ROTATION OF CROPS 



409 



air by rain and thus reach the soil. Exceedingly small quantities 

 of nitrates are formed in nature in this way. Commercial processes 

 of building nitrogen into 

 nitrates is now being con- 

 ducted in several sections 

 of the country. Com- 

 mercial nitrates are 

 generally used for ferti- 

 lizers. One of these 

 plants is at Muscle 

 Shoals, i\.labama. The 

 excellent water-power 

 facilities of that location 

 supply the necessary 

 energy for the project. 



Rotation of crops. 

 Crops are rotated, that 

 is, one crop succeeds a 

 different kind in the 



same field year after year. The reasons for crop rotation are as 

 follows : (1) Plants use up varying amounts of the different min- 

 eral matters in the soil. One crop might need a quantity of 

 calcium salts but not much iron. Another crop will require 

 considerable iron but fewer sulphates. If one crop is planted year 

 after year in the same plot of land, that land will grow poor in one 

 or more mineral salts and the quality of the crops will be greatly 

 impaired unless commercial fertilizers are used. Where rotation 

 of crops is practiced, the same soil can be used for years without 

 adding fertilizer. (2) Plants are subject to diseases caused by 

 bacteria and other fungi. If only one crop is planted, the soil 

 may become infected with spores of disease-producing organ- 

 isms. Another crop probably would not be affected by these 

 spores. For example, corn smut will not injure potatoes; po- 



WH. FITZ. AD. BIO. — 27 



There is a continual building and destruction of carbon 

 compounds. This is described diagranunatically in the 

 carbon cycle. 



