March, 1914.] 
Soil Bacteria. 
273- 
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. 
