Microorganisms and Plant Growth 333 



the organic matter, leading to increased carbon dioxide evolution 

 and greater liberation of the nitrogen a's ammonia. This explains the 

 favorable effect of fallowing upon the activities of the soil micro- 

 biological population. The "ripening" of soil in spring is a result of 

 treatments that are favorable to the activities of soil microorgan- 

 isms. 



Fertilization and crop rotation also have an important effect upon 

 the microbiological population. The nature of the fertilizer, the 

 residual effect upon the reaction of the soil, the nature of the crop 

 grown, and the treatment of the crop will influence in one way or 

 another the nature and abundance of microorganisms of the soil. 

 This is true especially of the addition of available energy in the 

 plant residues, the excretion by plants of substances favorable to 

 microbial development, the increase in soil nutrients, and the im- 

 provement in the buffering capacity and physical condition of the 

 soil. 



Microorganisms and Plant Growth 



Plants and microorganisms exert numerous effects upon one an- 

 other. Plants supply to the microorganisms most of the energy and 

 nutrients in the form of the numerous residues in the roots and 

 stubble. They also secrete soluble substances which affect in various 

 ways the growth of microorganisms. Plants control the chemical 

 composition of the soil solution, thus modifying the nature of the 

 medium in which most of the activities of microorganisms take place. 

 By removing some of the nutrients from the soil, plants may exert an 

 injurious effect upon the growth of microorganisms, or may actually 

 compete with them for some of these nutrients. Plant roots influ- 

 ence the structure of the soil and bring about an improvement in soil 

 aeration, thus affecting greatly the growth of microorganisms. 



Microorganisms, in their turn, exert numerous influences upon the 

 growth of higher plants. By decomposing the plant and animal 

 residues in the soil, thus bringing about their mineralization, micro- 

 organisms liberate the nutrients required for plant growth, espe- 

 cially the carbon dioxide, nitrate, phosphate, and sulfate. The sym- 

 biotic nitrogen-fixing bacteria, through their association with the 

 roots of leguminous plants, effect the fixation of large quantities of 

 nitrogen. Plants and microorganisms form a variety of other sym- 

 biotic associations, designated as mycorrhiza (roots and fungi) and 

 bacteriorrhiza (roots and bacteria). Although the importance of 



