SOILS 95 



and after they decay they leave little channels in the soil 

 which serve to carry down water laden with carbonic acid, 

 as well as to introduce air that is a factor in bringing about 

 chemical changes in the soil and in furthering work in soil 

 formation. 



We shall learn later that in the^process of soil formation 

 certain leguminous plants, such as clovers, vetches-, etc., are 

 introduced into the soil, and that upon the roots of these 

 plants are nitrogen-gathering bacteria, enabling the plant 

 to derive part of its food from the nitrogen of the atmosphere. 

 The return of these plants to the soil through their decay adds 

 the element of nitrogen in the further formation of soil. The 

 soil is not only alive with bacteria, but myriads of forms of 

 animal life live and burrow in the soil, and aid in soil forma- 

 tion. 



The various agencies concerned in the formation of the 

 soil do not act separately, nor necessarily in any particular 

 order. As a matter of fact, all the processes take place simul- 

 taneously. Neither is all the soil formed directly from the 

 original rock. The soil is almost constantly moving, for some 

 of the agencies which -form soil also carry it away. It is 

 always moving from higher to lower levels.. Consequently it 

 is thinnest at the top of the hill and deepest in the valley. 

 Nature undisturbed has many ways of adding to the supply 

 of available plant-food in the soil. The various forces dis- 

 cussed have all tended to change the food into forms that can 

 be assimilated by the plants. Man has reversed the process, 

 and while adding little to the soil has removed much from 

 it. A study of the formation of the soil suggests two things 



