Influence of vetch– Brachiaria intercropping and phosphorus fertilisation on soil nutrient dynamics and forage mineral composition of Brachiaria varieties under a Mediterranean climate

A field experiment consisted of a 4 × 4 × 2 factorial layout in a randomised complete block design with four replicates at the Mariendahl and Welgevallen sites, Western Cape province, South Africa. Treatments comprised four phosphorus (P) levels (0, 20, 40 and 60 kg P ha-1), four Brachiaria (syn. Urochloa) varieties and two cropping systems (grass monoculture and vetch intercropping). Soil samples collected at trial completion were analysed for pH, calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), P and ammonium ion (NH+4) contents, while forage samples harvested at elongation and maturity were analysed for macro- and micronutrient composition. At Mariendahl, P fertilisation significantly affected most soil nutrients except Na and K, with positive responses in pH, Ca, Mg and NH+4 at low to moderate P levels (20–40 kg P ha−1), while the highest soil P concentration occurred at 0 kg P ha-1. At Welgevallen, only Ca and Mg responded significantly to P application. Cropping system effects on soil nutrients were not significant at either site. For plant mineral composition, P fertilisation showed limited effects, whereas growth stage significantly influenced most minerals, with higher concentrations at elongation than maturity. Optimal P rates varied by site and nutrient, indicating that moderate P application can enhance soil fertility and forage quality in Brachiaria grasses.