636 PRINCIPLES OF SOIL MICROBIOLOGY 



of C0 2 and carbonates. 49 The reaction of the soil as a result of appli- 

 cation of fertilizer may change, according to the nature of the biological 

 transformation of the fertilizer. Urea, for example, causes the soil 

 reaction first to become alkaline because of the formation of ammonia 

 and then acid because of the oxidation of the ammonia to nitric acid, 

 as shown in table 67. 50 



The soil reaction is also influenced by the moisture content of the 

 soil, application of fertilizers, green manures, stable manures, plants 

 grown and other factors. 51 



The reaction of the soil has a definite influence upon the activities 

 of various microorganisms 52 and upon the very distribution of the 

 microflora and microfauna in the soil. An acid soil favors the develop- 

 ment of fungi and is distinctly injurious to development of certain 

 groups of bacteria, like Azotobacter, which has a limiting reaction at 

 pH 6.0. Nitrifying bacteria are limited in their activities to a maxi- 

 mum acid range of pH 4.0 to 4.6; Bad. radicicola has its limiting acid 

 reaction at pH 3.4 and pH 6.0. Actinomyces are inhibited in growth 

 by reactions more acid than pH 4.8; this fact is utilized for the control 

 of Act. scabies causing potato scab in the soil. The application of 

 lime to an acid soil has a favorable influence upon the bacteriological 



49 Coville, F. V. The formation of leafmold. Smithsonian Report for 1913: 

 333-343. 1914; Ayers, S. H., and Rupp, P. Simultaneous acid and alkali bac- 

 terial fermentations from dextrose and the salts of organic acids respectively. 

 Jour. Inf. Dis., 23: 188-216. 1924. 



50 Brioux, Ch. Influence de l'uree employee comme engrais, sur la reaction 

 du sol. Compt. Rend. Acad. Sci., 179: 914-917. 1924. 



51 Plummer, J. K. Studies in soil reaction as indicated by the hydrogen elec- 

 trode. Jour. Agr. Res., 12: 19. 1918; Knight, H. G. Acidity and alkalinity of 

 soils. Jour. Ind. Engin. Chem., 12: 559. 1920; Hardy, F. Soil sourness — its 

 meaning and significance. West Indian Bui. 19: 37-85. 1921; Salter, R. M., 

 and Morgan, M. F. Factors affecting soil reaction. I. The soil-water ratio. 

 Jour. Phys. Chem.. 27: 117-140. 1923; Fischer, E. A. Studies on soil reaction. 

 Jour. Agr. Sci., 11: 19-44. 1921; Sci. Progr., 16: 408. 1922; Connor, S. D. Soil 

 acidity as affected by moisture conditions in the soil. Jour. Agr. Res., 15: 

 321. 1918; Morse, F. W. Effect of fertilizers on hydrogen-ion concentration in 

 soils. Jour. Ind. Engin. Chem., 10: 125. 1918; Atkins, W. R. G. Some factors 

 affecting the hydrogen-ion concentration of the soil and its relation to plant dis- 

 tribution. Sci. Proc. Roy. Dublin Soc, 16: 369-413, 414-426, 429-434. 1922; 

 Kappen, H., and Zapfe, M. Uber Wasserstoffionenkonzentrationen in Auszligen 

 von Moorboden und von moor- und rohhumusbildenden Pflanzen. Landw. 

 Vers. Sta., 90: 321-374. 1917. 



52 Adam, A. Uber die Bedeutung der Eigenwasserstoffzahl (des H-Ionen 

 optimum) der Bakterien. Centrbl. Bakt. I, 87: 481-486. 1922. 



