Assessing Starch-Based Phosphorus Fertilizers in Calcareous Soil-Wheat System: Rhizosphere Inorganic P Fractionation and Transformation Kinetics

Efficient phosphorus (P) management in calcareous soils remains challenging due to rapid fixation into Ca-dominated, poorly available forms. Building on a previous bulk-soil study, this greenhouse experiment evaluated inorganic P (Pi) fractionation and transformation kinetics in the rhizosphere following application of starch-formulated organo-mineral fertilizers. Calcareous sandy-loam soil (0–20 cm) was amended with inorganic P (IP), starch-mixed P (SIP), or starch-bound organomineral P (SP) at 50 and 200 mg P kg−1, and wheat (Triticum aestivum) was grown for 16 weeks. Sequential P fractionation showed significant effects of treatment, time, and their interaction on all Pi pools (p < .001). The SP treatments produced lower initial Ca2-P but maintained it for longer, with half-lives of 165–578 days compared to 133–257 days for IP and 57–192 days for SIP. Ca8-P increased over time, particularly in IP200 (207.6 mg kg−1 at week 16), whereas SP reduced accumulation of stable Ca10-P, indicating lower P fixation. Fe- and Al-associated Pi increased in all treatments, but SP showed consistently lower Fe-P accumulation than IP and SIP. Biomass responses reflected these dynamics: SP200 produced the highest biomass at week 12 (0.277 g plant−1), while IP200 was highest at week 16 (0.719 g plant−1). Overall, SP prolonged labile P persistence, reduced conversion to non-labile forms, and enhanced early-season growth. These results demonstrate that starch-bound organo-mineral fertilizers improve P-use efficiency in calcareous rhizosphere systems by synchronizing P release with plant demand, which benefits managing P fertilization in the calcareous soils.