EFFECT OF LIMING ON CALCIUM AND MAGNESIUM CONCENTRATIONS IN HERBAGE

The effects of adding calcitic and dolomitic limestone on the Ca and M g concentration of perennial ryegrass grown in two acid UK soils were investigated in a greenhouse experiment. Additions of calcitic limesto ne and subsequent cropping caused decreases in exchangeable Mg of some soils to values considered to be deficient for many agricultural crop s (<50 mg Mg kg(-1)). However, such decreases in the predicted availab ility of Mg did not lead to Mg-deficient herbage (<2.00 g Mg kg(-1)) o r reduced yields in most cases. In one of the soils, additions of dolo mitic limestone increased soil exchangeable Mg to concentrations adequ ate for plant growth at the higher application rates and resulted in s ignificantly (P < 0.05) higher herbage Mg concentrations (2.05-3.55 g Mg kg(-1)) than were obtained with calcitic limestone (1.63-2.93 g Mg kg(-1)). Thus, the use of dolomitic limestone is more likely to provid e adequate Mg concentrations for grazing animals than is calcitic lime stone. No significant correlations were obtained between either exchan geable Mg or Ca:Mg ratios in the soil and Mg concentrations in the her bage following calcitic limestone treatments. This suggests that plant s 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 a vailability of Mg.