3o8 MICROBiaLOGY OF SOIL 



sterilization is responsible for increased plant growth, and to explain 

 this phenomenon the following theories have been advanced: 



1. R. Koch's theory of direct stimulation to plant growth- — a 

 physiological effect of the sterilizing agency. 



2. Hiltner and Stormer's theory of indirect stimulation — an altera- 

 tion of the bacteriological equilibrium resulting in a marked develop- . 

 ment of numbers after decimation. 



3. Liebscher's view that soil sterilization may be regarded in the 

 same light as a nitrogenous fertilizer. 



4. Russell and Hutchinson's protozoan theory of soil fertility. , 



5. Pickering and Schreiner's contention that the alteration in 

 chemical composition is largely responsible for increased plant growth. 



6. Greig-Smith and others adhering to the bacterio-toxin hypothesis. 

 Higher Plants. — Higher plants modify the soil as a culture 



medium for bacteria in at least three ways. The root-hairs come 

 into contact with the moisture films surrounding the soil grains and 

 not only modify the composition of the film water, by withdrawing a 

 portion of the dissolved matter, but also change its character by secre- 

 tions from the roots. The changes thus effected must, necessarily, 

 modify the character of the soil and the soil solution as a culture 

 medium. Again, the rapid removal of watej: from the soil by growing 

 crops causes the film water to become more concentrated in so far, at 

 least, as some salts are concerned. Modifications, are, also, introduced 

 thereby in the proportions of oxygen, nitrogen and carbon dioxide in 

 the soil air. Finally, higher plants modify the soil environment for 

 bacteria by their root and stubble residues. For example, residues of 

 leguminous plants, being richer in nitrogen and possessing a narrower 

 carbon-nitrogen ratio than the corresponding residues of non-legumes, 

 will affect the soil somewhat differently than the latter. 



Bacteria. — Occupying, as they do, the leading r61e, bacteria 

 demand a more detailed consideration, in fact, most^ of the biological 

 discussions of soil are based upon a knowledge of these organisms. 



Numbers and Distribution {Bacteria in Productive and Unproductive 

 Soils). — The numbers of bacteria in soils well supphed with organic 

 matter usually range from 3,000,000 to .15,000,000 per g., as shown 

 by the agar plate method. These numbers vary from soil to soil, and 

 from season to season for any particular soil. The numbers of fungi 

 are also variable and may reach a total of 1,000,000 per g., although 



