SOURCES OF NITROGEN IN SOIL 103 



nucleic acids, amino acids, amides, and urea, as well as the alka- 

 loids and heterocyclic compounds. These various substances 

 are attacked by soil microbes before they become available to 

 higher plants. In certain processes, microbes diminish the sup- 

 ply of combined nitrogen in the soil; they occasionally form molec- 

 ular nitrogen or nitrous oxide from nitrate and thus decrease the 

 quantity of nitrogen in the soil. 



These numerous interrelated processes resulting from the pres- 

 ence and activities of microscopic and higher forms of life in and 

 upon the soil for untold thousands of years have resulted in the 

 accumulation of considerable quantities of nitrogenous organic 

 matter in the soil. 



The nitrogen content of the superficial 8 to 12-inch layer of soils 

 of humid regions varies from 0.025 per cent, in the case of the very 

 poor sandy soils, to practically 4 per cent in the case of certain peat 

 and forest soils, 90 per cent or more of which may consist of organic 

 compounds. The nitrogen content of most field soils ranges from 

 0.05 to 0.5 per cent, with an average of about 0.1 to 0.2 per cent. 

 Practically all of this nitrogen is found in the soil in complex 

 organic forms, and only a fraction of 1 per cent of the total nitro- 

 gen of the soil is generally present in an inorganic form, either 

 as ammonia, nitrite or nitrate. The nitre spots of certain arid 

 irrigated soils are exceptions. 



The explanation for the lack of any abundance of nitrogen in 

 the form of inorganic salts is found in the following two facts: 



(1) Transformation of ammonia to nitrate occurs more rapidly 

 under most soil conditions than the formation of ammonia; and 



(2) nitrates are so soluble that they do not persist long in the soil 

 in humid regions, and are either absorbed by the root sj^stems of 

 plants or become washed out by percolating water. It is, there- 

 fore, obvious why it is necessary to provide some means for creating 

 a continuous supply of nitrate during the period of active 

 absorption by the growing plants. Nature has invested such a 

 mechanism in the microbial inhabitants of the soil. Their action 

 is generally supplemented or enhanced by cultural treatments of 

 intensive agricultural practice. 



Practically all of the nitrogen that is brought into the soil by 

 natural agencies, as in the roots and stubble of cultivated and 

 uncultivated plants, animal residues, and dead animals, is in the 

 form of organic compounds, largely proteins and their derivatives. 



