INFLUENCE OF MOISTURE 227 



must be diminished as the soil becomes richer in nitrates. As early 

 as 1887 Deherain found that the most active nitrification took place 

 when the soil was allowed to become partially dry between the 

 applications of water, and later he found that there was a rela- 

 tionship between the speed of nitrification and the moisture con- 

 tent of fallow soil, the nitrification increasing with the water. 

 Boussingault taught that when soils contain as much as 60 per 

 cent, of water they lose in a few weeks the greater part of their 

 nitrates. This teaching gave rise to the general belief that deni- 

 trification may take place to a great extent in soils, but recent 

 work has amply demonstrated that it is only extremely abnormal 

 conditions where this becomes an important factor. 



Deherain and Demoussy found that the bacterial action of a 

 soil was at its maximum when a rich soil contained 17 per cent, 

 of water, but that it decreased if the proportion of water fell to 

 10 per cent, or rose to 25 per cent. With soils less rich in humus 

 a somewhat higher proportion of water was necessary to retard 

 oxidation to any marked degree. 



The optimum moisture content for nitrification, according to 

 Deherain, is 25 per cent. An insufficient supply of moisture 

 checked both nitrification and nitrogen fixation. This occurred 

 when the water had been reduced to 16.5 per cent. This, however, 

 would vary with the soil, for Schlosing found bacterial activity 

 less in fine-grained soils than in lighter, coarse-grained soils. In 

 order that nitrification be equally active in both light and heavy 

 soils, the latter must have a higher percentage of water than the 

 former, a difference in moisture content of soil of 1 per cent., accord- 

 ing to Dafert and Bellinger, being sufficient to produce a marked 

 change in the oxidation going on in the soil. 



Fraps found that the number of nitrifying organisms in a soil 

 varies with the moisture and that their activity was periodic, rapid 

 nitrification being preceded and followed by periods of less activity. 

 Later he found nitrification to be at its height in soil containing 

 55.6 per cent, of its water-holding capacity. Excessive quantities of 

 water practically stopped nitrification and were much more injur- 

 ious than too small a quantity. The water requirements, however, 

 varied considerably with the soil. Coleman's work with a loam 

 soil showed nitrification to be most active when the soil contained 

 16 per cent, of water. It was greatly retarded when the water 

 content was reduced to 10 per cent, or increased to 26 per cent. 



Patterson and Scott's work is interesting in that they found 

 nitrification to be inactive in sand and clay soils which still con- 

 tained about three times as much moisture as in their average air- 

 dry condition. At the lower limits of moisture less water starts 

 nitrification in sand than in clay. At the higher limits of moisture 

 less water stops nitrification in sand than in clay, while the opti- 



