ILLINOIS FIELD EXPERIMENTS 461 



using three different fields for this rotation, every crop may be 

 grown every year, and the yields of corn reported are true three- 

 year averages. 



With no special soil treatment aside from the use of legume catch 

 crops, the yield of corn for 1905, 1906, and 1907 averaged 69 bushels. 

 Where the equivalent of ^ ton per acre of ground limestone was 

 applied (five years before) the corn has yielded 72 bushels per acre; 

 and, with the phosphorus added for six years at the rate per annum 

 of 25 pounds per acre of the element phosphorus (in 200 pounds of 

 steamed bone meal) the average yield of corn has been 90 bushels 

 per acre for the last three years. The yearly addition of 42 pounds 

 of potassium in 100 pounds of potassium sulfate has further in- 

 creased the yield to 94 bushels. 



Under the heading " Live-stock Farming," in Table 86, are re- 

 corded the average yields of corn secured during the same three 

 years where farm manure has been applied to the clover ground 

 to be plowed under for corn. The plan of this system is to remove 

 all crops from the land as usually harvested, including the corn 

 and stover, oats and straw, and both first and second crops of 

 clover. The amounts of manure applied to the different plots are 

 determined by the crop yields secured during the previous rotation. 

 While the system of cropping followed during the 13 years on these 

 plots, and on those just described under " Grain Farming," has 

 been approximately equivalent to a three-year rotation of corn, 

 oats, and clover, the applications of manure have been made only 

 for the three years, 1905, 1906, and 1907. If the average yields are 

 decreasing on plots that receive only the amounts of manure that 

 can be produced in practice from the crops grown, then the appli- 

 cations of manure must also be reduced on such land; whereas if 

 the crop yields are increasing where both manure and phosphorus 

 are applied, then the applications of manure for such plots may be 

 increased in direct proportion. 



Where manure alone has been used in this rotation, the corn has 

 averaged 81 bushels per acre for the three years; with lime added, 

 the average is 85 bushels; with lime and phosphorus, the manured 

 land has averaged 93 bushels of corn, and this was increased to 96 

 bushels by adding potassium. 



While potassium has usually made some increase in crop yields 



