220 MODIFICATION OF THE SOIL POPULATION 



ment of the microbial population. The greater abundance of 

 organisms in the limed soils is also reflected in crop yields, as will 

 be considered later. 



Many of the common cultural treatments of soils bring about 

 changes in the soil population in as varied a manner as lime. 

 Seldom can the influence of the treatment be ascribed to a single 

 effect or a single soil change. Even a slight modification of soil 

 causes a series of changes both in plant growth and microbial devel- 

 opment, but some treatments have more pronounced and extended 

 effects than others. Cultivation and the application of fertilizer 

 salts affect biological activity directly to some extent, but the 

 indirect effects of such treatments and the direct effects of the 

 incorporation of organic substances in soils cause a greater degree 

 of modification. 



Influence of Reaction upon Soil Microbes. — Not all 

 microorganisms are affected alike by acid and alkaline conditions 

 in soils. Some microbes will develop over a narrow range of soil 

 reaction with an optimum zone at pH 6.0 to 7.5. Others are tol- 

 erant to as extreme acid or alkaline conditions as ever appear in 

 soils; many of the soil fungi, for example, will grow well at a series 

 of reactions ranging from pH 2.0 to pH 9.0. In general, soil 

 microbes will tolerate greater concentrations of hydrogen-ions than 

 hydroxyl-ions, that is, greater degrees of acidity than alkalinity. 

 In humid regions, soils as acid as pH 5.0 are not uncommon, and 

 such soils may support good growth of a number of cultivated 

 crops and a large and varied microbial population. The concen- 

 tration of hydroxyl-ions in soils as alkaline as pH 9.0 to 10.0 is the 

 same as the concentration of hydrogen-ions in soils of pH 5.0 to 

 4.0. Such alkaline soils exist only in regions of deficient rainfall. 

 They are fairly unproductive and harbor relatively smaller num- 

 bers of organisms. 



Many mycorrhiza fungi grow well at pH 5.0 and make little 

 growth at neutrality (pH 7.0). Nitrifying bacteria have an acid 

 minimum at pH 4.0 and an alkaline maximum at pH 9.4. Most 

 species of Azotobacter have an acid minimum at pH 6.0, so that in 

 soils which are more acid than pH 6.0 these organisms are either 

 inactive or absent. However, the anaerobic nitrogen-fixing 

 organism, Clostridium pastorianum, will grow at a reaction as 

 acid as pH 5.2, and is, therefore, much more widely distributed in 

 acid soils than Azotobacter. 



