INFLUENCE OF ENVIRONMENTAL CONDITIONS 793 



plants secrete formic, oxalic and malic acids, as well as reducing and 

 non-reducing sugars. 103 Plant roots also secrete phosphatides and 

 stimulate the development of various soil fungi. 104 



2. They supply energy and nitrogen sources for the microorganisms 

 through their residues, in the form of dead roots, root cap cells, etc. 



3. They remove the various soluble materials from the soil through 

 their roots, especially the nitrates. This changes the composition of 

 the soil solution and modifies the activities of microorganisms. 



4. They excrete carbon dioxide into the soil, which, in addition to 

 that produced by the microorganisms, will tend to change the reaction 

 of the soil, increase the solubility of certain inorganic soil constituents 

 and change the composition of the soil atmosphere. 



5. They remove the moisture from the soil, exerting an injurious 

 influence upon the growth of microorganisms. 105 The importance of 

 this influence was, however, minimized by Lyon and associates. 106 



6. Plant roots modify the structure of the soil and produce a medium 

 more favorable for the development of microorganisms. 107 The re- 

 moval of nitrates from the soil, leaving the bases behind in the form 

 of carbonates, 108 would come under the third group of phenomena; 

 this may affect the activities of microorganisms favorably. 



The influence of growing plants upon bacterial activities may be 

 expressed in terms of (1) numbers of microorganisms, (2) nitrate accu- 

 mulation or nitrifying capacity of the soil, (3) oxidizing power of the 

 soil as expressed either in terms of oxygen absorption or carbon dioxide 

 production, (4) other biological activities. It is most difficult to 

 determine the influence of the growing plant upon the numbers of 



103 Czapek, F. Zur Lehre von der Wurzelausscheidungen. Jahrb. wiss. Bot., 

 29: 321-390. 1896; Maze, P. Recherches sur la physiologie vogi'tale. Ann. 

 Inst. Past., 25: 705-738. 1911; Schulov, I. C. Investigations on the physiology 

 of nutrition of higher plants (Russian). Moskau. 1913. 



104 Melin, E. Die Phosphatiden als okologischer Faktor im Boden. Svensk. 

 Bot. Tidskr., 18: 460-464. 1924. 



105 Deherain, P. P. Drainage des terres nues. Trait6 de Chimie Agricole., 

 58G-587. 1902. 



106 Lyon, T. L., Heinicke, A. J., and Wilson, B. D. The relation of soil mois- 

 ture and nitrates to the effects of sod on apple trees. Cornell Univ. Agr. Exp. 

 Sta. Mem. 63. 1923; Mem. 91. 1925. 



107 Berkmann, M. Untersuchungen iiber den Einflusz der Pflanzewurzeln 

 auf die Struktur des Bodens. Intern. Mitt. Bodenk., 3: 1-49. 1913. 



108 Hall, A. D., and Miller, N. H. J. The effects of plant growth and of man- 

 ures upon the retention of bases by the soil. Proc. Roy. Soc, 7B, 1-32. 1905; 

 (Exp. Sta. Reed., 27: 124. 1912.). 



