CHAPTER II 



ABSORPTION OF NITROGEN FROM THE SOIL AND THE 



AIR 



15. Sources of Nitrogen for Plants. — The amount of nitrogen 

 contained in the dry substance of a plant is rather small — as a 

 rule not exceeding 1 to 3 per cent. Nevertheless, it plays a very 

 important role in the life of a plant. Nitrogen is an indispensable 

 constituent of the protein molecule, which in turn is a vital part 

 of protoplasm. Therefore, the absorption of nitrogen is by no 

 means a less important factor in the nutrition and growth of plants 

 than the utilization of carbon dioxide, hydrogen, and oxygen, 

 which form 95 per cent of the dry substance. 



Nitrogen is found in the environment of plants in two different 

 forms: as elemental gaseous nitrogen, constituting almost 80 per 

 cent of the air and as a part of various inorganic and organic sub- 

 stances. Gaseous nitrogen is directly accessible to the leaves and 

 the roots of plants, since there is always a circulation of air through 

 the soil along the capillary cavities formed between the solid 

 particles. Excessively inundated or swampy soils contain prac- 

 tically no air. Nitrogen is present in the air and in the soil in the 

 form of various chemical compounds. In the atmosphere, nitro- 

 gen is found as ammonia gas, which is produced by the decay of 

 organic matter, and in the form of oxides of nitrogen formed by 

 the combustion of nitrogen-containing substances and by electric 

 discharges, as, for instance, during a thunderstorm. However, 

 the relative amount of these forms of nitrogen is quite small. A 

 much greater variety of nitrogenous compounds are present in the 

 soil. Here are found both the inorganic forms of nitrogen, such 

 as the salts of ammonium and of nitric acid, and the organic com- 

 pounds of incompletely decomposed tissues of plants and animals, 

 chiefly in the form of proteins and the products of their dissociation, 

 the amino acids. 



The question as to which compound of nitrogen may serve 



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