16 FIELD CROP PRODUCTION 



producing fully as well as that used for the rotation exper- 

 iment. During the last fifty years the falling off under 

 continuous culture has been comparatively little and has 

 now reached a low level at which it may be sustained 

 almost indefinitely. This is what usually takes place 

 under any poor system of farming ; yields fall off rapidly 

 at first and soon reach a low level below which they are 

 not easily reduced. Comparisons here have been made 

 only between the unfertilized plots in the two systems of 

 cropping. By means of commercial fertilizers the yields 

 of wheat and barley, even in continuous cropping, 

 have been maintained at a high level. But the 

 necessary quantity of fertilizer has been so great that 

 the increased yields resulting from their use have not 

 paid the cost. Also, when the fertilizers are used, the 

 crops are now so dependent upon them that if they are 

 discontinued, even for a single year, the yields drop 

 very low. 



20. Rotation experiments in Iowa and Illinois. — ■ A 

 most striking example of the rapid decline in produc- 

 tion under continuous cropping is shown in the experiments 

 with corn growing at both the Illinois and Iowa experiment 

 stations. In both cases the fields are located on typical 

 dark brown silt loam prairie soil which at the outset yields 

 without fertilization 70 to 80 bushels of corn per acre. 

 The Illinois experiments show that after 10 or 12 years of 

 continuous corn culture the yields of corn on this soil are 

 reduced to about 35 bushels per acre. Under a rotation 

 of corn and oats, which is a poor rotation, but better than 

 a single crop, the yield is 62 bushels. Under a four-year 

 rotation of corn, corn, oats, and clover the yield is 66 

 bushels. The corresponding figures for a similar experi- 

 ment on a smaller scale show still further reduced yields, 



