May 28, 1917 
Influence of Crop , Season , Water on Soil Bacteria 299 
aside from the influence of cultivation, the source of greatest difference 
in the nitrates under various crops may be sought in the inherent differ¬ 
ences in the plants of different species in their stimulating and inhibiting 
influence on the production of nitrates, as well as in their relative rates, 
amounts, and forms of nitrogen absorbed. Changes in the moisture 
content and temperature of the soil after early summer had no important 
effect on the nitrogen content of the soil under plants. On the uncropped 
soil an increase in moisture content was sometimes accompanied by an 
increase in nitrates and sometimes by a decrease. But, as pointed out 
by Kellerman and Wright (27), there may be a variation with different 
soils. But even the species of organisms found within the soil is con¬ 
trolled to a degree by the crop growing on the soil, as indicated by the 
work of Stoklasa and Vitek (58). 
INFLUENCE OF SEASON 
The season of the year has a marked influence upon the bacterial 
activities of the soil, but it is not necessarily correlated with the nitrate 
content of the soil. Schloesing (50) found the nitrates in the drain 
water from both manured and unmanured soil high in spring, as com¬ 
pared with midsummer, fall, or winter, thus confirming the results ob¬ 
tained at the Rothamsted station. Shutt (53) reports nearly five times 
the quantity of nitrates in fallow and cropped soil during June as dur¬ 
ing November. He does, however, find more during June than during 
May. The exact season of the year at which the maximum nitrate 
content is reached will vary with a number of factors, chief amongst 
which is the kind of crop growing on the soil, for King and Whitson (29) 
found that the nitrates in the surface foot start in the spring compara¬ 
tively low and increase rapidly until June 1 on clover and oat ground 
and until July on com and potato ground. From these dates they fall 
more or less rapidly, and the work at the Utah Station (56) demonstrates 
conclusively that there is a seasonal variation depending upon tempera¬ 
ture, crop, and quantity of irrigation water applied to the soil. 
Moreover, Andre (1) has shown that the insoluble nitrogenous com¬ 
pounds of the surface soil are largely transformed into soluble com¬ 
pounds during the summer, and these are widely diffused through the 
deeper layers of soil during the winter, so that in the spring the lower 
layers of soil contain more soluble nitrogen than the surface soil. At 
the end of summer, however, the distribution is quite uniform. This 
finding has been amply verified by the results reported by Stewart and 
Greaves (56),Vel'bel (59), Jensen (26), and Lyon and Bizzell (34). These 
results will vary, however, with different soils, as shown by Russel (49), 
who reports the fluctuations in nitrates more marked on loams than on 
clay or sands; moreover, he found the bacterial activities much greater 
in early summer than later. 
