282 SOILS. 



the formation of the gas. Such formation is of course, how- 

 ever, conditioned upon the presence of oxygen. In the case of 

 most bacteria, there is a certain limit beyond which the pres- 

 ence of their own product exerts an injurious or repressive 

 effect upon their activity ; so that if the gas accumulates beyond 

 that limit, the rate of its formation decreases despite of other- 

 wise favorable conditions. 



It follows that the best life-conditions of these organisms 

 (even when anerobic) cannot be fulfilled below a certain 

 limited depth in the soil; and all observations show that their 

 number decreases very rapidly with increasing depth (see 

 chap. 9, p. 142), varying with the perviousness of the soil, 

 but rarely exceeding four or five feet in the humid regions; 

 though doubtless found at greater depths in the arid climates. 

 It is also obvious that the use of any antiseptic or poisonous 

 materials on the field or in the manure pile will tend to disturb 

 and restrain the useful activity of these organisms. 



Putrefactive Processes. Carbonic gas is formed also, but to 

 a much more limited extent, in putrefactive processes, occur- 

 ring in the absence, or with only limited access, of air or 

 oxygen. These processes likewise are conditioned upon the 

 presence or activity of (largely anerobic) bacteria; but they 

 should not occur in normally constituted, and especially in tilled 

 soils, being as a rule inimical to the growth of cultivated 

 plants (see chap. 9, p. 145). 



