NITRATE REDUCTION 145 



Plant growth may greatly affect nitrate accumulation in soils 

 but will not necessarily inhibit nitrate formation. In fact, develop- 

 ment of plants may favor nitrification, as shown later. Through 

 the absorption of nitrate from the soil by the plants, the amount of 

 nitrate in the soil will decrease. It is apparent that under such 

 conditions, the presence of small amounts of nitrate is no indica- 

 tion of a depression of nitrification. Consequently, because soils 

 under plant growth contain less nitrate than soils free from vege- 

 tation, it should not be concluded that plant growth depresses 

 nitrification. Since nitrate in soils most commonly originates 

 from the organic nitrogen compounds, the soil containing the 

 greatest amounts of readily decomposable organic substances will 

 show the most rapid nitrification, other conditions being favorable 

 for the processes (Fig. 63) . Where plants are not growing on a soil 

 no organic substances become incorporated with the soil as resi- 

 dues of roots and other plant parts. On the other hand, consider- 

 able amounts of organic matter gain entrance into the soil which is 

 cropped. In an unplanted soil, the amounts of readily decom- 

 posable organic matter continuously decrease, while planted soils 

 receive periodic additions of such substances. These considera- 

 tions indicate why planted soils may be expected to show a more 

 active nitrifying population. 



Nitrate Reduction. — The nitrogen transformations already 

 considered do not cause losses of nitrogen from soils. In fact, such 

 reactions as nitrogen fixation even add nitrogen. With the excep- 

 tion of nitrogen assimilation by microorganisms, most of the reac- 

 tions may be interpreted as being quite favorable to plant growth; 

 even the assimilation processes only temporarily remove the 

 nitrogen from availability to plants, and may prove to be de- 

 sirable under some circumstances. 



There is at least one set of transformations of nitrogen com- 

 pounds associated with microbial activities which is generally not 

 beneficial to plant growth, and may be decidedly undesirable 

 from the point of view of economics of the soil. These transforma- 

 tions are concerned with reduction of nitrates, frequently leading 

 to the formation of gaseous nitrogen and consequent impoverish- 

 ment of the store of fixed nitrogen in the soil. Associated with the 

 formation of free nitrogen are a number of other reactions provoked 

 by microorganisms by which a large number of partly or com- 

 pletely reduced inorganic compounds of nitrogen are produced. 



