Hoagland —138— Plant Nutrition 



the solutions made with ammonium nitrogen rapidly 

 become more acid, for the reason that the ammonium 

 ion (if ammonia is absorbed by another mechanism 

 the result would be the same) is removed at a greater 

 rate than the associated anions, sulphate and phos- 

 phate. The solution may eventually become so acid as 

 to produce injury to the plant. Internally, the am- 

 monia can react quickly with organic substrates. 

 Whether they are sufficient in supply to take care of 

 the ammonia absorbed depends ultimately on the syn- 

 thesis of carbohydrates in the green parts of the plant. 

 On the other hand, as already stated, nitrate can be 

 stored, if necessary, in inert form. The adjustment of 

 a plant to ammonia is, therefore, especially dependent 

 on climatic environment as governing photosynthesis 

 and respiration. Arnon (1937) found definite evi- 

 dence of these interrelations in growing barley plants 

 with the two sources of nitrogen at various seasons of 

 the year under well controlled nutrient solution con- 

 ditions. 



Nitrate is utilized by plants only following its 

 reduction, apparently through the stages of nitrite and 

 ammonia. Knowledge of the mechanism and enzyme 

 systems involved in nitrate reduction is still very 

 meager. Light is not essential as a direct factor in its 

 reduction. Excised barley roots and various other 

 plant tissues can readily reduce nitrate in darkness. 

 The normal course of nitrate reduction for some kinds 

 of plants seems to proceed almost entirely in the root 

 system. Light, of course, is indirectly an indispensable 

 factor, in providing at some stage of the metabolic 

 cycle photosynthetic substrates. 



One feature of nitrate as a source of nitrogen is 

 that it contains oxygen as well as nitrogen, and so is 

 an effective oxidising agent. There are suggestions 

 that this action is of some value under circumstances 

 which limit the oxygen supply to roots. 



