ROTATION SYSTEMS FOR GRAIN FARMING 227 



would produce, the nitrogen contained in the roots and stubble 

 being no more than that furnished by the ordinary corn-belt soil. 

 If the two regular cuttings would make two tons of clover hay; 

 and if the growth of clover during the previous season (after wheat 

 or oats harvest) and during the autumn (after the clover-seed 

 harvest) and the following spring (before plowing for corn) would 

 make another half-ton of clover hay, or two and one half tons in 

 all, then 100 pounds of nitrogen would be secured from the air to 

 balance the 86 \ or 89 pounds removed in the grain and seed. 

 In other words, from 13 to 15 per cent more nitrogen is returned 

 by the clover than is removed in the grain and seed. 



On normal soils the only addition to this system that is neces- 

 sary in order to establish a permanent agriculture is the applica- 

 tion of 20 pounds of phosphorus for the lower yields, or 40 pounds 

 for the larger yields, these amounts being ample to replace the 

 phosphorus removed in the grain and seed and to cover all possible 

 loss by leaching. For the smaller yields, 200 pounds per acre of 

 steamed bone meal or 200 pounds of raw rock phosphate or 400 

 pounds of acid phosphate, every three years, will be more than 

 sufficient to maintain the phosphorus content of the soil; and twice 

 these quantities would be ample for the larger yields after the 

 productive power of the soil has been raised to that point. To do 

 this may require much heavier initial applications of phosphorus, 

 or moderately heavy applications for the first four or five rotations. 

 Thus, an application of one ton of good rock phosphate (12^ per 

 cent phosphorus) every three years would add 1250 pounds of 

 phosphorus per acre in 15 years, or more than 1000 pounds above 

 the amount removed in the grain or seed for the larger yields in 

 the rotation. In other words, the phosphorus content of the aver- 

 age Illinois surface soil should be doubled in 15 years under this 

 system, and the annual cost of phosphorus ($2.50) would be no more 

 than is commonly paid by farmers in the Eastern and Southern 

 states for so-called " complete " fertilizers. If the phosphorus 

 applied produced increased yields of 7 bushels of corn and equiva- 

 lent values of other crops, the cost would be covered by the in- 

 creased crops. (See actual results reported in later pages.) 



This system requires that the ears of corn shall be husked and 

 the stalks returned to the soil, that the oat straw and clover straw 



