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 0, 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.