RESPONSE OF POTATOES TO NITROGEN CONCENTRATIONS DIFFER WITH NITROGEN FORMS

Two separate experiments were conducted to investigate plant growth an d mineral composition of potatoes (Solanum tuberosum L.) at varied sol ution concentrations of nitrate (NO3-) and ammonium (NH4+). Each exper iment evaluated five nitrogen (N) concentrations of 0.5, 2, 4, 8, and 12 mM, which were maintained with a non-recirculating nutrient film sy stem in controlled environment. Plants were harvested on day 42 with N O3- and day 35 with NH4+ after transplanting of tissue culture plantle ts, and growth measurements were taken as leaf area, tuber number, and dry weights of different parts. With NO3-, plant growth was greatest and similar at 2, 4, and 8 mM of N whereas with NH4+ plant growth was best only at 2 and 4 mM of N. At 12 mM of N, plants exhibited intervei nal ammonium toxicity with NH4+ nutrition, but healthy growth appearan ce with NO3- nutrition. With either N form, total N concentrations in tissues tended to increase with increased N concentrations, and tissue phosphorus (P) concentrations were reduced at 0.5 and 2 mM of N. Tiss ue concentrations of calcium (Ca), magnesium (Mg), and sulfur (S) chan ged only slightly at particular N concentrations, yet changed substant ially with different N forms. The data indicate that the optimal range s of N concentrations in both solution and tissues are wider and highe r with NO3- than with NH4+ nutrition, and thus a careful control of NH 4+ concentrations is necessary to minimize possible ammonium toxicity to potato plants.