634 PRINCIPLES OF SOIL MICROBIOLOGY 



When neutral salts are added to a soil, the cations are adsorbed, re- 

 placing the hydrogen ions, thus making the soil even more acid. 42 

 It has also been suggested 43 that acidity in well aerated soils is due to 

 the hydrolysis of silicates; the bases are removed by plants or soil 

 water, while the acid silicates are left behind. 



Definite concentrations of free hydrogen-ions have actually been 

 demonstrated in soils, and have been measured electrometrically 44 and 

 colorimetrically. 45 A pH of 3.7 is the extreme value obtained for mineral 

 acid soils, while values of pH 3.2 have been reported for peat soils; 

 pH 9.7 to 10.0 were reported for alkaline soils (containing free sodium 

 carbonate); fertile soils usually give a range of pH values of 6.0 to 7.5. 



Soils are usually well buffered over considerable ranges of hydrogen- 

 ion concentrations. 46 By adding acid or base to a soil and titrating the 

 resulting hydrogen-ion concentrations, a linear titration curve is 



as a neutralizing agent (Russian). Zhur. Opit. Agron., 22: 3-27. 1924; see also 

 Hissink, D. J., and Van der Spek, J. The acidity of the soil. Verslag. Land. 

 Onderzoek, Rijksland., 27: 146-161. 1922 (Chem. Abstr., 16: 4293). 



42 Cameron, F. K. The soil solution. Easton, Pa. 1911; Kappen, H. Zu 

 den Ursachen der Aziditat der durch Ionenaustausch sauren Boden. Landw. 

 Vers. Sta., 89: 39-80. 1916; 96: 277-307. 1920; Wrangell, M. Phosphorsaur- 

 eaufnahme und Bodenreaktion. Landw. Vers. Sta., 96: 209-255. 1920; Harris, 

 J. E. Soil acidity. Mich. Agr. Exp. Sta., Tech. Bui. 19. 1914; Jour. Phys. 

 Chem., 18: 355. 1914. 



43 Truog, E. Cause and nature of soil acidity with special regard to colloids 

 and adsorption. Jour, physik. Chem., 20: 457-484. 1916. 



44 Sharp, L. T., and Hoagland, D. R. Acidity and adsorption in soils as meas- 

 ured by the hydrogen electrode. Jour. Agr. Res., 7: 123-145. 1916; Soil Sci., 

 7: 196-200. 1919. 



48 Gillespie, L. The reaction of soil and measurement of hydrogen-ion con- 

 centration. Jour. Wash. Acad. Sci., 6: 7-16. 1916; Soil Sci., 4: 313-319. 1917. 

 An excellent theoretical discussion of the hydrogen-ion concentration of soils 

 and culture media, methods for determination and applications to microbiological 

 processes is given by L. Michaelis, Die Wasserstoffionenkonzentration. Berlin. 

 1922; Clark, 1922 (p. 371). The use of the quinhydrone electrode for determining 

 the soil reaction is discussed in detail by H. Christensen and S. T. Jensen, Unter- 

 suchungen bezuglich der zur Bestimmung der Bodenreaktion benutzten elek- 

 trometrischen Methoden. Intern. Mitt. Bodenk., 14: 1-26. 1924; and Baver, 

 L. D. The use of the quinhydrone electrode for measuring the hydrogen-ion 

 concentration of soils. Soil Sci., 21: 167-180. 1926. 



46 Charlton, J. The buffer action of some Burma soils. Mem. Dept. Agr. 

 India, 7: 101-121. 1924; Jensen, S. T. Om bestemmelse af jordens stodpudevirk- 

 ning. 177 Ber. Statens Forsogs. i. Plantenk, 1924; Arrhenius, O. Clay as an 

 ampholyte. Jour. Amer. Chem. Soc, 44: 521-524. 1922; see also various papers 

 in the Trans, second Comm. of the Intern. Soc. Soil Sci. Groningen. 1926. 



