782 



PRINCIPLES OF SOIL MICROBIOLOGY 



equivalent to 55.6 per cent of its water-holding capacity. Excessive 

 quantities of water were found to be much more injurious than too 

 small quantities, due to the fact that in the first case soil conditions 

 become anaerobic and denitrification sets in. The water requirements 

 of the microorganisms vary considerably with the soil. Maximum 

 nitrification in a loam soil occurred with 16 per cent water; by reducing 

 the water content to 10 per cent or increasing to 26 per cent, nitrifi- 

 cation was greatly retarded. 47 



A definite favorable influence of increased moisture upon bacterial 

 numbers was recorded. 48 Moisture content of the soil was found 49 

 to have a greater influence upon bacterial numbers than the tempera- 

 ture. By increasing the moisture content of the soil from 6.5 to 14 per 

 cent, there was an increase in bacterial numbers from 10 to 16.4 mil- 

 lions per gram. By raising the moisture content of the soil from 6 to 



TABLE 89 

 Influence of moisture upon the evolution of COz from different soils 



15 per cent, van Suchtelen obtained an increase in C0 2 production 

 from 19 to 208 nigra., which led him to conclude that the latter was a 

 more sensitive method for studying changes in microbiological activi- 

 ties than determining numbers, as is shown elsewhere. Sixty to eighty 

 per cent of the total water holding capacity of the soil is an optimum 

 for bacterial activities. 50 Excessive soil moisture reduces both the 

 numbers of soil microorganisms and their activities. 51 A definite rela- 

 tion between moisture content of different soils and bacterial activities 



47 Coleman, L. C. Untersuchungen uber Nitrifikation. Centrbl. Bakt. II, 

 20: 401-420, 484-513. 190S. 



48 Fischer, H., Lemmermann, A., Kappen, H., and Blanek, E. Bakteriologisch- 

 chemische Untersuchungen. Lanchv. Jahrb., 38: 319-364. 1909. 



49 Engberding, 1909 (p. 771). 



60 Lohnis, F. Untersuchungen uber den Verlauf der Stickstoffumsetzungen 

 in der Ackererde. Mitt, landw. Inst. Leipzig, 7: 1-105. 1905. 



61 King and Doryland, 1909 (p. 771). 



