Assessing the Role of Phosphorus, Starter Nitrogen and Boron in Subterranean Clover and Annual Ryegrass Grown in Acidic Soils Under Mediterranean Conditions in Portugal

In Mediterranean pasture systems, maintaining legume persistence and productivity is often constrained by phosphorus (P) deficiency in acidic soils. Although small doses of starter nitrogen (N) are commonly used in pulse crops to support early growth before N fixation begins, this practice has not been widely tested in legume–grass mixtures. In addition, boron (B), a micronutrient particularly important for dicot species, may also influence legume performance under these conditions. Thus, this study investigated the effects of P application on dry matter yield (DMY) and nutrient uptake in subterranean clover ( Trifolium subterraneum L., cv. Denmark), annual ryegrass ( Lolium multiflorum Lam., cv. Falladino) and their mixture, using a pot experiment with acidic soils from two distinct locations in Portugal. The study also evaluated the impact of a small N starter dose and B application. Although Soil 2 had higher acidity (pH 5.0) than Soil 1 (pH 5.4), it supported greater biomass production, likely due to its higher organic carbon (C) content (50.8 g kg −1 ), which may have buffered soil acidity. P application significantly increased P uptake, DMY and N accumulation in legumes, suggesting a critical role in nodulation and/or nodule function. The N starter had no significant effect, likely because sufficient N was released from soil organic matter prior to the onset of biological N fixation. While the B application increased tissue B concentrations, it did not affect DMY, possibly because B was adequately available from organic matter. These findings highlight the importance of P in legume management in pasture systems and suggest that, in soils with sufficient organic matter, the application of starter N and B may not be necessary.