NITROGENOUS MANURES 267 



been treated with nitrate of soda every year for forty- 

 seven years in succession, has produced an average crop 

 of 35 cwts. of hay annually. Plot 3, which has been con- 

 tinuously unmanured, has produced an average crop of 

 only 22 cwts. annually. During the last ten years of 

 the period, the average yield of the unmanured plot was 

 16 cwts., and that of the nitrate of soda plot was 31 cwts., 

 or nearly double that of the former. In the year 1905 

 the yield from the unmanured plot was 19 cwts., and 

 from the nitrate of soda plot over 39 cwts., or more 

 than double that of the former. It will thus be seen 

 that even when applied alone for forty-seven years in 

 succession, the exhaustive effects of the nitrate of soda 

 are not apparent so long as the application is continued, 

 while there has been a marked diminution in the produce 

 of the unmanured plot. 



What would happen if the application of the nitrate 

 of soda were now discontinued is difficult to tell. In an 

 experiment conducted by the writer, two plots were 

 manured with nitrate of soda at the rate of 1 cwt. and 

 2 cwts. per acre respectively, for four years in suc- 

 cession, and then left unmanured for three years. Each 

 year that the manure was applied, a considerable increase 

 in the crop was obtained, but when the application was 

 discontinued, both plots gave a smaller yield than the 

 continuously unmanured plot, showing that a certain 

 exhaustion had been produced. In the second year the 

 1 cwt. plot had recovered, and in the third year both plots 

 again gave larger crops than the unmanured plot. When 

 the nitrate of soda was applied in conjunction with 

 superphosphate, there was no exhaustion ; both plots 

 gave a larger yield than the unmanured plot throughout 

 the whole period of the experiment, as is shown in the 

 table on p. 268. 



It has been pointed out that under like conditions other 



