Improvement of the agronomic effectiveness of low-grade phosphate rock by physical, biological and chemical approaches under calcareous soil conditions

Most agricultural soils in Morocco are alkaline with low organic matter and low water availability, which may limit direct application of phosphate rock (PR) and more specifically low-grade PR in agriculture. To enhance the agronomic effectiveness of low-grade PR as to make it more convenient to a wide range of soils and crops, different approaches can be adopted. In this study, physical, biological and chemical treatments of PR were carried out and evaluated via agronomic trials on faba bean (Vicia faba) in terms of growth parameters, nutrients uptake and phosphorus (P) use efficiency under alkaline soil (pH 8.1) conditions. Physical treatments consisted of blending PR with soluble P fertilizer (TSP) at 75:25 and 50:50 ratios. Biological treatments were a mixture of PR, compost and phosphocompost elaborated from PR, sewage sludge and crop residues with the following ratio 20:46:34. Chemical treatment consisted of PR that was acidulated at 30% by phosphoric acid (H3PO4). All treatments except the control were amended with 120 mg P.Kg-1. Plants were also fertilized with nitrogen (N) and potassium (K) at rates equivalent to 40 and 120 kg.ha-1, respectively. After two months of faba bean pot cultivation, results indicated that all PR treatments significantly improved plant growth, root nodulation and nutrients uptake compared to control treatments. The relative agronomic effectiveness of PR treatments was significantly higher with phosphocompost treatment (90%) than both PR partially acidulated (80%) and PR blended with TSP 50:50 (66%). Likewise, P uptake, P apparent recovery efficiency, root nodules number and N uptake were also improved. Our findings revealed that the biological treatment based on phosphocompost yielded better as compared to chemical and physical treatments.