POLYHALITE APPLICATION TO SORGHUM-SUDANGRASS AND LEACHING IN SOIL COLUMNS

Acid, infertile soils typically benefit from the addition of K, Ca, Mg, or S fertilizers. Polyhalite, K2MgCa2(SO4)4.2H2O, may provide a slow-release fertilizer source of these nutrients. Sorghum-sudangrass (Sorghum bicolor, L. Moench 'NB280S - S. sudanese (Piper) Stapf.) was grown in Red Feather loamy sand soil (Lithic Cryoboralf) in a greenhouse study to compare responses to polyhalite (< 0.15-mm particle size) to soluble sulfate sources of K, Mg, and Ca. Treatment rates for both fertilizer sources ranged from 0 to 600 mg K kg-1 soil in 100 mg K kg-1 increments. Eight plant harvests were obtained. Finely-ground polyhalite produced larger total dry matter yields and total K uptake than the soluble treatments. Increasing the fertilizer rate produced a positive quadratic dry matter yield (largest at the 400 mg K kg-1 soil rate) and significant plant K and S uptake responses. Electrical conductivity (EC) of saturated soil-pastes at the end of the study was higher in the soluble fertilizer treatments compared to the polyhalite nutrient source. A column leaching study showed that Ca and K were leached more readily while Mg and SO4-S were leached to a lesser extent in the polyhalite treatments than the soluble-fertilizer treatments. In these studies, finely-ground polyhalite provided adequate K, Ca, Mg, and SO4-S to the plants and performed somewhat like a slow-release fertilizer compared to more soluble fertilizer sources. This mineral should be an effective fertilizer in acid, infertile soils.