20 FIRST PRINCIPLES OF AGRICULTURE. 



tinction between soil and subsoil is that the soil contains 

 more organic or vegetable matter, is more finely divided, 

 and is less compact than the subsoil. The subsoil may 

 be regarded as something between the soil and rock, 

 and partaking in part of the characteristics of both. 

 The subsoil serves to gradually supply the constituents 

 that are removed by crops from the surface soil, and 

 also performs important functions as a reservoir of 

 moisture, and as a medium for the roots of plants. Its 

 character thus materially modifies the fertility and pro- 

 ductiveness of the soil proper. 



The Formation of SoiL Certain agencies are con- 

 stantly at work converting rock into soiL This gradual 

 conversion is termed "weathering," and is observed on 

 all sides. The rapid crumbling of shale, so familiar in 

 Xew Jersey, on exposure to the atmosphere, is an ex- 

 cellent illustration of thin process, the air, temperature, 

 and water all playing an important part. 



The Action of the Atmosphere. The atmosphere 

 causes what is termed " oxidation," a slow burning or 

 decay. The oxygen of the air combines with the miner- 

 als, forming new substances called " oxides." A familiar 

 illustration of this process is seen when iron is exposed 

 to the air; the red rust that forms is an oxide of iron, 

 a substance very different from the iron itself. 



The Influence of Water. The most powerful agent 

 in the rotting or breaking up of rocks is water. It acts 

 both mechanically and chemically. Most rocks contain 

 cracks or fissures into which the water readily penetrates ; 

 by freezing the water expands, and forces the rock apart. 

 Thus the solid rock is gradually separated into fragments 



