FARMING AND FERTILITY 



119 



crops to be turned under, in order to maintain the supply of 

 nitrogen and organic matter ; and, second, the purchase of 

 phosphorus and sometimes of potassium to replace that removed 

 in crops. It requires, further, that all straw and corn stover be 

 returned to the land. This may be illustrated with a rotation of 

 corn, oats, and clover, all grain being sold, and the corn stover 

 and oat straw being returned to the land. The first crop of clover 

 must be cut somewhat earlier than usual and left to lie on the 

 ground, the second crop coming up through it and being cut for 

 the seed, the threshed clover straw being returned to the land. 



146. Balance of gain and loss. The effect of such a system on 

 an acre of soil during one round of the rotation is shown below : 



LOSS IN CROPS 



RETURNED TO SOIL 



Under this system the nitrogen supply is nearly main- 

 tained by the clover, if we assume that the clover growing in 

 the oat stubble during the first year is checked against the 

 loss of this element through leaching and the loss through the 



