250 Higher Plants and Soil Microorganisms 



surface microorganisms in a root-rot-susceptible variety of tobacco 

 grown in infective soil, as compared with the same variety of tobacco 

 grown in healthy soil; similar effects for wheat were also demon- 

 strated. Starkey illustrated the intense development of microorgan- 

 isms in and about senescent and dying roots or portions of roots. 



According to West and Lochhead, there exists in every soil inves- 

 tigated a balance between two general nutritional classes of bacteria. 

 On the one hand are bacteria capable of rapid growth on a single 

 substrate, and in opposition to them are others dependent for their 

 development on an ample supply of certain specific compounds. If 

 conditions favor an increase in one group, the incidence of the other 

 must correspondingly fall. These two workers demonstrated strik- 

 ing differences in distribution of these nutritional groups between 

 rhizosphere and control soils: in the rhizosphere soil, the percentage 

 incidence of organisms capable of growth on the basal medium alone 

 declined, whereas that of organisms with more complex growth re- 

 quirements increased. The increase in percentage incidence of the 

 group requiring growth factors was especially marked. It was found 

 later that a requirement for amino acids, and not for growth factors, 

 was the most important characteristic of the rhizosphere population. 

 Fertile soil is usually well supplied with growth factors, and the 

 growth factor heterotrophy does not, therefore, necessarily limit 

 growth of a microorganism in soil. 



Influence of Soil Microorganisms upon Plant Growth 



Among the numerous soil processes that are carried out by micro- 

 organisms and that directly affect the growth of higher plants, the 

 following may be analyzed in further detail: 



Microorganisms decompose the plant and animal residues added 

 to the soil and the organic matter or humus in the soil itself. They 

 thus liberate the nitrogen and minerals necessary for the growth of 

 higher plants and produce considerable quantities of COo, which 

 is essential for plant growth. 



Microorganisms oxidize and otherwise transform into forms readily 

 available to plants various minerals either introduced into the soil, 

 such as ammonium salts and sulfur, or formed there in the decompo- 

 sition of the organic residues. 



Microorganisms synthesize a variety of organic substances from 

 the inorganic compounds in the soil and thus compete with higher 

 plants for available nutrients, notably the nitrogen and the minerals. 



