SOIL CONDITIONS AND PLANT GROWTH 



bacteria seems to be certain (140$), but it has never been shown whether 

 assimilation of the compounds as a whole takes place, or whether there 

 is decomposition at the surface of the root. Most of the supposed 

 assimilated compounds are as a matter of fact more or less easily 

 hydrolysable, or otherwise decomposable, with formation of ammonia, 

 and the decomposition will obviously proceed as fast as the ammonia is 

 removed by the plant. The 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 decomposi- 

 tion products : if these happen to be toxic the whole process stops as 

 soon as they have sufficiently 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 Wil- 

 farth's (130) experiments (Table IX.). 



TABLE IX. EFFECT OF NITROGENOUS FOOD SUPPLY ON THE GROWTH OF BARLEY 

 IN SAND CULTURES. HELLRIEGEL. 



The figures are plotted in Fig. 3. Similar results are obtained on 

 the field plots at Rothamsted (Table X.). 



O 312 224 336 



M.gms. N supplied as Ca (NO o ) 

 */ j, i. \ *y / & 



FIG. 3. Effect of Nitrogenous Food Supply on the Growth of Barley. (Hellriegel.) 



