254 Higher Plants and Soil Microorganisms 



ing of the green manure, there is no serious injury to germination. 



Various attempts have been made to explain the unproductiveness 

 of certain soils by the presence of substances that are injurious to 

 plant growth. These substances, designated as soil "toxins," were 

 believed to be formed in the soil as a result of activities of certain 

 groups of microorganisms. The "toxin" theory of soil fertility was 

 based upon the injurious action of such substances on the growth 

 of higher plants. The treatment of soil by heat, volatile antiseptics, 

 or simple liming was believed to overcome this injurious effect. 



Under certain conditions, the presence of decomposed organic 

 residues and of microbial cell substance has a favorable effect upon 

 the growth of plants and microorganisms. Whether this is due to 

 the production by microorganisms of plant-stimulating substances, in 

 the nature of auxins, "auximones," or "phytohormones," or to the 

 "buflfering" or "poising" effect upon the oxidation-reducing potential 

 of the medium, or to the production of stimulating substances, what- 

 ever their nature may be, remains to be determined. 



Krassilnikov and his associates have shown that various bacteria 

 and fungi may exert a highly stimulating or inhibiting effect upon 

 the growth of isolated roots of plants, depending on the nature of 

 the organism and of the plant. The inhibitory action may be due to 

 competition for nutrients, to injurious action of high concentrations 

 of growth-stimulating substances, or to the physical effect of bacterial 

 slimes in preventing the roots from obtaining sufficient nutrients. 



Influence of Plants upon Microorganisms 



The growing plant exerts a variety of influences upon the activi- 

 ties of the microorganisms in the soil. These can be briefly summar- 

 ized as follows: 



1. Plants secrete soluble organic and inorganic compounds that 

 offer a favorable medium for the growth of microorganisms. Among 

 the chemically defined products, it is sufficient to mention formic, 

 oxalic, and malic acids, certain reducing and nonreducing sugars, 

 phosphatides, and various nitrogenous compounds. All these favor 

 the growth of many soil fungi and bacteria. 



2. Plants supply large amounts of energy for the growth of micro- 

 organisms, in the form of dead roots and root hairs, root cap cells, 

 epidermal cells, and other waste products of plants. 



3. Through their roots, growing plants continuously remove from 

 the soil various soluble minerals, including nitrates, phosphates, and 



