NITRATE AMMONIUM RATIO REQUIRED FOR PH HOMEOSTASIS IN HYDROPONICALLY GROWN SOYBEAN

Plant acid-base homeostasis is achieved when the mmoles of hydroxyl ions produced in the plant equal the mmoles of protons. Reduction of nitrate to ammonia is the major source of hydroxyl ions whereas ammonium uptake-assimilation and the metabolism of neutral sugars to organic acids are the primary sources of protons. Soybean [Glycine max (L.) Merr.] plants were grown hydroponically on medium supplemented with 3.0 mol m-3 nitrogen provided as various combinations of KNO3 and NH4NO3. Plant growth consumed essentially all available nitrogen in each case; however, only in flasks supplemented with approximately 1.8 mmoles of KNO3 plus 0.6 mmole of NH4NO3 was the pH of the medium unchanged. Thus, for every mmole of nitrogen assimilated, approximately 0.6 mmole of dissociable protons must have been produced by the conversion of neutral sugars to carboxylic acids. Also, it was shown that a plant obtaining all of its nitrogen from nitrate must neutralize or excrete approximately 0.5 mmole of hydroxyl ion d-1. Conversely, the plant deriving all of its nitrogen from dinitrogen must excrete or neutralize at least 0.8 mmole of hydrogen ion d-1 whereas the plant deriving all of its nitrogen from ammonium must excrete or neutralize approximately 2.1 mmoles of hydrogen ion d-1. Nevertheless, plants grown on medium supplemented with 2.4 mol m-3 nitrate plus 0.6 mol m-3 ammonium did not achieve a higher growth rate than plants grown on 3.0 mol m-3 nitrate.