MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 357 



indicating neutrality. The product of the concentrations of these ions 

 (water) is constant, about i X io~ 14 . If the concentration of H ions is 

 greater than that of the OH ions, the solution is then acid. When the 

 concentration of OH ions is greater than of H ions, the solution is alkaline. 

 Total acidity (or potential acidity, to use the expression of Sharp and 

 Hoagland) or alkalinity may be due to undissolved substances or to 

 soluble compounds only partly hydrolyzed or dissociated. 



The hydrogen-ion concentration of the soil can be measured both 

 electrometrically ancf colorimetrically. The work of Gillespie, Sharp 

 and Hoagland and others has brought out the fact that soils vary greatly 

 in the hydrogen-ion concentration, from a high acidity to a high alkalin- 

 ity. There is a definite correlation between the hydrogen-ion concen- 

 tration of soils and the occurrence and activities of microorganisms. 

 Gillespie has shown that potato scab (Actinomyces scabies) rarely occurs 

 in soils having a hydrogen-ion exponent lower than 4.8 to 5.2. 



Gainey called attention to the fact that Azotobacter occurs in soils 

 having a hydrogen-ion exponent greater than 6.0, while the more acid 

 soils are practically free from this important group of nitrogen-fixing 

 organisms. Waksman demonstrated the occurrence of Azotobacter 

 in cranberry soils that received an application of lime and gave a de- 

 cided increase in crop, while the unlimed soil was too acid for the organ- 

 isms to act in; this limiting reaction for Azotobacter corresponded to a 

 hydrogen-ion exponent of about 6.0. 



CHANGE OF REACTION PRODUCED BY MICROORGANISMS IN THE 

 MEDIUM.- Microorganisms modify the reaction of the medium both 

 by their ability to produce organic and inorganic acids (in the case 

 of sulphur oxidizing bacteria) and also by their utilization of the 

 organic acids as sources of energy. 



EFFECT OF REACTION ON NUMBERS AND SPECIES. Some of the 

 important groups of soil bacteria including nitro, azoto and ammonify- 

 ing species will develop slowly or not at all, when the amount of acid in 

 the medium is increased beyond a certain point. Hence it is realized 

 by progressive farmers that a proper supply of lime is essential for the 

 satisfactory decomposition of organic matter in the soil, and the abund- 

 ant supply of available nitrogen compounds, as well as of other con- 

 stituents of plant food to growing crops. The influence of lime on the 

 multiplication of soil bacteria is well illustrated, for instance, by the 

 experiments of Fabricius and von Feilitzen. These investigators found 



