296 
Journal of Agricultural Research 
Vol. IX, No. 9 
medium water content. Sharp (52) found that the water content most 
favorable for ammonification was not the optimum condition for nitri¬ 
fication. 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 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. 
McBeth and Smith (38) found a slight variation in the number and 
nitrifying powers of soil, depending upon the moisture content. How¬ 
ever, Gainey (18) considers that among the factors controlling the bacterial 
activity of a soil the available moisture probably plays a leading part. 
But we (22) have 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 from the Utah Experiment 
Station (21) 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 number of bacteria 
in a soil and the ammonifying and nitrifying powers of the soil are func¬ 
tions of the moisture content of the soil, and that the optimum varies 
with the physical and possibly the chemical properties of the soil. But 
in all soil, to obtain the maximum count and ammonification, the mois¬ 
ture content should be about 6 per cent higher than to obtain maximum 
nitrification. While the optimum for nitrification varies with different 
soils, the average would seem to be between 18 and 20 per cent of 
moisture. 
INFLUENCE) OF CROP 
Even as early as 1855 the work at Rothamsted (48) had demonstrated 
that the beneficial effects of fallowing lies in the increase brought about 
in the available nitrogen compounds of the soil. Dehdrain and De- 
moussy’s (14) work indicated that there is a larger production of nitrates 
in fallow than in cropped soils, and Pfeiffer (44) considers fallowing an 
extreme form of soil robbery, for he found that it promotes the activity 
of the soil organisms and, hence, hastens the exhaustion of the nitrogen 
supply. But, as it is so clearly pointed out by Warington (63), these 
results may not hold in a dry climate or during dry seasons; for here 
bare fallow may not necessitate this loss, and much is to be gained by its 
practice. But it must always be borne in mind that, if there be sufficient 
moisture, the loss may be great. For instance, Schneidewind, Meyer, 
and Miinter (51) record a loss in fallow plots of 85.5 pounds per acre, 
which even exceeded the nitrogen removed by the growing plant on the 
cropped soil. 
