252 Missouri Agricultural Report. 



Let us first consider the phosphorus required for this rota- 

 tion. The two crops of corn will each require 23 pounds, 17 for 

 the grain and 6 for the stalks ; the oat crop will require at least 15 

 pounds of phosphorus, about 11 for the grain and 4 for the straw; 

 and the 4-ton crop of clover will require 20 pounds of phosphorous. 

 Thus we see that 81 pounds of the element phosphorus will be re- 

 quired for the rotation. If we leave the stalks on the land the re- 

 quirement is reduced to 69 pounds of phosphorus, or to about 17 

 pounds a year. 



Suppose the soil contains in the first 7 inches 1,200 pounds of 

 phosphorus per acre, which is about the average of the principal 

 type of soil in the Illinois corn belt, how many years would be re- 

 quired to remove this amount from the land if it could be drawn 

 upon at this rate? Only seventy years. On the other hand, sup- 

 pose with this crop rotation we can secure from the soil the equiv- 

 alent of only one per cent of the phosphorus contained in the first 

 7 inches. This would be only 12 pounds of phosphorus a year, 

 which would necessarily reduce the crop yields to about one-half 

 the amounts suggested above, and with the further reduction in 

 the total amount of phosphorus year by year, the crop yields must 

 be reduced accordingly. 



Ultimate failure is the only future for this system of farming, 

 even if we consider the phosphorus alone; although, as stated 

 above, the phosphorus may be returned in bone meal, in rock phos- 

 phate or in farm manure. 



If we consider the element nitrogen in this system of farming, 

 we find that 200 bushels of corn require about 200 pounds of nitro- 

 gen, aside from that required for the stalks, which must be re- 

 turned to the land without burning, otherv/ise the 96 pounds of 

 nitrogen required for the two crops of- stalks will also be removed 

 from the land. The oats crop will remove 90 pounds of nitrogen, mak- 

 ing 290 pounds for the corn and the oats. 



The four tons of clover will contain about 160 pounds of nitro- 

 gen, and the clover roots and stubble about one-half as much as the 

 tops, or 80 pounds per acre. If all of the nitrogen contained in the 

 entire clover crop is taken from the air, the rotation would add 

 only 80 pounds of nitrogen to the soil, while the corn and oats 

 would remove 290 pounds. 



How then is it possible to maintain the supply of nitrogen by 

 this rotation? It is not possible. Under such a rotation, with all 

 crops removed except the corn stalks, the supply of nitrogen grows 

 less and less. Where this rotation is successful for a time it is due 



