802 PRINCIPLES OF SOIL MICROBIOLOGY 



the growth of the plants, both in the case of ectotrophic and endotrophic 

 forms. 



Microorganisms affect the growth of higher plants not only directly, 

 but also indirectly. The formation of carbon dioxide and various or- 

 ganic acids brings about a greater solubility of the soil minerals, 

 particularly the carbonates and phosphates, as well as to some extent the 

 zeolitic materials. To this we must add, of course, the action of inor- 

 ganic acids, namely nitrous, nitric and sulfuric, which result directly 

 from the activities of microorganisms. The favorable influence of 

 bacteria upon the activities of the roots of plants, by increasing their 

 etching power, may also be referred to here. 1 The favorable influence of 

 legumes upon non-leguminous plants has also been noted above. 2 



When seeds are planted immediately after turning under a green man- 

 ure crop, the seedlings may be injured. As a result of the decomposi- 

 tion of the green manure, numerous fungi develop, some of which are 

 destructive to the seedlings, especially in the case of oil seeds. The 

 rapid evolution of C0 2 and utilization of oxygen produce conditions 

 unfavorable to oxidation, which is essential for the seeds in the process 

 of germination. However, when seeds are planted two weeks after the 

 addition of the green manure, no serious injury is caused to their ger- 

 mination. 3 An attempt was made to explain unproductiveness of soils 

 not by a lack of proper nutrients but by the presence of substances 

 actually injurious to plant growth; 4 these substances were presumably 

 formed in the soil partly at least as a result of activities of microorgan- 

 isms. The hypothesis that protozoa are concerned in the destruction of 

 bacteria and, therefore, bring about the formation of "sick" and unpro- 

 ductive soils, as well as other biological theories which attempt to explain 

 this phenomenon, have been discussed above. 



1 Fred and Hass, 1919 (p. 647). 



2 See also Koch, A. Die Pflanzennahrstoffe des Bodens unter dem Einflusz 

 der Bakterien. Chem. Ztg., 36: 726. 1911; Koch, A. "Uber die Einwirkung 

 des Laub- und Nadelwaldes auf den Boden und die ihn bewohnenden Pflanzen. 

 Centrbl. Bakt. II, 41 : 545-572. 1913; Gibbs, W. M., and Werkman, C. H. Effect 

 of tree products on bacteriological activities in soil. I. Ammonification and 

 nitrification. Soil Sci., 13: 303-322. 1922; Bondorff, A. The use of micro- 

 organisms for the determination of the content in the soil of plant food available 

 for higher plants. Den. Kgl. Verteriner. Z. Lanbohojskoles aarskrift. 1918, 

 339-362 (Physiol. Abstr., 6: 137). 



3 Fred, E. B. Relation of green manures to the failure of certain seedlings. 

 Jour. Agr. Res., 5: 1161-1176. 1916. 



4 Schreiner, O., and Reed, H. S. Some factors influencing soil fertility. 

 U. S. Dept. Agr. Bur. of Soils, Bui. 40. 1907. 



