THE MICRO-ORGANIC POPULATION OF THE SOIL 275 



certain soils/ but not on all. Nitrates increase the amount of 

 ammonification,^ the growth of the organisms causing decay 

 (176a), the nitrogen fixing azotobacter (p. 199) and of nitrate 

 assimilating organisms (113^), though in higher quantities 

 they become toxic. 



Table LXXII. — Percentages of Various Salts in Loam Soil which are 

 Necessary to Reduce Ammonification, Germination and Dry Matter 

 Produced in Wheat to about Half Normal. J. E. Greaves, E. G. 

 Carter, and H. C. Goldthorpe (113a and c). 



Effect of Dissolved Oxygen Supply. — Russell and Apple- 

 yard (241^) found that rainfall has a more definite effect than 

 moisture in increasing bacterial numbers and biochemical 

 changes in the soil. They attribute this action to the oxygen 

 dissolved in the rain-water which renews the dissolved atmos- 

 phere in the soil and gives the organisms a new lease of 

 activity. 



Effect of Previous Treatment of the Soil. — Prolonged drought 

 affects the soil even after it has passed away and the soil has 

 become moistened. The rate of production of nitrate and the 

 bacterial numbers both increase (24^ and also Prescott ^). 



Field Observations. — The general phenomena observed in 

 the laboratory can be seen also in the field, but it is less easy 

 to disentangle the various factors. 



^J. Dumont, Compt. Rend., 1897, 125, 469-472. 



' D. A. Coleman, Soil Sci., 1917, 4, 345. 



^ A note on the Sheraqi soils of Egypt (yourn. Ag. Set., 1920, 10, 177.) 



