RESPONSES OF POTATOES TO SOLUTION PH LEVELS WITH DIFFERENT FORMS OF NITROGEN

Four nutrient culture experiments were conducted to determine the responses of potatoes (Solanum tuberosum L.) to varied solution pH levels with nitrogen supplied as NO3-, NH4+, and mixed NO3-/NH4+ (1/1) at the same total N concentration of 4 mM. In the first and second experiments, pH levels were maintained at 3.5, 4, 5, 6, 7, and 7.5 with separate NO3- and NH4+. In the third experiment, pH levels were provided at 4, 4.5, 5, 6, 6.5, 7 with mixed NO3-/NH4+. In the fourth experiment, pH levels of 5 and 6 were maintained in combination with NO3-, NH4+, and mixed NO3-/NH4+. Plants were grown in trays filled with quartz gravel and fed with non-recirculating nutrient solutions at the flow rate of 4 mi min(-1). The giants were grown for 28 days after transplanting of tissue culture plantlets. With mixed nitrogen, plant growth as total dry weight, leaf area and tuber number per plant was essentially similar at pH 4.5 to 7, and decreased only at pH 4. With either nitrogen form alone, however, plant growth peaked at a particular pH level, pH 5 with NO3- and pH 6 with NH4+, and decreased at other pH levels. The shoot concentrations of N, K, Ca, Mg, and P changed little from pH 4 to 7 with mixed nitrogen whereas they varied significantly between pH 4 and 7 with either NO3- or NH4+. The interacting effects of pH levels and nitrogen forms on plant growth and tissue mineral composition were essentially duplicated when pH 5 and 6 were maintained with NO3-, NH4+, and mixed NO3-/NH4+ in a combined experiment. The data in this paper indicate that mixed nitrogen forms can broaden the useful pH range and enhance growth for potato plants as compared to a single nitrogen form.