Availability of magnesium to ryegrass from soils during intensive cropping in the glasshouse

Abstract

SUMMARYTen Scottish soils were cropped in a glasshouse with three sowings of perennial ryegrass (Lolium perenne, cv. Dutch Barenza) without added magnesium. After 25 cuts (185 weeks) growth was poor in some soils, but soil Mg was not limiting growth in others.Uptake of Mg and the Mg concentration in the ryegrass correlated well over the entire cropping period with the equilibrium Mg-concentration ratio Mg/(Ca, Mg) = CR0, determined from the Mg quantity/intensity isotherms measured before cropping in 0·01 m chloride solutions. A CR0value of 0·16 was required in the early stages of cropping to give 0·2 % Mg in grass dry matter.Seven soils released 12–37 mg non-exchangeable Mg/kg, between one-fifth and one-third of the total Mg uptake. These reserves probably came from vermiculite minerals, but the release was too slow to maintain Mg concentrations of 0·2 % in the ryegrass.