Maize–Urochloa Intercropping Enhances Soybean Yield Across Phosphorus Rates

Phosphorus (P) management and residue production are important challenges for sustainable crop intensification in highly weathered tropical soils. This study evaluated maize–Urochloa ruziziensis intercropping and P fertilization effects on aboveground biomass, residue nutrient pools, soil chemical attributes, and maize and subsequent soybean performance over three growing seasons (2017/2018–2019/2020). Maize monoculture and maize–Urochloa intercropping, both followed by soybean, were evaluated under three P rates (40, 60, and 80 kg ha−1 of P2O5 applied separately to each crop per season). Intercropping increased the measured aboveground dry biomass by 26–30% and resulted in greater macronutrient pools in the combined plant material without significantly reducing maize grain yield. Differences in nutrient pools between maize harvest and soybean sowing were greatest for K, but independent sampling did not allow nutrient-release rates to be determined. Soybean following intercropping yielded 8.3% more grain, accompanied by a 6.8% greater number of pods per plant and higher leaf P and Mg concentrations, although the underlying mechanisms were not established. Higher P rates were associated with greater Mehlich-1-extractable P concentrations at the final soil sampling but provided no consistent additional grain yield benefit. Intercropping was also associated with lower soil pH and exchangeable base cations in surface layers at the final sampling. These findings support maize–Urochloa intercropping as a means of increasing aboveground biomass and subsequent soybean yield under the evaluated conditions. However, the absence of a zero-P treatment and the previous fertilization history limit conclusions regarding minimum P requirements.