ORGANISMS IN THE SOIL 423 



it is in soil of this character that they are most needed 

 because of the stirring and aeration they accomplish. 

 Sandy soil and the soils of arid regions, in which are 

 found few or no earthworms, are not usually in need of 

 their activities. 



344. Insects. There is a less definite, and probably 

 less effective, action of a similar kind produced by insects. 

 Ants, beetles, and the myriads of other burrowing insects 

 and their larvse effect a considerable movement of soil 

 particles, with a consequent aeration of the soil. At the 

 same time they incorporate into the soil a considerable 

 quantity of organic matter. 



345. Large fungi. The larger fungi are chiefly con- 

 cerned in bringing about the first stages in the decom- 

 position of woody matter, which is disintegrated through 

 the growth in its tissues of the root mycelia of the fungi. 

 These break down the structure, and thus greatly facili- 

 tate the work of the decay bacteria. Action of this kind 

 is largely confined to the forest and is not of great im- 

 portance in cultivated soil. 



Another function of the large fungi is exercised in the 

 intimate, and possibly symbiotic, relation of the fungal 

 hyphse to the roots of many forest trees, in soil where 

 nitrification proceeds very slowly, if at all, for nitrates are 

 apparently not abundant in forest soils. This envelop- 

 ing system of hyphse, which may consist of masses in a 

 definite zone of the cortex with occasional filaments pass- 

 ing outward into the soil, or which may surround the root 

 with a dense mass of interwoven hyphse, is called mycor- 

 rhiza. 



The cereal, cruciferous, leguminous, and solanaceous 

 plants are not associated with mycorrhiza. Mycotrophic 

 plants are usually those that live in a humous soil filled 



