Release dynamics and plant availability of POLY4 fertilizer nutrients in tropical acidic soils

POLY4 is a polyhalite-based mineral fertilizer containing essential nutrients for crop growth and development. Dissolution of POLY4 and its nutrient availability to crops in acidic tropical soils are currently deemed inadequate. The objective of this study was thus to evaluate the fertilizers’ release dynamics of nutrients under controlled conditions, using an open leaching system in the laboratory and in a shade house with Napier grass as the test crop. In the open leaching system, Gondang, Selangor and Tawar soils were packed in soil columns similar to their bulk density in the field. Fertilizers were then added at a rate of 100 kg K2O/ha. Subsequently, the soil columns were leached with distilled water for approximately 30 days, and the nutrients present in the leachate measured. Napier grass was planted in polybags filled with soil and treated at 0 and 100 kg K2O/ha for 280–627 days. The plants were subsequently harvested, and the dry weights recorded, and nutrient uptake calculated. The quantity of K2O released from POLY4 was greater from the POLY4 standard or chipped (POLY4S) versions than from the POLY4 granule (POLY4G) version. The values ranged from 135.2 μg K2O, with a pore volume (PV) of 8.4, for the Tawar soils to 46.3 μg K2O, with a PV of 11.4, for the Gondang soils. Based on nutrient uptake by the Napier grass, for each unit of K2O release, the efficiency of Mg uptake (80.8%) was greater than that of Ca (32.5%). This result is in agreement with a common Mg release pattern as in an open leaching system, suggesting that Mg is an important factor in fertilizer formulation when POLY4 is used for crop production.