224 PLANT PHYSIOLOGY 



the ammonium ion become manifest. Besides the direct toxicity 

 of the ammonium ion, the harmful effect of these salts is also 

 due to the fact that their anions are assimilated by plants in 

 smaller proportion than the cations. They accumulate in the 

 nutritive solution, causing its reaction to shift toward increasing 

 acidity. Thus, for instance, if ammonium sulphate, (NH4)2S04, 

 is added to the nutritive solution, it will be observed that, as 

 the plant absorbs the ammonium, sulphuric acid accumulates 

 in the solution, injuring the roots of the plant, checking its 

 growth, and even causing its death. In general, salts whose 

 cations are consumed by the plant with greater intensity than 

 their anions are termed ''physiologically acid salts." Their 

 introduction into the nutritive solution leads to its gradual 

 acidulation, which progresses in proportion to the growth of the 

 plant. On the other hand, there are salts whose anions are 

 absorbed with greater avidity than are their cations. Such, 

 for instance, is Chile saltpeter, NaNOs- Its Na+ ion is hardly 

 used at all by plants, while the NOa" anion is absorbed with 

 great rapidity. The introduction of such a salt into a nutritive 

 solution produces a gradually increasing alkalinity. Salts of 

 this kind are called ''physiologically alkaline salts." It goes 

 without saying that this physiological acidity or alkalinity has 

 no connection with the purely chemical idea of acid and basic 

 salts. 



The harmful effect of such physiological acidity in the nutritive 

 solution may be avoided by changing the solution frequently. 

 In soil cultures, the effect is avoided by introducing calcium 

 carbonate (limestone or marl) into the soil, which neutralizes 

 the free acid formed. Experiments carried out by Prianishnikov 

 have shown that when ammonium sulphate is added to a soil 

 simultaneously with limestone, its fertilizing effect is no less than 

 that of nitrates. 



Ammonium nitrate, NH4NO3, is one of the best sources of 

 nitrogen for plants, as both of its ions are utilized at an almost 

 equal rate. The introduction of this salt into a nutritive solution 

 does not cause its reaction to shift in either direction. 



51. The Products of Nitrogen Assimilation and Its Con- 

 nection with Photosynthesis. Primary Synthesis of Protein 

 Substances. — The plant requires nitrogen for the purpose of 

 forming protein substances, which are an indispensable part of 



