PART III. SOIL 



CHAPTER XV 

 NATURE OF SOIL 



IN our discussion of plants and crops we have constantly 

 referred to the soil. What is soil? Whence does 

 it come? What relation does it bear to plant life, and hence 

 to our own lives? Take a handful of "dirt" and crumble 

 it between your fingers; of what does it consist, what is 

 its nature? 



1. Origin of the Soil 



Soil did not always exist as it is to-day. When the earth 

 was young and the crust was forming there was no soil. 

 There was only rock. And it is out of the weathering of 

 this surface rock that the soil has come ; and the process is 

 still going on whenever rock is exposed. Soil is but par- 

 ticles of rock, to which has been added organic matter com- 

 ing from the plants and animals that have lived on it or in it. 



The weathering of rock. Rock is made into soil by 

 two different processes, disintegration and decomposition. 

 By disintegration is meant the breaking up of rock into 

 small particles without changing their nature. By decom- 

 position is meant such breaking up by chemical action that 

 the nature of the particles is changed. 



The chief agencies causing the disintegration of rocks 

 are sudden changes of temperature and the action of frost. 



225 



