24 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 



weeks after favorable climatic conditions had been established and 

 that after this period of growth they underwent a resting period, or 

 at least their physiological activity was much reduced, just as is the 

 case with an annual flora of higher plants. From this explanation 

 one would expect that their general activity would be more intense 

 and begin earlier in the fallow plat than in the wheat plat, as the 

 former w T as spring-plowed to a good depth, while the latter was not 

 so treated — and this is the case. As -already noted, the maximum 

 nitrate content in the respective soil layers occurred in general about 

 one week earlier in the fallow than in the wheat plat, but the general 

 "maturity of the crop," to continue the analogy of the higher flora, 

 was effected at about the same time in the two instances, viz, about 

 June 19. This seasonal decrease in nitrification may also be caused 

 by a decrease in food readily available for the nitrifying organisms. 

 Whether the seasonal variation in amounts of nitrates is due to 

 periodicity of the activity of the nitrif} T ing organisms in the soil, 

 unaffected b} r any climatic or food factor, or whether it is due to 

 lack of sufficient food for continued activity would be hard to decide 

 from the various curves. As somewhat similar variations may be 

 obtained in the laboratory under controlled food conditions, this 

 factor must be kept in mind. Schneider found that the fixation of 

 nitrogen in the soil was greatly increased by the addition of organic 

 food, as also by the addition of potassium phosphate and of lime; 

 and Bazarewski 6 found that the addition of small amounts of organic 

 food promoted nitrification, but that the addition of larger amounts 

 decreased the activity of the organisms. 



Notwithstanding the fact that neither nitrites nor ammonia were 

 ever found in sufficient amounts to account for the actual decrease 

 in amounts of nitrates, other organisms active in the production of 

 these compounds may conceivably have increased vegetatively at the 

 expense of the nitrates already present. Whether the food supply 

 itself could be entirely responsible for the variations in the amounts 

 of nitrates it would be hard to decide, but certainly the prairie soil in 

 question is considered quite fertile so far as the ordinary farm crops 

 are concerned, and the plats on which these determinations were 

 made had never been cropped before. 



That this rather definite and comparatively short seasonal period of 

 activity of nitrification has not been limited by lack of moisture in 

 the soil would seem to be indicated by the moisture conditions in the 

 summer-fallow plat. The drying up of the soil in the wheat plat, due 

 to the demands made upon the moisture by the plants, w T ould of 



a Schneider, P. Landwirtscliaftliche Jahrbiicher, vol. 35, sup. 4, 1906, pp. 63-83; 

 abstract in Experiment Station Record, vol. 18, no. 8, April, 1907, p. 722. 



b Bazarewski, L. von. Nitrification and Denitrification in Soils. Reviewed 

 in Experiment Station Record, vol. 21, no. 1, July, 1909, pp. 20-21. 

 173 



