food more rapidly than takes place solely through the activities of micro-organisms.
Soil insects and other arthropods, by their burrowing activities, are also instrumental in loosening the soil texture and thereby facilitating soil aeration and the percolation of water. The action of termites in warmer countries is discussed by Drummond in his “Tropical Africa,” who compares the rôle of subterranean termites to that of earthworms. The great abundance of ants renders them also significant in this same respect, and very few species are direct enemies of the agriculturist.
Fig. 23.—Diagram showing the Relation of the Soil Invertebrata (other than Protozoa) to the Nitrogen Cycle.
(b) In their relation to the nitrogen cycle (vide [p. 174]), the activities of the soil invertebrates may be expressed diagrammatically, as a side-chain in the process ([Fig. 23]). The proteins, elaborated by plants, are utilised as nitrogenous food by the phytophagous animals present. The waste products of the latter, which contain the nitrogen not used for growth or the replacement of loss by wear and tear, are returned to the soil. Here they disintegrate, and are ultimately converted into ammonium salts, mainly by bacterial action. The dead bodies of these animals are also broken down by various means, becoming eventually chemically dissociated and available as plant food. Animal (and plant) residues serve, however, as food for the large number of saprophagous invertebrates present in the soil. In this event the nitrogen contained in such residues becomes “locked up,” as it were, for the time being in their bodies. Both saprophagous and phytophagous animals are preyed upon by carnivorous species, but ultimately the nitrogen is returned to the soil upon the death of those organisms. The amount present in the bodies of the whole invertebrate fauna has been calculated by Morris (1922) upon analyses furnished by chemists at Rothamsted. It is estimated that the fauna of manured land contains about 7349 grm., or 16·2 lb. of nitrogen per acre, and that of untreated land, 3490 grm., or 7·5 lb. per acre. These amounts are equal respectively to the nitrogen content of 103·6 lb. and 48 lb. of nitrate of soda.
The primary question affecting agriculture is, whether any notable loss of nitrogen is occasioned by the presence of these organisms in the soil. It has been mentioned that their nitrogenous waste material, and their dead bodies, ultimately undergo disintegration; any loss, if any, takes place during the latter process. With the more complex compounds it probably consists in the production of amino-acids and their subsequent hydrolysis or oxidation. During this process an appreciable loss of nitrogen in the gaseous form occurs. This loss, which is discussed on p. 173 would represent the net deficit occasioned by the incidence of invertebrates in the soil. Against this loss must be placed the beneficial action of such organisms as earthworms, which, in all probability, more than counterbalances it.
(c) Many soil insects, on account of their phytophagous habits, are well-known to be some of the most serious enemies of agriculture. Certain of these, and also other classes of invertebrates, which are likewise directly injurious, have been instanced in the earlier pages of this chapter. Detailed information on this subject will be found in textbooks of economic zoology, notably the volume by Reh (1913).