EVOLUTION OF CARBON DIOXIDE 



189 



activity about plant roots are numerous. There are physical 

 effects produced by the addition of organic matter from plant 

 roots, by the liberation of carbon dioxide from the plant roots and 

 microorganisms, by root penetration opening passages in the soil, 

 by absorption of inorganic nutrients and large quantities of water. 

 The dominating influence of the plants on soil organisms is most 

 commonly exerted by the addition of organic materials in the form 

 of root excretions and degenerated root tissues. These are sub- 

 sequently used by the soil microbes as food. Such effects are most 

 apparent in the zone of root development, frequently referred to 

 as the rhizosphere, since it is here that the root residues become 

 introduced. Table 46 indicates that the abundance of bacteria 

 in soils supporting different plants is different, and that, in general, 

 the abundance of bacteria about the roots is closely correlated 

 with the activity of the microbes, as indicated by the formation of 

 carbon dioxide. 



TABLE 46 



Influence of Plant Growth upon the Number of Bacteria in Soil and 

 Production of CO2 (from Stoklasa) 



Not all elements of the soil population are affected alike by the 

 development of plants. The influences of any one plant on the soil 

 population at different stages of plant growth are characteristic, 

 as shown by Fig. 71. Different plants affect the microbes in dis- 

 tinct ways. Further, the various microbes in the soil respond 



