ROTATION OF CROPS 



117 



Rotation to add vegetable matter. No soil can be kept 

 fertile unless vegetable matter is added to it, either by 

 the decay of large amounts of plants grown on the land or 

 by other means. Place must be made in any rotation for 

 an occasional crop that leaves much vegetable matter to be 

 plowed under. Such crops are hay plants, which com- 

 pletely cover the surface and leave a mass of roots and 

 fallen leaves. The stubble of wheat and oats, together 

 with the growth of weeds 

 that usually follows these 

 grains, also affords consider- 

 able vegetable matter. 



Rotation to add nitrogen. 

 Plants like cowpeas and 

 clovers get nitrogen from 

 the air. This nitrogen is 

 added to the soil when the 

 growth of cowpeas or clover 

 is plowed into the land. 

 Even the stubble and roots 

 of these crops, when plowed 

 under, increase the nitrogen 

 in the soil. Hence a wisely 

 planned rotation makes room 

 for the growing, as fre- 

 quently as can well be done, 

 of some leguminous crop, such as cowpeas, clover, peanuts, 

 or velvet beans. When oats or wheat are grown, cow- 

 peas ought usually to be sown in June after the grain crop 

 is removed. Cowpeas ought generally to be sown among 



FIG. 72. TWENTY-FIVE CORN PLANTS 

 IN EACH BUNDLE 



On left, grown after plowing under the 

 stubble of mixed vetch and oats; on 

 right, after oat stubble. Weights, 33 

 and 18 pounds. 



