Abstract
The effects of adding calcitic and dolomitic limestone on the Ca and Mg concentration of perennial ryegrass grown in two acid UK soils were investigated in a greenhouse experiment. Additions of calcitic limestone and subsequent cropping caused decreases in exchangeable Mg of some soils to values considered to be deficient for many agricultural crops (<50 mg Mg kgβ^1^). However, such decreases in the predicted availability of Mg did not lead to Mgβdeficient herbage (<2.00 g Mg kgβ^1^) or reduced yields in most cases. In one of the soils, additions of dolomitic limestone increased soil exchangeable Mg to concentrations adequate for plant growth at the higher application rates and resulted in significantly (P<0.05) higher herbage Mg concentrations (2.05β3.55 g Mg kgβ^1^) than were obtained with calcitic limestone (1.63β2.93 g Mg kgβ^1^). Thus, the use of dolomitic limestone is more likely to provide adequate Mg concentrations for grazing animals than is calcitic limestone. No significant correlations were obtained between either exchangeable Mg or Ca : Mg ratios in the soil and Mg concentrations in the herbage following calcitic limestone treatments. This suggests that plants may be able to take up soil Mg which is not readily exchangeable and, therefore, soil exchangeable Mg may be a poor measure of the plant availability of Mg.