MOISTURE AND NITRIFICATION 143 



the gases between the soil and the atmosphere would be the factor 

 determining the concentration of oxygen. The concentrations of 

 oxygen most favorable to nitrification lie between 30 and 55 per 

 cent (Fig. 61), while in soils this gas exists more commonly in 

 concentrations between 15 and 20 per cent. In the atmosphere, 

 oxygen comprises close to 20 per cent of the volume. Carbon 

 dioxide occurs in the atmosphere in concentrations close to 0.03 

 per cent. However, this gas makes up 0.2 to 2.0 per cent of the 

 soil air. 



It is due to these relationships of the gases to nitrification that 

 factors favoring the rapid exchange of gases between the soil air 

 and the atmosphere lead to more rapid nitrate formation. Culti- 

 vation of soils and other treatments creating a more porous medium 

 increase the rate of nitrification. Sandy soils nitrify more rapidly 

 than hea\'y clay soils, other conditions being equal. 



Organic Matter and Nitrate Formation. — The early work 

 of Winogradsky and his colleague Omeliansky indicated that very 

 small amounts of some organic materials may completely inhibit 

 nitrification, as determined by studies in solution cultures. Such 

 substances as glucose, peptone, asparagine, glycerol, and urea were 

 distinctly injurious in concentrations of 0.2 to 1 per cent. Under 

 most conditions, the soil solution does not contain such large quan- 

 tities of these soluble organic substances. Further, the so-called 

 humic substances which exist in the soil are not nearly as toxic as 

 many of the organic materials given above. In fact, soil extract 

 accelerates nitrification rather than depresses it. In the more 

 fertile soils, which contain relatively greater amounts of organic 

 materials, nitrification is appreciably more rapid than in the less 

 fertile soils. Nitrification will also proceed in heaps of animal 

 manures which contain large amounts of organic matter in solution. 

 In the preservation of manure from losses of nitrogen one of the 

 necessities is to keep nitrification down to a minimum, for it is 

 subsequent to nitrate formation that large amounts of nitrogen are 

 lost by reduction of nitrate to gaseous nitrogen. 



Moisture and Nitrification. — [Moisture conditions most 

 favorable for nitrification are much the same as are favorable for 

 many aerobic soil processes, being close to 50 per cent of the amount 

 of water the soil holds when saturated. The influence of high 

 moisture content is determined to a considerable degree by the 

 retarding effect of the water on the circulation of the gases. High 



