SOIL ORGANISMS 385 



ties of soil. While the effect is rather beneficial and is analo- 

 gous to tillage, the activities of these animals are generally 

 unfavorable to agricultural operations and such soil inhabi- 

 tants have been more or less exterminated in arable land. 



The common earthworm is the most conspicuous example of 

 the benefits that may accrue from the presence of animals. 

 Darwin, as the result of careful measurements, states that the 

 quantity of soil passed through these creatures may under 

 favorable conditions in a humid climate, amount to ten tons 

 of dry earth to the acre annually. The earthworm obtains its 

 nourishment from the organic matter of the soil, but takes 

 into its alimentary canal the inorganic matter as well, ex- 

 pelling the latter in the form of casts after it has passed en- 

 tirely through the body. The ejected material is to some ex- 

 tent disintegrated, and is in a flocculated condition. The holes 

 left in the soil serve to increase aeration and drainage. The 

 activities of the worms bring about a notable transportation 

 of lower soil to the surface, which aids still more in effecting 

 aeration. Darwin's studies led him to state that from one- 

 tenth to two-tenths of an inch of soil is yearly brought to the 

 surface of land in which earthworms exist in numbers normal 

 to fertile soil. 



Earthworms naturally seek a heavy compact soil, and it is 

 in soil of this character that they are most needed because of 

 the stirring and aeration that they accomplish. Sandy soil 

 and that of arid regions, in which are found few or no earth- 

 worms, are not usually in need of their activities. 



There is a less definite, and probably a less effective, action 

 of a similar kind produced by insects. Ants, beetles, and the 

 myriads of other burrowing insects and their larvae 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. 



210. Macro-organisms — plant forms. — The larger fungi 

 are chiefly concerned in bringing about the first stages in the 



