SOIL FORMATION AND TRANSPORTATION 15 



their growth causing the rock to decay and organic matter 

 to accumulate in the crevices. These plants are followed by 

 higher vegetation, the roots of which are larger ; when these 

 roots extend themselves into cracks in the rock they exert 

 a prying action when wind gives the plant a swaying motion. 



After rock becomes sufficiently pulverized to produce 

 soil, plants are active agents in decomposing soil particles 

 by the solvent action of the acid secreted by their roots 

 and formed by their decay. 



Very small plants, included among the microorganisms 

 because they are too small to be seen without a microscope, 

 are also concerned in rock decay. Their action is exerted 

 principally in. soil, and is due to the production of acids even 

 stronger than that secreted by the roots of higher plants. 



17. Powdered rock is not soil. — We have seen that in 

 the process of soil formation the rock is pulverized, but the 

 process of weathering to which nature resorts is different 

 in its result from merely grinding rock in a crusher or mortar. 

 At the same time that the particles are becoming smaller, 

 certain chemical changes are going on that produce a ma- 

 terial having a different composition from the original rock. 

 One result of the transition is the removal of a part or some- 

 times all of the more soluble constituents of the rock. The 

 percentage loss of some of the constituents of granite and 

 of limestone in the process of forming a clay is as follows : 



Table 1. — Percentage Loss of Plant-Food Materials in 

 Granite and Limestone in Process of Soil Formation 



