II.] 



ACTION OF SODA IN NITRATE OF SODA 



53 



one-half or more. For twenty-five years, then, the use 

 of nitrate of soda alone has enabled the soil to supply 

 a mangold crop with the large amount of potash it 

 wants, though the store of potash in the soil apparently 

 soon becomes exhausted when a manure is used which 

 cannot bring it into solution. With other crops the 

 same results are obtained, though the lack of potash 

 does not become manifest so quickly as in the case of 

 mangolds. For example, we may compare the yield of 

 barley (Table XI.) for successive ten-year periods, the 

 yield of each plot being calculated as a percentage of 

 that on the completely manured plot receiving nitrate 

 of soda, to eliminate seasonal influences. 



It will be seen that when the manure contains 

 potash the ammonium salts yield practically the same 

 crops as nitrate of soda. When the nitrogenous 

 manure is nitrate of soda, the omission of potash 

 causes no diminution in the yield ; but with ammonium 

 salts and no potash the crop after the third decade 

 becomes unable to satisfy its potash requirements 

 from the soil alone, and the yield declines. In other 

 words, nitrate of soda has dispensed with the necessity 



