SOIL CONDITIONS AFFECTING PLANT GRO WTH 59 



up into protein when suflficient carbohydrate is present. This 

 accumulation would account for the fact on which all experi- 

 menters agree, that plants fed on ammonium salts contain a 

 higher percentage of nitrogen than those fed on nitrates (Table 

 XIV.). 



Table XIV. — Percentage of Nitrogen in Dry Matter of Plants. 



Nitrites are also assimilated so long as the solution is not 

 too concentrated or too acid.^ 



In spite of a considerable amount of work it is not known 

 whether nitrogen compounds other than nitrates and am- 

 monia are assimilated by plants. That many other com- 

 pounds serve as nitrogen nutrients, even without the inter- 

 vention of bacteria, seems to be certain (140^), but it has 

 never been shown whether assimilation of the compound 

 as a whole takes place, or whether there is decomposition at 

 the surface of the root. Many of the supposed assimilated 

 compounds are as a matter of fact more or less easily hydrolys- 

 able, or otherwise decomposable, with formation of ammonia, 

 and the decomposition will obviously proceed as fast as the 

 ammonia is removed by the plant. Two factors that determine 

 how far a given compound serves as a nitrogen nutrient are : 

 (l) the ease with which it splits off ammonia, (2) the effect on 

 the plant of the other decomposition products : if these happen 

 to be toxic the whole process stops as soon as they have suf- 

 ficiently accumulated. 



The normal nitrogenous food of plants is, however, a 

 nitrate, and there is a close connection between the amount 

 supplied and the amount of plant growth, which is well shown 

 in Hellriegel and Wilfarth's (i30f) experiments (Table XV.). 



1 See Perciabosco and Rosso, Staz. Speriment. A^rar. ital., 1909, xlii., 5, - 



