240 MICROBIOLOGY OF SOIL. 



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 secretions 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 

 water from the soil by growing crops causes the film water to become 

 more concentrated in so far, at least, as some salts are concerned. Modi- 

 fications 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 role, 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) . Among the various factors that affect the numbers of bacteria in 

 the soil moisture, aeration, food supply and reaction have already been 

 discussed as important. Sandy soils because of their relative dryness and 

 poverty in organic matter contain a comparatively small number of bac- 

 teria, at times less than 100,000 per g. and usually less than 1,000,000 

 per g. But when such sandy soils receive deep and thorough cultivation 

 and are enriched in organic matter the numbers increase to several mil- 

 lions per g. In most loam soils in a good state of cultivation the num- 

 bers range from about 1,000,000 to several millions per g. In heavy, 

 compact soils the aeration conditions are unsatisfactory, acid substances 

 and reduction products accumulate and the numbers are greatly reduced. 

 Again, in peat and muck soils the numbers of bacteria are small when 

 the reaction is markedly acid. It has been found that under such condi- 

 tions the molds are more numerous than the bacteria and that the latter 

 often occur to the extent of less than 100,000 per g. Generally speak- 

 ing, the higher the state of cultivation the greater the number of bacteria 

 in the soil. It increases as we pass from forest and prairie to plowed 

 fields, and from extensive field practice to intensive garden practice. 

 Under special conditions, as in green-house soils generously supplied with 

 manure and moisture, in sewage polluted soils, and in soils partly steri- 



