Exchange Fertilizer (Phosphate Rock plus Ammonium‐Zeolite) Effects on Sorghum‐Sudangrass

Fertilizer systems that use an exchange medium in combination with slightly soluble phosphate rock could provide a slow, sustained release of P and avoid the potential problems (e.g., fixation) associated with a large, single application of an element. Consequently, the effectiveness of one NH 4 ‐exchanged zeolite (clinoptilolite) in increasing phosphate‐rock solubility in two soils (Red Feather loamy sand, a Lithic Cryoboralf and Weld loam, an Aridic Paleustoll) was evaluated by measuring increases in dry matter, nutrient content, and nutrient uptake by sorghum‐sudangrass [ Sorghum bicolor (L.) Moench‐ S. sudanese (Piper) Stapf, ‘NB280S’]. Two phosphate‐rock applicationr ates of 170 and 340 mg P kg −1 soil were used in factorial combination with zeolite/phosphate‐rock ratios of 0, 1.5, 3, 4.5, 6, and 7.5:1. The largest dry‐matter yields through four plant cuttings from the Weld soil generally were found with the 340 mg P kg −1 rate and with increasing zeolite/phosphate‐rock ratio (linear response). Increasing zeolite/phosphate‐rock ratio in this soil generally increased plant P concentrations and uptake. The exchanger fertilizer system increased P availability in the Weld soil by producing lower soil pH (nitrification of NH 4 released by the zeolite) than the 0:1 zeolite/phosphate‐rock ratio control and by the adsorption of Ca in the zeolite. The Red Feather soil, which was deficient in K, did not provide definite yield responses because the zeolite sequestered K, thereby reducing plant K concentrations and uptake even though the zeolite increased total P uptake. The NH 4 ‐exchanged zeolite/phosphate rock combination used in the present study seems to be a promising P‐fertilizer system in nonalkaline soils if K or other plant‐essential cations are not limiting.