THE ASSIMILATION BY PLANTS OF VARIOUS FORMS OF NITROGEN

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SOME YEARS AGO (1926) the author became interested in the possible effect of hydrogen-ion concentration on the assimilation by plants of nitrogen as ammonia or as nitrate. In an experiment 2 designed to attack this problem, mineral nutrient solutions containing dextrose were prepared in which the nitrogen was furnished as ammonium nitrate, ammonium sulfate, or sodium nitrate, and the hydrogen-ion concentration of the solutions was varied by intervals of 0.2 or 0.3 pH from pH 4.0 to pH 7.6. The organism used, Rhizopus nigricans, grew well at all reactions in the solutions which contained ammonium sulfate or ammonium nitrate (somewhat better in the more alkaline solutions) but not at all in those which contained sodium nitrate. This confirmed earlier reports (Bach, 1927; Ritter, 1909) of the inability of Rhizopus nigricans to assimilate nitrogen as nitrate (at least under ordinary laboratory conditions) and encouraged the writer to plan an investigation of the ability of organisms to use various forms of nitrogen. Circumstances made it impossible to carry out the investigation as planned. Nevertheless, the preliminary work may be of sufficient interest to encourage further investigations in this field. A survey of the literature (space precludes summarizing all the available literature) suggested to the author that organisms may be arranged in four groups 3 according to their ability to assimilate various forms of nitrogen. GROIJP I. NITROGEN-FIXING ORGANISMS. These organisms are capable of assimilating gaseous nitrogen and also nitrates, ammonium salts, and certain organic nitrogenous compounds. Attention should be called to the assumption that everv organism capable of fixing gaseous nitrogen is capable also of assimilating nitrogen in the form of nitrates, ammonium compounds, and 'suitable forms of organic nitrogen. Though the evidence available supports it, this assumption needs further confirmation since considerably more attention has been devoted to the relation to gaseous nitrogen of the organisms generally accepted as nitrogen fixers than to their ability to assimilate combined nitrogen. However, it is generally agreed (Waksman, 1927; Lohnis, 1926; Stephenson, 1930) that Azotobacter can assimilate nitrogen as nitrates, as ammonium salts. and as orgLanic nitrogen (for