256 Higher Plants and Soil Microorganisms 



violet medium is used for plating out the soil. The dye inhibits the 

 growth of the actinomycetes and the Gram-positive bacteria, but not 

 of the Gram-negative bacteria. Radiobacter colonies are raised, 

 smooth, glistening, with opaque centers and transparent edges. 

 When legumes are grown in a given soil, there is an increase in the 

 number of Radiobacter in that soil. The greatest numbers of this 

 group of organisms are found close to the plant, but frequently none 

 at all one foot away from the plant. Cowpeas, field peas, vetch, and 

 soybeans stimulate these bacteria, the increase being accompanied 

 by an increase in the consumption of nitrates and in the evolution 

 of CO.. 



When a single type of plant is continuously grown in a soil, it 

 leaves residues that may result in a change in the chemical compo- 

 sition of the soil, which in turn influences the bacterial population. 

 Some kinds of plant species favor certain types of bacteria and 

 inhibit others, and thus bring about a change in the bacterial equi- 

 librium in the soil. The new bacterial population produces a change 

 in the composition of the soil which may subsequently affect plant 

 growth, whereby some plant species are favored and others are 

 retarded. The continuous growth of a single type of plant, such as 

 wheat, flax, or clover, will bring about the development of fungi 

 pathogenic to this plant, making the soil "sick" for the particular 

 plant. 



Starkey has shown that higher plants may affect some groups of 

 microorganisms differently from other groups; the extent of the 

 influence of various plants upon different organisms may vary. The 

 greatest increase appears to be brought about in the Radiobacter 

 group, although some striking effects may be observed upon the 

 general bacterial population. Potato increased the numbers of bac- 

 teria only to a slight extent, whereas rape produced some striking 

 changes. The influence of various plants upon the soil population 

 as measured by the number of Radiobacter per gram of soil depends 

 upon the stages of growth of the plant ( Table 56 ) . Slight effects are 

 observed in the early stages. The maximum occurs when plants 

 reach considerable size. Because of their longer growing period, 

 bieimials show a much more prolonged effect upon the soil organisms 

 than do annuals. The effect of higher plants upon the microbio- 

 logical population of the soil may be an important factor in bring- 

 ing about the seasonal fluctuations of microorganisms. 



Neller measured the total CO- liberated from oxidation processes 

 taking place in the soil during plant growth. He found much more 



