Ch. 2] NITROGEN AND ORGANIC MATTER IN PROFILES 



53 



Nitrogen and Organic Matter in Profiles 



Soluble nitrogen compounds in the rain water increase the nitrogen 

 content of the soils to the extent of a few pounds of nitrogen 

 per acre per year. More important is the fixation of atmospheric ni- 

 trogen by soil bacteria, living either non-symbiotically (Azotobacter 

 group and Clostridium group), or symbiotically in association with leg- 

 umes (nodule bacteria). The contribution of biological fixation of 

 atmospheric nitrogen may amount to a hundred pounds or more of ni- 

 trogen per acre per year. 



The total amount of nitrogen and organic matter * in soils assumes 

 substantial magnitudes (see Table 3). Its rate of accumulation is 



TABLE 3 



Total Amounts of Nitrogen and Organic Matter in Selected Profiles 



conditioned by the rate of addition of organic matter by vegetation 

 and by the rate of decomposition by soil microorganisms. Under con- 

 ditions of relatively constant vegetation, a quasi equilibrium of soil 

 organic matter is reached in a few centuries. The annual rate of de- 

 composition is then nearly equal to the annual addition of vegetative 

 material. It is estimated that annual decomposition rates of soil or- 



* Humus represents the dark fraction of organic matter (Waksman, 1938) . It 

 consists of compounds synthesized by microorganisms from dead plant remains. 

 As there is no standardized method of determining humus, soil scientists prefer 

 to report total organic matter, obtained by multiplying organic carbon by the 

 factor 1.742. 



