Influence of Concentrations of K and Mg in Nutrient Solutions on Sorghum

Several interactive effects of K and Mg are present both in plants and soils. Very limited information is available on the interaction of these two elements in sorghum [Sorghum bicolar (L.) Moench] plants. Thus a greenhouse experiment was conducted in sand culture to determine the main and interactive effects of K and Mg in the medium on growth and composition of sorghum plants. Nutrient treatments included seven concentrations each of K as KCI, and Mg as MgCI₂. Seventeen of the possible 49 treatment combinations were used in a central composite factorial design with two replications. A sorghum hybrid (‘RS 671’), its male (‘Tx 415’), and female (‘Redlan’) lines were used. Aboveground parts of plants were harvested at 25, 50, and 75 days after emergence. The three sorghums showed similar effects for most treatments. Dry matter production was not affected by concentrations of K and Mg until 50 and 75 days after emergence, respectively. At final harvest, maximum top dry matter production was obtained at the optimum rates of 195 and 38 ppm of K and Mg, respectively. Effects of K were not affected by concentrations of Mg and vice versa. As long as the absolute amounts of K and Mg in the growth medium were adequate to meet the demands of the plants, the K/Mg ratio in the growth medium varied widely without causing any adverse effect on dry matter production of the plants. Plant nutrient concentrations were determined by rate of growth and consequent nutrient dilution. However, high levels of K in the growth medium reduced plant Mg concentrations. Also increased rates of applied Mg increased the concentrations of P in the plants. Increases of K/Mg ratio in the growth medium were associated with reductions in Mg, Ca, and P concentrations in the plant, but the sum (K + Mg + Ca) remained constant. These results show that K is probably more critical than Mg in the nutrition of sorghum, but low levels of both created growth problems particularly as the growth period of the plant exceeded 50 days. Two parent lines and a hybrid showed limited differences in K and Mg nutrition. The K/Mg ratio in the growth medium was not useful in predicting amount of growth.