18 SOILS: PROPERTIES AND MANAGEMENT 



must result most disastrously to the physical condition 

 of the rock. This action of frost is by no means com- 

 plete when the rock is fined mechanically to a soil, but is 

 continued on the soil itself. Such further fining is of 

 the greatest importance in bettering the physical condi- 

 tion of the soil, and is usually designated as a wetting and 

 drying and freezing and thawing process. It is to such 

 forces, more than to any other action, that the farmer 

 owes the good tilth of his soil. 



15. Plants and animals. Plants and animals unite 

 their forces with those already mentioned to bring about 

 further physical change. Unlike the modifications due to 

 erosion, denudation, and temperature, these agencies affect 

 the soil to a greater extent than they affect the parent rock. 

 In other words, they begin their work after the minerals 

 have been reduced, at least partially, to the form of a 

 soil. Simple plants, as mosses and lichens, will develop 

 readily on rock ledges and coarse rock fragments. They 

 send their rootlets into the crevices and exert a prying 

 and loosening effect. They also catch dust, provide 

 humus, and gradually accumulate a soil in which higher 

 and still higher species of plants may grow. Their 

 chemical effects, especially regarding solution and oxida- 

 tion, aid in this disintegration. The distribution of 

 organic matter through the soil by the extension and 

 death of plant roots is of no mean importance in soil 

 fertility. Bacteria also may be a factor in rock decay, 

 not only through their action on the humus material 

 but also through a direct attack on the rocks themselves. 

 Their influence, however, is probably mostly chemical. 



Animals also effect the fining of rock fragments and 

 soils, from their burrowing and mixing tendencies. Such 

 rodents as gophers and squirrels open up the soil, thus 



