Low soil available phosphorus (P) severely limits crop production in sub-Saharan Africa. The present study evaluated phosphate rock-enriched composts as locally available low-cost fertilizers for sorghum production. The treatments consisted of sorghum straw, compost (COMP), phosphate rock (BPR), BPR-enriched compost (P-COMP), BPR-rhizosphere soil-enriched compost (P-COMP-SOIL), nitrogen-phosphorus-potassium treatment (NPK, 60–39–25), and control (NK, 60–25). Sorghum straw and compost were applied at 1.34 tons ha−1. N, P, and K in all treatments, excluding the control, were adjusted to 60, 39, and 25 kg ha−1, with urea, BPR, and KCl, respectively. Sorghum vr. kapelga was cultivated and soil samples were collected at the S5, S8, and S9 growth stages. P-COMP-SOIL and NPK yielded better sorghum yields than the other treatments. The rhizosphere soil of P-COMP-SOIL had high abundance of soil bacteria and AMF, and genes involved in P solubilization, such as: acid phosphatase (aphA), phosphonatase (phnX), glucose dehydrogenase (gcd), pyrroloquinoline quinone (pqqE), phosphate-specific transporter (pstS). The superior performance of the P-COMP-SOIL was associated with its higher available P content and microbial abundance. Multivariate analysis also revealed vital contributions of N, carbon, and exchangeable cations to sorghum growth. Soils could be amended with phosphate rock-rhizosphere soil-enriched composts, as an alternative to chemical fertilizers.

Purpose The low availability of phosphorus (P) severely limits crop production in sub-Saharan Africa. We evaluated phosphate rock-enriched composts on soil properties and sorghum growth for use as environment-friendly fertilizers. Methods Treatments were sorghum straw, compost (Comp), Phosphate Rock (BPR), BPR-enriched compost (P-Comp), BPR-soil-enriched compost (P-Comp-Soil), nitrogen-phosphorus-potassium (NPK, 60-90-30), and control without phosphorus and organic material (CT). Sorgum straw and composts were applied at 1.34 tons ha-1. The amounts of nitrogen, phosphorus, and potassium in treatments, except in CT, were adjusted to 60, 90, 30 kg ha-1, with urea, BPR, and KCl, respectively. Sorghum vr. Kapelga was cultivated and soil samples were collected on days 52, 93, and 115 (harvest) for analysis. Results NPK and P-Comp-Soil provided the best sorghum yields. Soil available P was less in these treatments. P-Comp-Soil-amended soils recorded higher populations of bacteria (16S rRNA), acid phosphatase (aphA), phosphonatase (phnX), glucose dehydrogenase (gcd) and its cofactor pyrroloquinoline quinone (pqqE) genes. Phosphate-specific transporter (pstS) and arbuscular mycorrhizal fungi (AMF) abundances were generally higher in P-Comp-Soil soils, especially at the early growth stage. This active microbial activity in the P-Comp-Soil added to its initially higher available P justified a better nutrient uptake and yields comparable to NPK. Multivariate analysis also revealed the contribution of nitrogen, carbon, and exchangeable cations in sorghum growth. Conclusion This study demonstrated that direct phosphate rock application is not effective in sub-Saharan African upland cultivation. Alternative to chemical fertilizers, soils may be amended with phosphate rock-enriched composts, a niche of beneficial microbes improving soil health.