334 Microorganisms and Soil Fertility 



mycorrhiza in plant development has been definitely established, 

 the effect of the bacteriorrhiza formations is still under discussion. 

 Various microorganisms are believed to produce plant-growth- 

 stimulating substances, including vitamins and hormones. Although 

 this is still open to debate, the fact remains that plants may benefit 

 considerably from addition of certain hormones and vitamins to the 

 soil. To what extent bacteria and other organisms are responsible 

 for the production of such substances has not been estabhshed as 

 yet. There is no doubt, however, that addition of organic matter, 

 especially stable manures, to the soil results in favorable effects upon 

 plant growth, which cannot be ascribed to the mere inorganic fer- 



Table 77. 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. 



tilizer constituents of the manure. The favorable action, resulting 

 from decomposition of the organic residues, upon plant growth must 

 be definitely ascribed to the activities of microorganisms. 



The effect of antibiotics produced by microorganisms upon plants 

 is another debatable question. Certain compounds, like actinomycin 

 and clavacin, are formed in artificial media by soil-inhabiting micro- 

 organisms. These compounds have a toxic effect upon plant growth, 

 resulting in a type of wilting. The question remains, however, to 

 what extent these substances are produced in the soil itself and 

 how their activities are modified by the inorganic and organic soil 

 colloids. 



