Modeling Magnesium Uptake from an Acid Soil: I. Nutrient Relationships at the Soil‐Root Interface

Abstract Annual ryegrass ( Lolium multiflorum Lam.), an important forage crop in the southeastern USA, is often grown on soils with potentially toxic levels of soluble Al. Effects of Al on nutrient uptake are poorly understood. Two ryegrass cultivars were grown on an acid Stough soil (a coarse‐loamy, siliceous, thermic Fragiaquic Paleudult) in a greenhouse. Lime (CaO) at three rates to give soil pH values of 3.8, 4.4, and 5.6, and Mg (as MgCl 2 ) at rates of 0, 12, and 36 mg kg −1 were applied in factorial combination. Magnesium application increased shoot Mg concentration, especially in plants grown on unlimed soil. Net Mg influx increased with increased Mg and lime application but decreased with plant age. Liming and plant uptake each caused significant decreases in soil‐solution Mg concentrations. Calculated concentrations of Mg and Ca were higher and K concentrations lower at the root surface than were corresponding concentrations in the bulk soil solution. Concentrations of monomeric hydroxy‐Al species (Al mon ) at the root surface varied widely. Concentrations of Mg at the root surface were significantly related to net Mg influx only for plants grown in unlimed soil ( r 2 = 0.89, P < 0.005, and r 2 = 0.81, P < 0.014, for 23‐ and 35‐d‐old plants, respectively). For plants grown on limed soil, the ratios of concentrations at the root surface (Mg/Ca, Mg/K, and Mg/Al mon ), rather than Mg concentration alone, were significantly related to net Mg influx, substantiating existence of the complementary‐ion effect.