SOIL CONDITIONS AFFECTING PLANT GRO WTH 59 



up into protein when sufficient 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. 1 



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 : 

 (i) 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 (130*) experiments (Table XV.). 



1 See Perciabosco and Rosso, Staz. Speriment. Agrar. ital., 1909, xlii., 5. 



