Response of Millet and Sorghum ✕ Sudangrass Crosses to Magnesium Fertilization 1

In the southeastern United States, millet ( Pennisetum glacum ) often produces more forage than sorghum ✕ sudangrass crosses [ Sorghum bicolor (L.) Moench] on sandy soils, while the reverse is often true on clay soils. Based on the hypothesis that difference in production may be related to different Mg requirements of the two crops, a study was made to (a) determine the critical levels of Mg for growth of millet and sorghum ✕ sundangrass crosses in the greenhouse, and (b) relate these results to field production. One greenhouse study was grown in sand using Hoagland's solution to develop different plant Mg levels. A second greenhouse study was grown on a Magnolia sandy loam with Mg rates ranging from 0 to 200 ppm. Two field studies were conducted with Mg rates ranging from 0 to 67.2 kg/ha, one on a Magnolia sandy loam and the other on a Lynchburg sand. In the greenhouse sand‐solution study, maximum yields occurred at 0.13 to 0.14% mg (whole plant samples) with both millet and sorghum ✕ sudangrass, whereas maximum yields in the greenhouse soil study occurred at 0.45% Mg (whole plant samples) in both crops. Maximum yields were higher in the sand‐solution study and forage contained lower Mg and higher K levels, compared to the soil study in which maximum yields were lower and were accompanied by higher Mg and lower K levels. Significant yield increases were not obtained due to added Mg in either field study, but Mg content in the plants increased with added Mg on the Magnolia soil. In the field, higher yields (the Magnolia soil) were accompanied by lower Mg and higher K levels in the plant tissue, whereas lower yields (the Lynchburg soil) were accompanied by higher Mg and lower K in the plant. Millet and sorghum ✕ sudangrass crosses used in these studies did not differ in their Mg requirements. Mg required for maximum growth varies according to environmental conditions affecting growth and K availability to the plants.