INFLUENCE OF POTASSIUM AND MAGNESIUM CONCENTRATIONS IN NUTRIENT CULTURE ON GROWTH AND COMPOSITION OF SORGHUM (SORGHUM BICOLOR (L.) MOENCH) PLANTS

Greenhouse experiments were conducted to evaluate the main effects and interactions between applied K and Mg on dry matter yield and nutrient contents of sorghum {Sorghum bicolor (L.) Moench} plants harvested at three different growth stages, using sand culture. Effects of variations in K/Mg ratio in the growth medium on yield were also evaluated. Nutrient treatments consisted of 7 levels each of K as KCl from 0-300 ppm in 50 ppm increments and Mg applied as MgCl(,2) at 0-60 ppm in 10 ppm increments. A sorghum hybrid, \u27RS 671\u27 and its male and female lines, \u27Tx 415\u27 and \u27Redlan,\u27 respectively, were used. Whole plants were harvested at 25, 50 and 75 days after seedling emergence, and dried samples of top and root tissues were subjected to chemical analyses. Variables measured included top and root dry weights at each harvest. Top tissue concentrations of N, P, K, Mg, Ca, Zn, Fe and Mn in the 50- and 75-day-old plants were also measured. There was no yield response to applied K and Mg until 50 and 75 days after seedling emergence, respectively. At final harvest, maximum top dry matter yield occurred when K and Mg were supplied at a concentration of 195 and 38 ppm, respectively. Significant interaction effects were not observed. Applied K increased K but decreased top tissue N, P, Ca, Zn and Mn concentrations. Since top dry matter yield increased with increased K rates, both N and P uptake increased with low but decreased with high K levels. Magnesium concentration and uptake decreased at all rates of K, indicating a strong antagonistic effect of K on Mg absorption. Potassium application had no effect on Zn, Fe and Mn uptake. Applied K was negatively correlated with tissue N, P, Mg, Ca, Zn and Mn concentration but positively correlated with tissue K. Plant K concentration was negatively correlated with tissue N, Mg, Ca and Zn but had no relationship with Mn concentration. Applied Mg decreased N in RS 671 at both harvests but had no effect on N content of the parent lines. At 75 days, applied Mg had a slight depressive effect on K and a synergistic effect on P absorption. Applied Mg increased tissue Mg concentration and uptake, decreased Ca concentration but had no apparent effect on Ca uptake. Magnesium had a synergistic effect on Zn and an antagonistic effect on Mn absorption. A positive correlation was observed between applied Mg and each of tissue P, Mg and Zn concentration but negatively correlated with tissue Ca and Mn. Plant Mg concentration was positively correlated with each of tissue N, P, Ca and Zn concentration but negatively correlated with tissue K. No relationship was observed between plant Mg and each of plant Fe and Mn. Plant N concentration was positively correlated with tissue P, Mg, Ca, Zn, Mn and Fe but negatively correlated with K. Plant P was positively correlated with tissue Mg, Zn and Ca but had no relationship with tissue K, Fe and Mn. Plant Ca was positively correlated with plant Zn, Fe and Mn concentration. Significant negative K-mg interactions were observed for Mg and Mn concentrations but positive interactions were observed for Ca concentration and forage cation ratio. Plant N, P, Mg, Ca, Zn, Fe and Mn concentrations were each negatively correlated with yield. However, plant N concentration affected yield of the 50-day-old plants more than did the other nutrients. At 75 days, however, plant K and Ca contents affected yield more than did the other nutrients. The wide variations in K/Mg ratio of the growth medium had no adverse effect on dry matter yield of sorghum plants