APPLICABILITY OF NUTRIENT-SOLUTION PURIFICATION TO THE STUDY OF TRACE-ELEMENT REQUIREMENTS OF RHIZOBIUM AND AZOTOBACTER ' By ROBERT A. STEINBERG Associate physiologist, Division of Tobacco and Plant Nutrition, Bureau of Plant

Though almost 20 years have elapsed since the introduction by the writer of the method of nutrient-solution purification for studying trace-element requirements of plants, no application of this procedure to studies with bacteria has as yet been made. For reasons readily apparent, it has long seemed desirable to the writer to obtain data affording a direct comparison of the effects of this technique on bacteria and on Aspergillus niger Van Tiegh. (14)-^ A direct comparison involves the use of the same equipment, chemicals, and technique by the same investigator, so far as possible. The organisms selected for this comparison were Bhizobium trifolii Dangeard and Azotobacter chroococcum Beij. The procedures employed necessitated, as usual, the ascertainment of the minimum quantities of the usual ash constituents required for maximum growth, together with the identification and study of the necessary trace elements. Again, it was found essential to devote some attention to the accessory growth substance required by rhizobium in order to limit the extent to which any mineral impurity it might contain would alter or vitiate the results on mineral nutrition. Though these experiments are, in a manner, incomplete, it should be recalled that their primary objective was a study of the applicability of the method of nutrient-solution purification to bacterial studies. It was considered undesirable, therefore, to go more deeply into the study of these organisms than was necessary for a general comparison with aspergillus. To have done so, even for the purpose of obtaining improved results with the trace elements, would have necessitated changes in equipment, to provide adequate aeration for azotobacter, besides considerable additional work on purification of the accessory growth bodies required by rhizobium.