756 



PRINCIPLES OF SOIL MICROBIOLOGY 



adversely the production of plant food. The partial sterilization of 

 the soil results in the destruction of protozoa, thus removing the agent 

 injurious to normal bacterial development. 



The various theories and hypotheses that have been proposed in 

 explanation of the favorable influence of partial sterilization of soil 

 upon its fertility can be summarized as follows: 



1. Direct stimulation theory. Plant roots and microorganisms may be stimu- 

 lated directly by small quantities of antiseptics; 61 it has been suggested 62 that the 

 latter are used directly as a nutrient by microorganisms. 



o o h* J 



8*0 36 



SJO 30 *0 

 V 



eo" 



Fig. 69. Influence of CS 2 upon the numbers and activities of microorganisms 

 in the soil (from Waksman and Starkey). 



2. Indirect stimulation of bacteria. The organic matter in the soil may be 

 modified in such a manner, as a result of partial sterilization, as to make it more 



81 Maassen, A., and Behn, H. Das Verhalten der Bakterien, insbesondere der 

 Bodenbakterien gegeniiber dem Schwefelkohlenstoff, und die Beeinfluszung des 

 Pflanzenwachstums durch eine Schwefelkohlenstoffbehandlung des Bodens. 

 Mitt. K. Biol. Anst. Land. u. Forstwirt., 12: 285-338. 1924. 



82 Matthews, A. Partial sterilization of soil by antiseptics. Jour. Agr. Sci., 

 14: 1-5. 1924. 



