Effect of NPK Slow-Release Fertilizer Diameter and Zeolite Particle Size on Phosphorus Uptake and Yield Quality of Shallot (Allium ascalonicum L.) Grown in Inceptisol

Abstract— Shallot (Allium ascalonicum L.) is a high-value horticultural crop with increasing demand due to population growth. However, the availability of productive agricultural land to expand shallot cultivation is declining. Inceptisols are widely distributed and have potential for cultivation, but their physical and chemical limitations often restrict crop productivity. The application of slow-release NPK fertilizer (NPK-SR) combined with natural zeolite is expected to improve nutrient availability and enhance soil quality in Inceptisol. This study aimed to determine the optimal diameter of NPK-SR fertilizer and zeolite particle size, as well as their interaction effects on phosphorus (P) uptake and yield quality of shallot. The experiment was conducted in pots using a randomized complete block design (RCBD) with two factors: NPK-SR diameter (control, 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm) and zeolite particle size (60 mesh and 100 mesh), each with three replications. Observed variables included P uptake, bulb diameter, bulb volume, bulb firmness, bulb color, bulb aroma, vitamin C content, and bulb water content. The results showed that different diameters of NPK-SR fertilizer did not significantly affect all observed variables. Zeolite particle size significantly affected bulb firmness. A significant interaction between NPK-SR diameter and zeolite particle size was observed for vitamin C content, with the best combination obtained from 5 mm NPK-SR fertilizer and 60 mesh zeolite, producing the highest vitamin C content of 0.96%.