Coapplication of humic acid and gypsum affects soil chemical properties, rice yield, and phosphorus use efficiency in acidic paddy soils

This study investigated the effects of humic acid (HA), flue gas desulfurization gypsum (FG), and their combined application on soil chemical properties, rice yield, phosphorus use efficiency (PUE), and phosphorus agronomic efficiency (PAE) during rice production in acidic paddy soil, which has not been previously studied. A greenhouse experiment was conducted in a completely randomized block design with four treatments, including a control (no soil amendments), HA (975 kg ha^-1), FG (636 kg ha^-1), and HA plus FG. HA application increased the soil pH; cation exchange capacity (CEC); total concentrations of P, Ca, and S; and exchangeable Ca and S. Additionally, HA application maximized the rice yield, total P uptake, PUE, and PAE. FG application alone increased the soil total Ca, whereas coapplication of HA and FG increased the total organic carbon (TOC), CEC, available P, exchangeable Ca, exchangeable S, and total S. However, FG application, both alone and combined with HA, failed to increase the rice yield, PUE, and PAE owing to lower total P uptake by the plants. Therefore, HA application alone is a sustainable soil management practice for acidic paddy soils, whereas FG application must be carefully considered, as it produces disappointing agronomic effects.