NITROGEN FIXATION IN SOIL 247 



and the process of mineralization continues even in the case of 

 microbial cell constituents. A part of the nutrients may remain 

 for a more or less extended period of time, thus locked up in the 

 bodies of the microbes, or in the form of disintegration products of 

 their cells. 



Disappearance of Nitrogen from the Soil. — There are 

 other ways whereby microorganisms may bring about processes 

 which tend to injure the growth of the green plants, as by the 

 reduction of nitrates to atmospheric nitrogen carried out under 

 certain conditions and by certain organisms. Once the nitrogen is 

 changed into its elementary or gaseous form, it is lost from the soil 

 as far as the green plants are concerned. Notwithstanding the 

 fact that nearly 80 per cent of the gases of the atmosphere is made 

 up of nitrogen, which amounts to many thousands of tons (31,250) 

 over each acre of soil, nitrogen is usually the limiting element in 

 the growth of green plants. This is simply due to the fact that 

 this gaseous nitrogen is inert as far as green plants are concerned. 

 Any activities of microorganisms which tend to change even a 

 small fraction of the limited supply of combined nitrogen into the 

 gaseous form are distinctly injurious to plant growth. As a result 

 of continued cropping, certain farm practices, and natural environ- 

 mental conditions, these losses of nitrogen may be far from negli- 

 gible. 



Nitrogen may be removed in crop plants, later fed to animals 

 and a small portion returned to the soil as excretion products of 

 these animals. It may be lost as volatilized ammonia from manure 

 heaps. This may later become precipitated upon the soil and 

 become available to plants, but, from a practical standpoint, the 

 nitrogen volatilized is of httle value. Nitrogen may become 

 stored away in the cells of many microorganisms at least tem- 

 porarily. Nitrates may be removed from soils in the natural 

 leaching waters in humid cHmates and in the irrigation waters in 

 arid regions. Nitrogen may also disappear in some cases in the 

 gaseous form through the reduction of nitrates. 



Nitrogen Fixation in Soil. — To counterbalance such losses 

 of the combined nitrogen from the soil, nature has provided certain 

 groups of organisms with the capacity of fixing atmospheric nitro- 

 gen, i.e., using the nitrogen in its gaseous elemental form and build- 

 ing from it compounds of nitrogen which sooner or later become 

 available to green plants. These microbes are known as nitrogen- 



