368 INVESTIGATION BY CULTURE EXPERIMENTS 



5 bushels) as potassium sulfate. It is commonly assumed that the 

 effect of the sodium and magnesium salts is due to their reaction 

 with insoluble potassium silicates with liberation of soluble potas- 

 sium, and the results of the later years certainly strongly support 

 that view. The regularity with which potassium is surpassing so- 

 dium and magnesium in its influence upon crop yields would 

 even lead one to imagine that the 1907 yields of plots 12 and 13 

 might have been interchanged, except that the exceedingly care- 

 ful methods of the Rothamsted Station makes such an error prac- 

 tically impossible, and that the more certain explanation lies in 

 the enormous variation (which every experimenter is familiar 

 with) in different seasons among field plots, subject to so many 

 uncontrolled and uncontrollable influences. Compare, for example, 

 plot ii with plot 17 (minerals) in 1904 and 1906. 



In studying plots 17 and 18, it should be understood that for the 

 1907 crop (for example) the minerals only were applied to plot 17 

 and the ammonium salts only to plot 18, while for the 1908 crop 

 the ammonium salts only were applied to plot 17 and the minerals 

 only to plot 18, this system of alternating having been followed 

 since 1852, and the amounts recorded in Table 60 for these two 

 plots are thus applied biennially and not annually. 



The data prove conclusively that almost none of the applied 

 nitrogen remains to benefit the second crop, while the minerals 

 remain and exert marked benefit on the succeeding crop. Compare 

 with plot 7 (for example), which receives twice as much minerals 

 during the biennium. 



It is of interest to note that in the dry season of 1893 (see 

 rainfall record, Table 65) the farm manure plot produced 12.5 

 bushels more wheat than the best fertilized plot (No. 8), while as 

 an average the heaviest applications of commercial plant food have 

 given slightly larger yields than the farm manure, and the difference 

 in favor of the commercial materials seems to be greater in wet sea- 

 sons. Thus, in 1903, plot 8 produced 6.1 bushels more than plot 2. 

 Compare also the wet year of 1879 with the dry year of 1898. 



It must be understood, of course, that Broadbalk field is de- 

 signed to secure knowledge and establish principles rather than 

 to serve as a model for agricultural practice. Nevertheless, it is of 

 interest to apply some financial measurements to the results. 



