ARTIFICIAL FERTILIZERS AND SOIL MICROBES 221 



In most soils, practically all actinomyces fail to grow if the 

 acidity is greater than pH 4.0. Some of these organisms, such as 

 Actinomyces scabies, the causative agent of potato scab, will not 

 grow at a pH less than 4.8. Control of potato scab and of sweet 

 potato pox by treatment of soil with sulfur and other acid-reacting 

 fertihzers is based upon this sensitivity of actinomyces to high 

 acidity. Sulfur becomes oxidized by biological agents to sulfuric 

 acid, which increases the acidity of the soil. If sufficient sulfur is 

 added to change the reaction of the soil to pH 4.6-4.8 (the amount 

 to be used depending upon the initial reaction and the buffer 

 content of the soil), the development of Actinomyces scabies will 

 be checked, but the potato plants will still make sufficient growth 

 to produce a good crop. When the soil is to be used the following 

 year for another crop, it should be properly limed before that crop 

 is planted, so as to make the reaction less acid. Certain other 

 microbes which cause plant diseases are inhibited in their develop- 

 ment by an alkaline reaction of the soil. Such an organism is 

 Plasmodiophora brassicae, which produces club-root on cabbage. 

 For the control of this disease, soils are liberally treated with lime. 

 Thus, a thorough knowledge of the characteristics of the patho- 

 genic organisms and their response to differences in reaction some- 

 times enables one to control plant diseases by judicious soil man- 

 agement. 



Different cells of microorganisms of a certain species are not all 

 alike in their tolerance to different degrees of acidity and alkalinity. 

 The variabihty among the different individuals results in the 

 selective development of the forms which are best adapted to the 

 environment in which they exist. Consequently strains of an 

 organism obtained from one soil may show certain characteristics 

 which are different from those of another strain obtained from a 

 different soil. Some organisms natural to acid soils may show 

 greater tolerance to acid reactions than strains of the same organ- 

 isms present in alkaline soils. Even under laboratory conditions, 

 by growth in certain media, cultures may be obtained which are 

 more tolerant to acid or alkahne reactions than the mother culture 

 from which the modified strains were obtained. By gradual suc- 

 cessive growth of nitrifying bacteria on alkahne media, strains 

 were obtained which grew at a reaction more alkaline than pH 10.0. 



Influence of Artificial Fertilizers upon Soil Microbes. — 

 Although not as pronounced as many other effects on soil organ- 



