190 



HIGHER PLANTS AND SOIL MICROORGANISMS 



differently to plant growth. As shown by Fig. 73, the general 

 bacterial population is affected to a much greater extent that either 

 fungi or actinomyces; however, some species of bacteria develop 

 to a greater extent than others in proximity to root systems of 

 plants. This appears from the fact that the organisms closely 

 related to the Radiobacter group increase many times more in 

 response to plant development than do the microbes of the general 

 bacterial population. 



It is on the immediate root surfaces and within the superficial 

 tissues of the roots that the greatest influences of plants are 

 exerted, and there are progressively decreasing effects the greater 

 the distance from the region of soil penetrated by the roots (see 

 Table 47). 



TABLE 47 



Influence of Plant Development upon the Abundance of Microbes 



AND Their Activity in Soils at Different Distances from the Plant 



Roots* (from Starkey) 



* Age of plants — 113 days. 



Since all plants require some oxygen about their roots and since 

 microbial activities and root excretions tend to lower the oxygen 

 concentration and raise the concentration of carbon dioxide, there 



