18 THE PRINCIPLES OP AGRICULTURE 



usually proceeds more slowly than on inclined 

 surfaces. 



28. There are, then, two sets of forces con- 

 cerned in the original formation of soils, — the 

 disintegration or wearing away of the rock, and 

 the transfer or moving of the particles to other 

 places. 



26. The organic elements and agents 



29. Plants are important agents in the forma- 

 tion of soil. Their action is of two kinds : the 

 roots corrode and break up the surfaces of rock 

 and particles of soil, and the plant finally 

 decays and adds some of its tissue to the soil. 



30. In the disintegration of rock and the 

 fining of soil, the root acts in two ways : it ex- 

 erts a mechanical force or pressure as it grows, 

 cracking and cleaving the rock ; and it has a 

 chemical action in dissolving out certain ma- 

 terials, and thereby consuming and weakening 

 the rock. 



31. Animals contribute to the formation of 

 soil by their excrement and the decay of their 

 carcasses. Burrowing and digging animals also 

 expose rocks and soils to weathering, and con- 

 tribute to the transportation of the particles. 

 Some animals are even more directly concerned 

 in soil- making. Of these, the chief are the 



