SOIL IMPROVEMENT 83 



for about every third rotation. Where the growth of 

 corn is not too rank, cowpeas or soy beans make a satis- 

 factory catch crop and these may well be used in suc- 

 cessive rotations to prevent insect or fungous pests ob- 

 taining a foothold through the too continuous use of 

 clover. It should be remembered that the roots of clover 

 contain one-half as much nitrogen as the tops and the 

 roots of cowpeas only about one-tenth as much as the 

 tops. In grain crops about two-thirds of the nitrogen 

 is deposited in the grain and one-third in the stalk and 



roots. 



On all lands not subject to overflow phosphorus should 

 be applied in considerably larger amounts than are re- 

 quired for the need of the crop actually growing at that 

 time. The fine ground natural rock phosphate can be 

 used successfully and is the most economical form of 

 phosphorus in all crop systems. The first application 

 should be at least one-half ton per acre, and a ton would 

 be better. Subsequently one-half ton applied every four 

 to six years will suffice until the total phosphorus con- 

 tained in the soil reaches 2,000 pounds per acre. This 

 will require a total application of five to six tons of raw 

 phosphate. 



For quick action and in emergencies steamed bone meal 

 or acid phosphate may be used, but this is a much more 

 expensive form than the ground natural rock. Good 

 phosphate direct from the mine in carload lots costs about 

 3 cents per pound, while steamed bone meal costs 12 

 cents per pound, and acid phosphate 12 cents. 



The loss of phosphorus by leaching is very small un- 

 less the land is subject to overflow or excessive drain- 

 age, so that erosion losses occur. Phosphorus applied 

 is not removed except in the form of mature crops. 

 Phosphorus and limestone may be applied at any time 

 during the rotation, but the limestone is best applied on 

 plowed land so that it may be worked into the soil dur- 



