784 PRINCIPLES OF SOIL MICROBIOLOGY 



nitrogen fixation, one under aerobic and the other under anaerobic 

 conditions. 



Ammonia formation from proteins was found' 50 to be at an optimum 

 when the soil contained water equivalent to 60 per cent of its total 

 moisture holding capacity. The formation of ammonia may also be 

 very intensive even in saturated soils. 61,62 This is due to active de- 

 composition of proteins by anaerobic bacteria. The mechanism of 

 ammonia formation is different of course under aerobic and anaerobic 

 conditions. 63 



In arid regions, the application of irrigation water has 64 a definite 

 beneficial effect upon the number of organisms in fallow soils and upon 

 the ammonifying and nitrifying capacities of both fallow and cropped 

 soils. These activities result in an increase in soluble nitrogen. An 

 excess of water may result in the washing out of the nitrates from the 

 soil. 



Greaves and Carter, 00 using the Briggs formula for the moisture 

 equivalent and the wilting and hygroscopic coefficients, found that 

 the following equations represent the water requirements for maximum 

 bacterial activities: 



M am = 0.6c = 0.942e + 12.6 = 1.74u> + 12.6 = 2.55^ + 12.6 

 M n = 1.55c = 0.8525e + 11.55 = 1.472m; + 11.55 - 2.163/i + 11.55 

 M n/ = 0.7c = 1.049e +14.7 = 1.947u> + 1.47 = 2.84SA + 14.7 



c = moisture capacity as defined by Hillgard, w = wilting coefficient, e = the 

 moisture equivalent, h — the hygroscopic coefficient, am = ammonification, 

 n = nitrification, nf = nitrogen fixation: 



Influence of soil cultivation. Cultivation tends to conserve the soil 

 moisture at a time of the year when it is most needed; it brings about 

 a better aeration of the soil, it influences the soil temperature and 



60 Greaves, J. E., and Carter, E. G. Influence of moisture on the bacterial 

 activities of the soil. Soil Sci., 10: 361-387. 1920; 13: 251-270. 1922. 



61 Lipman, J. G., and Brown, P. E. Report of the soil chemist and bacteri- 

 ologists. 29th Ann. Rpt., N. J. Agr. Exp. Sta. 105-115. 1908. 



62 Murray, T. J. The oxygen requirements of biological soil processes. Jour. 

 Bact., 1: 547-614. 1916. 



63 See also Miinter, F., and Robson, W. P. tJber den Einflusz der Boden und 

 des Wassergehaltes auf die Stickstoffumsetzungen. Centrbl. Bakt. II, 39: 

 419-440. 1916; Rahn, O. Bacterial activities in soil as a function of grain size 

 and moisture content. Mich. Agr. Exp. Sta. Bui. 16. 1912; Centrbl. Bakt. II, 

 35: 429-465. 1912; 38: 484. 1913. 



64 Prescott, J. A. A note on the Sheraqi soils of Egypt. Jour. Agr. Sci., 

 10: 177-181. 1920. 



