CHAPTER VI 

 ASSIMILATION OF NITROGEN BY PLANTS 



50. Absorption of Nitrogen from the Soil. Nitrates and 

 Ammonium Salts as Sources of Nitrogen. — The amount of 

 nitrogen contained in the dry substance of a plant is rather small, 

 as a rule not exceeding 1 to 5 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, hydrogen, 

 and oxygen, which form approximately 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 

 substances. Gaseous nitrogen is directly accessible to the leaves 

 and roots of plants, since there is always a circulation of air 

 through the soil along the capillary spaces formed between the 

 solid particles. Insufficient penetration of air is found only in 

 excessively dense and swampy soils. In the form of various 

 chemical compounds, nitrogen is also present in both the air and 

 soil. In the atmosphere, nitrogen 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, such as occur, 

 for instance, during a thunderstorm. The relative amount of 

 these forms of nitrogen is quite small. A much greater variety 

 of nitrogenous compounds is present in the soil. Here are found 

 both the inorganic forms of nitrogen, such as the salts of ammo- 

 nium and of nitric acid, and the organic compounds of incom- 

 pletely decomposed tissues of plants and animals, chiefly in the 

 form of proteins and the products of their decomposition, the 

 amino acids. 



The question as to which compound of nitrogen may serve 

 as a source of this element for the nutrition of plants may be 



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