396 THE PRINCIPLES OF SOIL MANAGEMENT 



the practical problem is to maintain a condition of 

 soil most favorable to such activity. 



266. Distribution. Bacteria are found almost uni- 

 versally in soils, although they are much more numer- 

 ous in some soils than in others. A number of investi- 

 gators have stated that in soils from different locali- 

 ties and of different types that they have examined, 

 the numbers of bacteria were proportional to the 

 productiveness of the soils. The number of bacteria 

 present has, in some cases, been shown to be propor- 

 tional to the amount of humus contained in the soil. 

 It is natural to expect that within certain limits 

 both of these findings will hold. The conditions ob- 

 taining in a productive soil are those favorable to 

 the development of certain forms of bacteria, and these 

 kinds constitute a very large proportion of those gen- 

 erally found in soils. However, there is evidence 

 that comparatively unproductive soils may contain 

 a large number of bacteria which are presumably 

 not favorable to plant-growth. 



Samples of soil taken from certain productive 

 and relatively unproductive portions of a field on 

 Cornell University farm contained a larger number 

 of bacteria in the poor soil, although the two soils 

 were equally well drained, and the good soil had slightly 

 more organic matter. They had also received practi- 

 cally the same treatment during the preceding few 

 years. 



Character of Number of bacteria 



soil per gram of dry soil 



Good 1,200,000 



Poor 1,600,000 



