CHAPTER V 

 CROPPING SYSTEMS 



Where Did Soils Acquire Productiveness? Dig into the 

 earth any place and " bed rock " will be found. Sometimes bed rock 

 is exposed above the surface and sometimes buried under many hun- 

 dred feet of soil. Originally, all the surface was rock, but a portion 

 has been slowly and gradually reduced, by such agencies as glaciers, 

 freezing, and rain water containing carbonic acid. Gradually as 

 this rock was pulverized, it accumulated where other agencies, as 

 plants and bacteria, worked on it further. 



Pulverized rock might contain all the minerals needed by plants, 

 but our common field crops would not grow in pulverized rock, as 

 may be easily demonstrated. There are two reasons for this : First, 

 the minerals are not soluble in water ; and second, the rock is devoid 

 of nitrogen. 



How Rock Minerals Become Soluble. Rain water contains 

 some carbon dioxide, which is a weak acid and has a slow solvent 

 effect on certain minerals of the soil. Enough minerals would be 

 leached out to support aquatic vegetation. This aquatic vegetation 

 mixed with soil would decay and produce organic acids, that in 

 turn would break down the insoluble minerals. 



Certain plants, as lichens, also live on rocks, and excrete acids 

 that liberate enough minerals for their own use. When some or- 

 ganic matter was finally mixed into sterile soils, other agencies, such 

 as bacteria and a variety of plant life, could live, and finally heavy 

 vegetation could flourish. On our forest lands, trees have grown 

 for ages, and the decaying leaves, branches, and roots have added 

 large quantities of organic matter. On the prairies, grasses have 

 grown up, to die on the land each year. In this way the available 

 mineral supply of the soil has been slowly accumulated. 



Where Nitrogen Came From. While nitrogen was not found 

 in the rock from which soil was made, it is the most important ele- 

 ment of the air, constituting about four-fifths of the atmosphere, or 

 35,000 tons over each acre of land. Very small quantities of 



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