MOISTURE 201 



Owen found ammonification to increase as the water added increased 

 up to a certain percentage, which varies with the physical nature 

 of the soil, but larger quantities of water reduced the ammonia 

 recovered. The work clearly demonstrated that the optimum 

 moisture content for maximum ammonification is higher than it is 

 for maximum nitrification. The quantitative difference between 

 the two processes in the same soil was found by Sharp. Ammoni- 

 fication was most rapid with a 25 per cent, moisture content and was 

 not markedly affected by 3 per cent, differences. Nitrification was 

 at its maximum 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. 



The work of Greaves and Carter shows that ammonification is 

 at its maximum in a soil containing 60 per cent, of its water-holding 

 capacity, as is illustrated in Fig. 29 which gives the results obtained 

 with twenty-two soils. Moreover, according to Briggs the moisture- 

 holding capacity, the wilting coefficient, the moisture equivalent, 

 and the hygroscopic coefficient are related by linear equations : 



C = 2.9 W + 21 



C = 1.57E + 21 

 C = 4.26H + 21 



where C is written for the moisture capacity as defined by Hilgard, 

 W for wilting coefficient, E for moisture equivalent, and H for 

 hygroscopic coefficient. Therefore, taking the maximum ammoni- 

 cation at 0.6 of the water-holding capacity we could relate these 

 other soil constants to maximum ammonification by a similar set of 

 linear equations. Thus: 



Ma = .942E + 12.6 

 Ma = 1.74 W + 12.6 

 Ma = 2.55 H + 12.6 



writing Ma for per cent, water for maximum ammonification. 



Variation in moisture content of a soil changes its ammonifying 

 flora, for soils held at various moisture contents for several months 

 show different ammonifying efficiency, as may be seen from the 

 following results obtained by Greaves and Carter. These soils 

 were held at the indicated moisture content for three months and 

 then all brought to 20 per cent, moisture and after four days the 

 ammonia determined. 



Per cent. Ammonia 

 Moisture Added Produced 



12.5 per cent, water 100 



15.0 per cent, water Ill 



17.5 per cent, water 113 



20.0 per cent, water 123 



22 . 5 per cent, water 119 



