234 NITRIFICATION 



upon the insoluble nitrogen of the soil by increasing its nitrifying 

 powers. 



Season. The season of the year has a marked influence upon 

 the bacterial activities of the soil, but it is not necessarily corre- 

 lated with the nitrate content of the soil. Schlosing found the 

 nitrates in the drain water from both manured and unmanured soil 

 high in spring, as compared with midsummer, fall, or winter, thus 

 confirming the results obtained at the Rothamsted station. Shutt 

 reports nearly five times the quantity of nitrates in fallow and 

 cropped soil during June as during 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 among which is the kind of 

 crop growing on the soil, for King and Whitson found that the 

 nitrates in the surface foot start in the spring comparatively low 

 and increase rapidly until June 1 on clover and oat ground, and 

 until July on corn and potato ground. From these dates they fall 

 more or less rapidly and the work at the Utah Station demonstrates 

 conclusively that there is a seasonal variation, depending upon 

 temperature, crop and quantity of irrigation water applied to 

 the soil. 



Moreover, Andre has shown that the insoluble nitrogenous 

 compounds of the surface soil are largely transformed into soluble 

 compounds 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, Welbel, Jensen, and 

 Lyon and Bizzell. The results will vary, however, with different 

 soils, as shown by Russell who reports the fluctuations in nitrates 

 more marked on loams than on clays or sands. Moreover, he 

 found the bacterial activities much greater in early summer than 

 later. 



Moll even goes so far as to claim from his work that the season 

 of the year is the principal factor in determining the biochemical 

 transformation in a soil, and Heinze found that the number of 

 organisms in a soil was highest in the summer months and lowest 

 in the fall and spring. As already pointed out, the highest nitri- 

 fying power of a soil is not necessarily correlated with the highest 

 nitrate content. The latter is highest in spring or early summer, 

 while Vogel found the former to be highest in October and Novem- 

 ber, after which there was a falling off until April, when it rose 

 again, but not so high as in autumn. This corresponds fairly well 

 with the findings of Green, for the ammonifying powers of the 



