Mineral composition and yield of tall fescue as affected by ammonium and nitrate nitrogen fertilization

A greenhouse experiment was conducted with tall fescue (Festuca arunidacea Schreb.) to evaluate the effects of NO₃ and NH₄ fertilization on mineral composition, yield and components related to grass tetany potential. A nitrification inhibitor [2-chloro-6-(trichloromethyl)- pyridine] was used to slow the conversion of NH₄ ions to the NO₃ form. In the experiment a factorial combination of three levels of N; 34, 67, and 134 kg N/ha; three levels of K; 56, 112, 224 kg K/ha; two levels of Mg; 0 and 112 kg Mg/ha; at two Soil pH levels; pH 5.2 and 6.2; was evaluated with and without a nitrification inhibitor. Two cuttings of plant material were analyzed for Mg, Ca, K, total N, and NO₃-N. The use of a nitrification inhibitor such that primary N fertilization was from the NH₄ form was found to lower Mg, Ca, and NO₃-N concentration of first cutting forage. However, N in the NH₄ form increased Ca and K in the second cutting. The ratio of K/(Ca + Mg) was not affected at the chosen probability level by the NH₄ form of N fertilization. Total Mg uptake by forage was reduced when the NH₄ ion was the primary form of available N. Yield was not affected by NH₄ fertilization in either of the two cuttings, but was lowered when combined yield data were statistically analyzed. First cutting forage grown at the 224 kg rate of added K/ha, at the 0 kg/ha rate of added Mg, with NH₄ as the primary form of N, contained Mg concentrations below 0.20%. Also in the first cutting, forage contained less than 0.20% Mg when grown at pH 5.2, at the 0 kg/ha rate of Mg, and with the 67 and 134 kg/ha rates of N fertilization. Addition of the higher levels of K used in the experiment increased K concentration and the K/(Ca + Mg) ratio of forage while lowering Mg concentrations in both cuttings. However, K concentrations were not high enough to limit Mg availability to animals as some authors have suggested. The ratio of K/(Ca + Mg) did not approach or exceed 2.2 in any cutting. The addition of Mg at the 112 kg Mg/ha rate was found to increase Mg concentration but did not significantly affect K/(Ca + Mg) ratios. Fertilization with the higher levels of N increased total N concentration of both cuttings, with total N being higher in the first cutting than the second. In neither cutting was total N concentration great enough to decrease Mg availability to animals consuming the forage. First cutting results revealed higher NO₃-N levels in forage from pots receiving NO₃-N as the primary form of N fertilization when compared to the NH₄ form. Levels of NO₃-N did approach or exceed the 0.35% level some have suggested as toxic to animals consuming forages in a few treatments. An increase in soil pH from 5.2 to 6.2 significantly lowered the K/(Ca + Mg) ratio in the first cutting and increased forage Mg concentration in both cuttings.