90 PLANT PHYSIOLOGY 



plenty of lime, ammonium salts can be used more advantageously 

 than if the soil is acid, which may be explained by assuming that 

 the ammonia, before absorption by plants, is first oxidized to 

 nitric compounds and forms too much acid in the soil in case no 

 lime is present to neutralize it. Another explanation, however, is 

 that the ammonium salts are absorbed as such, but the basic 

 part of the salt is absorbed more than the acid part, leaving an 

 excess of acid in the soil, which the lime, if present, neutralizes. 

 In low concentrations ammonium salts seem to be as easily used 

 as nitrates, but in high concentrations ammonium salts are toxic. 

 As to the third point, i. e., the nature of the salt, ammonium sul- 

 phate is less available than ammonium nitrate. In the case of the 

 former, the ammonium ion is taken up more rapidly than the sul- 

 phate ion, with the result that the soil solution becomes physiolog- 

 ically acid. The nitrate ion, however, is absorbed at about the 

 same rate, so that when ammonium nitrate is used, the soil solution 

 does not change appreciably towards either the alkaline or acid 

 side. There is no doubt but that plants may thrive on ammonium 

 compounds, but it is still doubtful whether these salts in nature 

 are absorbed as ammonia or only after transformation to nitrates. 

 Probably most of the compounds are changed into nitrates before 

 being absorbed, although Muntz and Maze (1898) as well as later 

 workers showed that plants are able to grow and increase in nitro- 

 gen content when nourished with ammonium salts on sterile 

 soil where no transformation into nitrates can occur. And Prian- 

 ishnikov (1923), with a revolutionary spirit characteristic of his 

 country, has asserted that nitrates before being absorbed are 

 always reduced to ammonia! 



Nitrates, which in general are the best source of nitrogen for 

 plants, are constantly being formed at the expense of the other 

 nitrogen compounds present. Boussingault sealed up a carboy of 

 soil in 1859 and reopened it 12 years later. He found that both 

 the nitric acid and nitrates had increased to more than 200 times 

 the original amount, while the total nitrogen remained approxi- 

 mately the same, showing that this increase of nitric nitrogen had 

 been at the expense of the ammonia and the organic compounds. 



Although nitrates are constantly being formed, there is always 

 only a small amount present in the soil. To be sure, some is being 

 removed by growing plants, but this does not account for all the 

 nitrate loss. If a solution of ammonia and nitrates is filtered 



