INFLUENCE OF ENVIRONMENTAL CONDITIONS 781 



nitrogen-fixing powers of the soil reached a maximum in the spring, a 

 minimum in summer and a maximum again in autumn. Similar 

 results were obtained in the study of the influence of season of year on 

 ammonia formation from peptone in solution. 41 Maximum nitrifying 

 power of the soil was found in spring. 42 Lemmermann and Wichers, 43 

 however, found that the season of year, outside of influence of tempera- 

 ture and other physical weathering conditions, has no appreciable 

 influence on nitrification. The seasonal course of ammonia and nitrate 

 formation, with all other factors alike, were found 44 to be influenced 

 primarily by the temperature course. The season of year, independent 

 of temperature and other physical conditions, had no influence at all; 

 the maximum activity is found in the spring and is due to outer influ- 

 ences, primarily change in temperature, but is independent of inner 

 causes. 



Influence of moisture. Some bacteria, like the nitrifying organisms, 

 are very sensitive to drying, while others, like Azotobacter, can resist 

 drying for a long period of time. The addition of moisture to a soil, 

 which contains less water than is the optimum for biological activities, 

 brings about an increase in numbers and activities of microorganisms. 

 Most active nitrification takes place when the soil is allowed to become 

 partially dry between the applications of water; 45 a direct relationship 

 was observed between the speed of nitrification and the moisture 

 content of fallow soil. Every soil has an optimum for nitrification; 

 higher or lower amounts of moisture reduce this optimum. Nitri- 

 fication was found 46 to be at its highest when the soil contains moisture 



41 Moll, R. Beitrage zur Biochemie des Bodens. Diss. Leipzig. 1909. 



42 Miintz, A., and Gaudechon, H. Le reveil de la terre. Compt. Rend. Acad. 

 Sci., 154: 163-168. 1912; see also Lumiere, A. Le rythme saisonnier et le 

 rdveil de la terre. Rev. Gen. Bot., 33: 545-557. 1921; Nitrate content of soil 

 at different seasons of the year has been studied by Jensen, C. A. Seasonal 

 nitrification as influenced by crops and tillage. U. S. Dept. Agr. Bur. PI. Ind. 

 Bui. 173. 1910; Leather, J. W. Water requirements of crops in India. Mem. 

 Dept. Agr. India, Chem. ser., 1: 133-184. 1910; 205-281; 2: 63-140. 1912. 



43 Lemmermann, O., and Wichers, L. tJber den periodischen Einflusz der 

 Jahreszeit auf den Verlauf der Nitrifikation. Centrbl. Bakt. II, 53: 33-43. 

 1920. 



44 Schonbrunn, B. Uber den zeitlichen Verlauf der Nitrifikation, unter be- 

 sonderer BeriAcksichtigung der Frage nach dem periodischen Einflusz der Jah- 

 reszeit. Centrbl. Bakt. II, 56: 545-565. 1922. 



45 Deherain, P. P. Sur la production des nitrates dans la terre arable. Ann. 

 Agron., 13: 241-261. 1887; also 22: 515-523. 1896. 



46 Fraps, G. S. The production of active nitrogen in the soil. Texas Agr. 

 Exp. Sta. Bui. 106. 1908. 



