276 
The Ohio Naturalist. 
[Vol. XIV, No. 5, 
air. This explains the reduction of nitrate in water-logged surface 
soil and the tendency to denitrification in heavy compact soils as 
compared with the more open sandy loams. 
Drainage, liming, and thorough tillage, greatly lessen the 
danger from denitrification by improving the circulation of air in 
the soil. 
The Symbiotic Root Tubercle Nitrogen 
Fixing Bacteria. 
Much has been written regarding the relation of legumes to 
the tubercle forming bacteria that grow upon their roots. But 
because of their importance to a permanent system of agriculture, 
it is well to call attention to them in this short review of the soil 
bacteria. 
These root tubercle bacteria (Bacillus radiciola) are parasites. 
They require carbohydrate material and are unable to manufacture 
it from the elements of carbon, hydrogen, and oxygen; conse¬ 
quently they derive it by growing upon the roots of leguminous 
plants. The agricultural plants included under the term legumes 
are: alfalfa or lucerne; clover; melilotus or sweet clover; peas; 
beans; and vetches. 
The bacteria can enter the roots of legumes when the latter 
are in a weakened condition, such a state resulting when the 
nitrogen supply of the soil is deficient. In a weakened state, 
they have slight power of resistence, and the nodule bacteria, 
seeking carbohydrate material, gain enterance to the root through 
the tip of the root hairs. The bacteria may possibly secrete an 
enzyme which dissolves the substance of the tip of the root. 
After they have entered the root, the bacteria cause excessive 
reproduction of the plant tissue about it, which results in the 
formation of the tubercles. The bacteria are not found in all 
parts of the plants, but are confined to the nodules and rootlets. 
The presence of bacteria upon seeds results from the contam¬ 
ination of the seed with soil. 
The symbiotic bacteria, developing in the nodules, absorb 
nitrogen from the air circulating in the porespaces of the soil. 
The nitrogen absorbed by these bacteria becomes immediately 
available to the plant. Soil fertility, however, is only increased 
when these plants become dead and have passed through the 
cycle of decomposition (humus, ammonia, nitrous salts, nitric 
salts). 
Although past attempts to develop these bacteria to grow upon 
non leguminous plants have been unsuccessful, it may yet be 
possible to do so. 
