Farmyard manure increased phosphate fertilizer use efficiency beyond soil pH effects in multi-year field experiments in Madagascar

Context and objectives The combined application of farmyard manure (FYM) and mineral phosphorus (P) fertilizers is widely promoted within the integrated soil fertility management framework to enhance the P use efficiency (PUE) in highly P-deficient soils. This study analyzed the magnitude and mechanisms of FYM × P interactions across 18 field × year combinations using an ensemble of field trials conducted between 2007 and 2025 in the Malagasy highlands. In parallel, a liming field trial was conducted to test earlier hypotheses that FYM × P interactions are primarily related to the alleviation of aluminium (Al) toxicity. Methods The field trials had been established at four locations using a common design with graded TSP applications in the presence or absence of FYM, combined with non-limiting applications of nitrogen (N) and potassium (K). Treatments were maintained on the same plots over multiple years. The different trials included rice, maize, soybean and Bambara groundnut. Results Crop yield largely responded to P applications, with a mean PUE (grain P uptake above control per unit of P applied) of only 4% in the absence of FYM. On average, FYM addition increased the total PUE (including FYM-derived P) by a factor of 1.6, indicating an overall synergistic effect. This effect was not related to watering regime (rainfed vs irrigated) or initial soil properties such as soil pH, organic carbon content, P status, and clay content. The annual PUE based on cumulative P inputs increased 1.6-fold in later application years compared with the first year, independently of FYM application. This indicates that the PUE is generally underestimated during the establishment phase. The new liming trial showed that soil pH increases may explain the FYM × P synergism in maize, but not in upland rice, which is more Al-tolerant than maize. Foliar composition indicated that FYM alleviated rice zinc deficiency in one field. Conclusions These findings demonstrate that combining mineral P fertilizers with FYM can provide a clear agronomic advantage across a wide range of field conditions in the Malagasy highlands. The benefits of FYM on the PUE are likely related to a combination of mechanisms, including the supply of nutrients beyond N, P, and K and the alleviation of Al toxicity in acid-sensitive crops. Other mechanisms such as reduced P sorption may also play a role, although not directly assessed in this study.