37 6 INVESTIGATION BY CULTURE EXPERIMENTS 



There are a preliminary and a final period of 8 years each, and 

 three i6-year periods intervening. These figures show that the 

 middle i6-year period (1868-1883) gives averages clearly below the 

 normal, and that the average of the four years within that period 

 are still lower, thus proving that even two i6-year periods may 

 not positively establish by crop yields whether land is growing 

 better or poorer. A comparison of the first and second 1 6-year 

 periods indicates that all plots are growing poorer; while a com- 

 parison of the second and third 1 6-year periods indicates that all 

 plots are growing better. 



In the lower part of Table 64 are recorded the average yields for 

 all wheat crops grown in two 32-year periods, and these figures are 

 the best that can be secured. They show decreases of 6.7 bushels 

 with the wheat and fallow plot (Hoos field), 5.7 bushels with the 

 fallow system, and 4.5 bushels with the legume system, on Agdell 

 field, and 3.3 bushels decrease with unfertilized continuous wheat, 

 which, however, is a greater percentage decrease than on either of 

 the Agdell plots. It should be kept in mind, however, that wheat 

 is the only profitable crop now grown on the unfertilized Agdell 

 plots. The yields increased slightly on the farm manure plot and 

 very considerably where minerals and legumes were used on Agdell 

 field. 



Finally, in the last line, are recorded the general average of all 

 wheat crops grown on these plots since the experiments were be- 

 gun, with extremes differing by 24 bushels, a difference which in 

 64 years amounts to 1500 bushels more wheat from the applica- 

 tion of plant food than could be obtained without it, in the same 

 system of cropping. 



Table 65 gives, in brief, some of the very interesting and valuable 

 weather records of Rothamsted, and for comparison is given the 

 very trustworthy average rainfall records for northern, central, 

 and southern Illinois, and Tennessee, as representing a wide range 

 of latitude in central United States, with the average precipitation 

 (including snow measured as water) varying from 33.48 inches in 

 northern Illinois to 53.69 in Tennessee. (See also map showing 

 average annual precipitation in the various parts of the United 

 States.) 



The 5o-year record gives practically 28 inches as the average 



