Comparative Reduction of Calcium and Magnesium Composition of Corn Tissue by NH4-N and K Fertilization

Ammonium-N nutrition in nutrient culture has been shown to reduce Ca and Mg uptake to near-deficient levels in many plants. The addition of K to soils also reduces Ca and Mg contents of plants. Since NH₄-N is often present during the seedling growth period of plants a study was set up on soils with high and low exchangeable Ca and Mg levels to determine the comparative effect of NH₄-N and rate of K on Ca and Mg absorption by corn (Zea mays L.) plants. Knowledge of these effects could be of benefit in formulating fertilizer programs in crop production where Ca and Mg content is a factor in forage or crop quality. A greenhouse study was conducted to compare the influence of N form and K levels on the absorption of Ca and Mg by corn plants grown in Princeton sand with 67 kg Mg/ha and Fincastle silt loam with 800 kg Mg/ha. The treatments used were 100 ppm N as NH₄ or NO₃, and O, 50 and 100 ppm K in factorial design. Aerial portions of the plants were analysed for N, P, K, Ca, and Mg. Increasing levels of K in the soils under NO₃ nutrition reduced the concentrations of Ca and Mg in the corn tissue. However, NH₄-N at the zero K rate reduced Ca and Mg concentrations in the corn tissue to less than that at the highest K rate with NO₃-N. K rate did not significantly change Ca or Mg uptake under NH₄ nutrition on either soil. The exchangeable Mg level of the soil directly affected the Mg composition of the tissue with both NO₃-N and NH₄-N nutrition. The ammonium-N source reduced Mg uptake by corn on the soil with exchangeable Mg of 67 kg/ha to a deficient level of less than 0.20% in this tissue, which is the critical Mg level of forage associated with incidence of hypomagnesemia in ruminant animals. Thus NH₄-N must be considered an important factor in its influence on cation concentration of tissue grown on low Mg soils during periods when NH₄-N is the predominant N source.