228 NITRIFICATION 



mum amount of water probably varies for each soil; it is higher 

 for clay, yet for both soils it lies within the range of from 14 to 

 18 per cent. A rise above the optimum amount of water is more 

 harmful than an equal fall below it. 



The work of the Utah Experiment Station demonstrated that 

 the application of irrigation water to a soil has a distinct beneficial 

 effect upon nitrification, being greatest where 15 inches of water 

 were applied when the nitric nitrogen formed amounted to 28.5 

 pounds per acre-foot of soil. The greatest benefit per inch of 

 water, however, was obtained where only 7.5 inches of water were 

 applied, resulting in 3.8 pounds of nitric nitrogen per inch of water, 

 while where 15 inches were applied it was 1.1 pounds of nitric nitro- 

 gen per inch of water applied, and when 25 inches of water were 

 applied to the soil the nitric nitrogen produced was only 0.7 pound. 



Miinter and Robson found that hornmeal decomposed more 

 rapidly in dry sandy soil than in clay or loam, whereas with higher 

 moisture content there was little difference. Ammonia sulphate 

 transformation increased with a higher water content. The best 

 nitrate formation from hornmeal occurred in sandy soils. In clay 

 and loam it was best with a medium water content. Sharp found 

 that the water content most favorable for ammonification was 

 not the optimum condition for nitrification. The former was 

 most rapid with a 25 per cent, water content and was not markedly 

 affected by 3 per cent, differences. Nitrification was at its maxi- 

 mum when the soil contained 19 per cent, of water. When it was 

 increased to 25 per cent, the rate of nitrification was decreased 

 50 per cent. 



McBeth and Smith found a slight variation in the number and 

 nitrifying powers of soil, depending upon the moisture content. 

 However, Gainey considers that among the factors controlling the 

 bacterial activity of a soil the available moisture probably plays 

 a leading part. But the author has reported results which indicate 

 that the nitrous nitrogen content of a soil is independent of the 

 irrigation water applied up to 37.5 inches a year. Results recently 

 published by the Utah Experiment Station clearly demonstrate 

 that the influence exerted by water upon ammonifying, nitrifying, 

 and nitrogen-fixing activities of the soil varies greatly with the 

 organic matter in the soil and is much more marked in effect on 

 soils recently manured than on those which have received no 

 manure. 



From the literature cited it may be seen that the nitrifying power 

 of the soil is a function of the moisture content of the soil, and 

 that the optimum varies with the physical and possibly with the 

 chemical properties of the soil. Recent work at the Utah Experi- 

 ment Station shows a close correlation between the nitrifying 

 powers of a soil and its water-holding capacity and varies only 



