Global mines of phosphates, which are a main source for the manufacturing of phosphorus (P)-fertilizers, are running out. To solve this problem, the recovery of P by struvite (NH4MgPO4 center dot 6H2O) precipitation as a slow-release P-fertilizer (SRPF) has been a growing interest of recent research. This study aimed to assess the effect of struvite/biochar composites as SRPFs on the growth yield, nutrient uptake responses of maize in two soils (with different amounts of calcium carbonate equivalent), and calculation of P use efficiency indices. The greenhouse experiment was designed with six P fertilizer treatments, i.e. triple super-phosphate (TSP), four types of struvite/biochar composites (S/G0, S/G25, S/G50, and S/G75), and control (no P fertilizer applied) with three replication. Results showed that struvite/biochar composite with high amounts of biochar i.e. S/G75 is the best treatment for improving biomass's of maize (63 and 39%), plant height (25 and 16%), SPAD value (31 and 26%) compared to control and TSP in soil-1 with the low amount of calcium carbonate equivalent, respectively. Applying S/G75, S/G50, S/G25, and S/G0 composites, and TSP concentration of P increased by 2, 1.85, 1.76, 1.73, and 1.34 times over control in soils, respectively. Using of composites significantly influenced P uptake, and recovery efficiency of P by plants, while the S/G75 composite was showed superior to other P-fertilizers. Overall, the struvite/biochar composites showed great potential as efficient SRPF for enhanced maize growth, and the presence of biochar in struvite composite with high amount (S/G75) increased the efficiency of fertilization in calcareous soil.
