ROTATION OF CROPS 



189 



first crop. For this reason, clover is often grown on land in which 

 it is proposed to plant corn later, the nitrates left in the soil giving 

 nourishment to the young corn plants. In well-managed farms, 

 different crops are planted in succession in a given field in different 

 years so that one crop may replace some of the elements taken from 

 the soil by the previous crop. This is known as rotation of crops. 1 

 Five of the elements necessary to the life of the plant which 

 may be taken out of the soil by constant use are calcium, nitrogen, 

 phosphorus, potassium, and sulphur. Several methods are used 

 by the farmer to prevent the exhaustion of these and other raw 

 food materials from the soil. One method, known as fallowing, 

 is to allow the soil to remain idle until bacteria and oxidation have 

 renewed the chemical materials used by the plants. This is an 

 expensive method if land is high priced. The more common 

 method of enriching 

 soil is by means of fer- 

 tilizers and materials 

 rich in plant food. 

 Manure is most fre- 

 quently used, but 

 many artificial ferti- 

 lizers, most of which 

 contain nitrogen in the 

 form of some nitrate, 

 are used because they 

 can be more easily 

 transported and sold. 

 Such are ground bone, 

 guano (bird manure), 

 nitrate of soda, and 



many others. Most Is this a g00d plan for rotating crops ] 



fertilizers contain other important raw food materials for plants, 

 especially potash and phosphoric acid. Both of these substances 

 are made soluble by the action of the carbon dioxide in the soil, 

 and in this form they can be taken into the roots. 



1 Crop rotation is not only a process to conserve the fertility of the soil, but 

 also a sanitary measure to prevent infection of the soil. 



Why? 



