462 ANNUAIi REPORT OF THE Off. Doc. 



fertility. Suppose we are practicing a four-year rotation, including 

 corn for two years, oats with clover seeding the third year, and 

 clover for hay and seed crops the fourth year. Let us' assume such 

 crop yields as have been produced, and as can be produced, in nor- 

 mal seasons on the richest, best-treated land with good seed and 

 good farming namely, 100 bushels of corn per acre, 100 bushels of 

 oats, and 4 tons per acre of clover including, perhaps, 3 tons in the 

 hay crop and one ton in the seed crop. If we do not succeed in secur- 

 ing these yields we should at least try to make such yields possible 

 and we should approach as near to them as we can. 



Let us first consider the phosphorus required for this rotation. 

 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 phosphorus. Thus 

 wo see that 81 pounds of the element phosphorus will be required 

 for the rotation. If we leave the stalks on the land the require- 

 ment is reduced to 69 pounds of phosphorus or to about 17 pounds 

 a year per acre. 



Supfjose the soil contains in the first seven 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- 

 qi.ired to remove this amount from the land if it could be drawn 

 upon at this rate? Only 70 years. On the other hand, suppose 

 with this crop rotation we can secure from the soil the equivalent 

 of only 1 per cent, of the phosphorus contained in the first seven 

 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. 



On the ordinary soils of Illinois ultimate failure is the only fu- 

 ture for this system of farming, even if we consider tjie phos- 

 phorus alone, although, as stated above, the phosphorus may be 

 returned in bone meal, in rock phosphate or in sufficient amounts of 

 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 nitrogen, 

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

 turned to the land without burning, otherwise the 96 pounds of ni- 

 trogen required for the two crops of stalks will also be removed from 

 the land. The oats crop will remove 90 pounds of nitrogen, making 

 290 pounds per acre for the corn and oats. 



The 4 tons of clover will contain about 160 pounds of nitrogen 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 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 to 

 the fact that the soil nitrogen has been drawn upon year by year 



