Modeling magnesium uptake from an acid soil. III. Determination of root magnesium concentration

Research on the nutrient interactions in the rhizosphere of soil-grown plants would benefit from a reliable estimate of root-nutrient concentrations. Ryegrass plants (Lolium multiflorum Lam., 'Wilo') were grown for 35 d in a greenhouse on an acid Stough soil (coarse-loamy, siliceous, thermic Fragiaquic Paleudult) amended with three rates of CaO and two rates of MgCl₇. Careful washing of roots in water was followed by a digestion procedure involving HF. Root Mg concentration was corrected for soil contamination based on Ti content in the root tissue. Titanium-corrected root Mg concentrations were lower than root Mg concentrations calculated by multiplying shoot Mg concentrations of soil-grown plants by the ratio of root to shoot Mg concentrations of solution-grown plants. Regression analysis showed that the Barber-Cushman model overpredicted total Mg uptake when root Mg content was calculated either from Ti-corrected root Mg concentrations (2.88 and 2.39 times for 0 and 36 mg Mg kg^-1, respectively) or from root/shoot Mg concentration ratios in solution-grown plants (1.98 and 1.66 times for 0 and 36 mg Mg kg^-1, respectively). Partitioning of Mg between roots and shoots of rye-grass may be quite different in soil- and solution-grown plants.