458 INVESTIGATION BY CULTURE EXPERIMENTS 



as compared with previous years. Thus, on the continuous corn 

 plot the yield was 18.1 bushels in 1898 and 60.2 bushels four years 

 later, and the largest recorded corn yield in the corn-oats-clover 

 rotation was 80.5 bushels in the wet season of 1907. 



A fair comparison between different systems can usually be made 

 in the same years, and the change in productive power under any 

 system can best be ascertained by comparing the results from these 

 old experiments with those from newer experiments, as shown in 

 Table 86, when the effect of sixteen years' cropping can be noted. 

 Every plot in the newer experiments produced more than 75 

 bushels of corn per acre in 1896, and the average in 1897 was about 

 70 bushels. Upon these facts is based the assumption that all of 

 the older plots originally produced 70 bushels or more per acre. 



It is apparent that the legume catch crops (chiefly cowpeas) 

 seeded in the corn decrease the yield for the first year at least, as 

 shown in 1904 on plot 3 and, even in spite of the light manuring, 

 on plot 4 in 1905. 



The general effect of the system of soil improvement adopted for 

 the south half of each of these old plots is already very marked, an 

 increase of 40 bushels of corn per acre being secured in 1907 from the 

 treatment on plot 4, where the most marked effect is to be expected 

 because no clover or other legumes had been grown previous to 

 1904 in this rotation, and the frequent change from corn to oats has 

 helped to avoid the development of corn insects. 



Table 86 gives, for comparison, three-year averages for corn, 

 including the 1901-7 corn crops grown in the three-year rotation on 

 the old field. 



As an average of the three years where corn has been grown every 

 year, the yield has been 27 bushels in the 29-year experiments and 

 35 bushels in the 13-year experiments. The lesson of these experi- 

 ments is that 12 years of cropping where corn follows corn every 

 year reduces the yield from more than 70 bushels to 35 bushels 

 per acre, after which the decrease is much less rapid, amounting 

 to only 8 bushels' reduction during the next 16 years. Undoubtedly 

 the rapid reduction during the first 12 years of continuous corn- 

 growing is due in large part to the destruction of the more active 

 decaying organic matter, resulting ultimately in insufficient libera- 

 tion of plant food within the feeding range of the corn roots. 



