54 TEXTBOOK OF PLANT PHYSIOLOGY 



17. Nitrates and Ammonium Salts as Sources of Nitrogen. 

 Physiological Acidity of Salts. — The salts of ammonium and 

 those of nitric acid are the inorganic compounds of nitrogen com- 

 monly found in soil. In water cultures the salts of nitric acid pro- 

 duce the better growth. Consequently, it was believed for a long 

 time that nitrogen was assimilated by the plant only in this form. 

 In cases where fertilization with ammonium salts (a sulphate is 

 most frequently used for this purpose) produced fairly good 

 results in soil cultures, this was attributed to the fact that ammo- 

 nia is first oxidized by bacteria into nitric acid, only then becoming 

 available to the plant. 



Experiments carried out with sterile cultures have destroyed 

 this conception. The root system of plants can absorb ammo- 

 nium salts directly, providing they are in a sufficiently dilute 

 solution. They cannot be considered a more inferior source of 

 nitrogen for plants than the nitrates. In higher concentrations, 

 however, ammonium salts are toxic. They cause a weaker and 

 more irregular growth of the roots and, hence, have a harmful 

 effect on the general development of a plant. 



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 a smaller proportion than the cations. 

 They accumulate in the nutritive solution, causing its reaction to 

 shift in the direction of increasing acidity. Thus, for instance, if 

 ammonium sulphate, (NH.O2 SO4, is added to the nutritive solution, 

 it will be observed that, as the plant absorbs the ammonium, sul- 

 phuric acid accumulates in the solution, poisoning the roots of the 

 plant, checking its growth and even leading to its death. In gen- 

 eral, salts whose cations are consumed by the plant with greater 

 intensity than their anions have come to be known by the name 

 of physiologically acid salts. Their introduction into the nutritive 

 solution leads to its gradual acidulation, which progresses in pro- 

 portion 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 NO3 anion is absorbed 

 with great rapidity. The introduction of such a salt into a nutri- 

 tive 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 stands in no 



