March, 1914.] Soil Bacteria. 275 



SOIL BACTERIA. 



M. C. Sewell. 



To one interested in the manifold works of nature, the impor- 

 tant factors concerned in crop production, and the maintenance 

 of soil fertility, a resume of present day knowledge of soil bacteria, 

 may well claim attention. 



Scientists have demonstrated the presence of fossilized bacteria 

 in the beds of ancient geological periods. We may then believe 

 that long ages before man himself came to this earth, their existed 

 microscopic forms of life, which found their food and energy in 

 the destruction of organic matter. 



The largest numbers of bacteria are found just beneath the 

 first three inches of soil. From that point, with increasing depth, 

 the numbers diminish, until at a depth of six feet but few bacteria 

 exist. At the surface, bacteria are few in number because they 

 are destroyed by snow and dryness. 



Most bacteria require organic matter as a source of food, a 

 certain degree of moisture, and a condition of aeration. The 

 factors then influencing their growth are : 



The character of the soil; 



Tilth of the soil; 



Percentage of moisture ; 



The reaction of the soil. 



The pathogenic bacteria in the soil are present only tem- 

 porarily. They do not increase in numbers and tend at all times to 

 disappear, due to the lack of proper environment and the compe- 

 tition of soil bacteria. 



The normal soil inhabitants are those which are particularly 

 active with reference to nitrogen; carbon; sulphur; hydrogen; and 

 iron. 



Reaction of Bacteria to Nitrogen of the Soil. 



Plants absorb nitrogen most readily in the form of a nitrate. 

 To what extent they can absorb nitrogen in the form of amido- 

 compounds we do not know. Nitrogen compounds are unstable. 

 They are derived from organic sources, excepting such small 

 amounts as may be combined by atmospheric electrical discharges 

 and the larger amounts of ammonia vapor which some bacteria 

 take from the air. 



Four-fifths of the atmosphere is composed of nitrogen, so bac- 

 teria that can use this free nitrogen as it circulates with the air 

 in the porespaces of the soil, have an abundant source. 



There are present in the soil, two classes of bacteria, which, 

 independently of green plants, absorb free nitrogen. They are 

 nonsymbiotic and are unlike the well known leguminous bacteria. 



