GEOLOGICAL STUDY OF SOILS 6 1 



porating the initial stores of carbon and nitrogen of the 

 air with the disintegrated rock materials. 



61. Action of Vegetation. Some of the lower forms 

 of plants, as lichens, do not require soil for growth, but 

 are capable of living on the bare surface of rocks, 

 obtaining food from the air, and leaving a certain 

 amount of vegetable matter which undergoes decay and 

 is incorporated with the rock particles, preparing the 

 way for higher orders of plants which take their food 

 from the soil. When this vegetable matter decays, it 

 enters into chemical combination with the pulverized 

 rock, forming humates. 18 The disintegrating action of 

 plant roots and vegetable matter upon rocks has been 

 an important factor in soil formation. The action of 

 vegetable remains in soil production is discussed in 

 Chapter III. 



62. Earthworms. Many soils have been greatly 

 modified by the action of earthworms. The soil in 

 passing through their digestive tract is ground into finer 

 particles and is intimately mixed with the indigestible 

 matter excreted by the worms. In the case of rich loam 

 soils it is estimated that all of the particles have at 

 some time passed through the digestive tract of earth- 

 worms. Where they have been active, air and water 

 are admitted into the soil more readily. The action of 

 earthworms in soils has been extensively studied by 

 Darwin. 



