Coal Humic Acid and SP-36 Effects on Phosphorus Availability and Uptake by Pakcoy in Ultisol

Abstract

Ultisol is a type of soil that has limited availability of phosphorus. The low availability of phosphorus is due to its strong fixation by aluminum and iron. This study aims to test the effects of coal humic acid and SP-36 fertilizer on increasing phosphorus availability and its uptake by pakcoy plants. The study was conducted as a greenhouse pot experiment using a factorial completely randomized design with treatments for humic acid and SP-36 fertilizer doses.  The coal humic acid dose treatments included H0 (0 g/plant), H1 (2 g/plant), and the SP-36 fertilizer dose treatments included P0 (0 kg/ha), P1 (100 kg/ha), and P2 (200 kg/ha). The treatment combinations are arranged in three replicates, and each treatment consists of three pots. The soil parameters analyzed included pH H₂O, available P, exchangeable Al, available Fe, and phosphorus uptake by pakcoy plants. The results showed that the application of coal humic acid significantly increased P availability and decreased Fe availability in the soil. The application of SP-36 fertilizer significantly increased pakcoy plants' phosphorus uptake. There is a significant interaction between the effects of coal humic acid and SP-36 fertilizer on a reduction of Al-exchangeable in Ultisol soil. In conclusion, the combined use of coal humic acid and SP-36 fertilizer offers a practical strategy for improving phosphorus management in marginal Ultisol soils.