316 SOIL PHYSICS AND MANAGEMENT 



erals are acted upon by the acids of the alimentary canal, producing 

 chemical changes resulting in the liberation of plant food. They 

 carry large amounts of soil from the subsurface and subsoil and 

 deposit it on the surface of the ground, where it may be seen as 

 casts, especially in the morning after a rain. Darwin states that 

 where earthworms abound the amount brought by them forms a 

 layer from 0.1 to 0.2 inch in thickness each year. This amounts to 

 from 15 to 30 tons per acre. Some comparative experiments have 

 been conducted which show that earthworms increase the yield of 

 crops. 



4. Plants. The soil is modified to a large extent by the roots 

 of all plants, whether large or small. The short-lived annuals and 

 biennials have the greater effect, because new roots are formed every 

 one or two years. The roots of perennial prairie grasses are great 

 factors in modifying soil because of their great abundance and 

 deep penetration. While most of the roots of trees and shrubs live 

 for years, yet many die every season. Eoots of all plants add some 

 organic matter to the soil, but they have another important effect. 

 They make the soil more porous after they decay and thus improve 

 aeration and drainage. Frequently when timber spreads over a 

 prairie area having a tight clay subsoil the ultimate effect of the 

 roots is to lessen the impervious character of the subsoil so that 

 drainage takes place with much less difficulty. 



Many fungi live on and in the soil and affect it to some extent. 

 Some aid in the early decomposition of vegetable matter. Others 

 are diseases, such as some smut and scab. Others live in sym- 

 biotic relation to certain higher plants. These through the agency 

 of large numbers of hypha? or fungi rootlets, called michoriza, trans- 

 fer the food to the companion plant. 



MICRO-ORGANISMS 



The group of micro-organisms, consisting of bacteria, fungi, 

 protozoa, algae and yeasts, is of special importance in soils. They 

 aid in the transformation of the vegetable and animal remains into 

 the humus-like residue, which really constitutes part of the soil. 

 They carry on many other operations that benefit soils chemically 

 and to some extent physically. However, the physical condition of 

 soils and the phenomena that occur in them which influence the 

 work and development of these micro-organisms are so important 

 that they merit considerable attention. How the farmer may in- 



