[Cir. 120—A.] 
TESTING CULTURES OF NODULE-FORMING BACTERIA? 
By Ikari EF. KELLERMAN, Physiologist in Charge of Soil-Bacteriology and Plant- 
Nutrition Investigations, 
INTRODUCTION. 
The importance of leguminous crops 1n maintaining soil fertility 
or in rejuvenating overcropped and worn-out fields has long been 
recognized by practical farmers. Critical investigations carried 
on both in the laboratory and in the field have shown that this 
method of improving soil conditions is dependent chiefly upon the 
simultaneous development of a leguminous crop and the variety of 
bacteria which can produce root nodules and fix atmospheric nitrogen 
in suitable form for plant food. 
Although these nodule-forming bacteria are widely distributed in 
nature, there are regions where they are lacking, and they should be 
artificially introduced into the soil in these regions. This may be ac- 
complished either by transferring from 200 to 500 pounds of soil per 
acre from a field where the leguminous crop shows abundant root 
nodules to the new field where the same legume is to be planted, or by 
the application of artificially prepared cultures of bacteria to the seed 
before planting or directly to the field itself. Though inoculation 
by the pure-culture method has proved less universally successful 
than the soil-transfer method, the artificial cultures possess certain 
important advantages, especially the greater ease of their trans- 
portation and application, as well as their freedom from danger of 
intreducing weeds or plant diseases.? 
VARIATION IN INOCULATING POWER. 
A plausible explanation for the occasiona: failure of cultures of 
Bacillus radicicola to properly inoculate a crop is that the bacteria, 
though able to grow vigorously in the culture medium, have actually 
deteriorated in the essential quality of being able to infect the 
leguminous roots and to produce nodules. Testing the cultures at 
frequent intervals on potted plants in greenhouses or on small plats 
1 Issued Apr. 5, 1913. 
Messrs. F. L. Goll and L. T. Leonard, scientific assistants, have assisted in working out 
_ the details of construction of the testing apparatus and its utilization in the laboratory. ~ 
2 These facts are discussed in more detail in previous publications of the Bureau of 
Plant Industry. 
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[Cir. 120] 
