Magnesium in New Zealand soils

Eleven soils, with varying levels of exchangeable Mg and of Mg extractable by boiling Im HCl (“reserve” Mg), were mixed with perlite (expanded rhyolite, used as a diluent and growth medium), placed in pots, and exhaustively cropped with white clover. All major and trace nutrients except Mg were supplied, and the plants were grown until symptoms of acute Mg deficiency had appeared on all but one soil (10–30 months). In four soils with a relatively high content of reserve Mg (= 30 me./100 g) a small release of non‐exchangeable Mg (up to 0.17 me./100 g) was found. Soils with a relatively low content of reserve Mg (< 15 me./100 g) generally showed no measurable release. Release of non‐exchangeable Mg was significantly, although not highly, correlated with reserve Mg, but not with initial exchangeable Mg. However, Mg concentrations in dry matter (DM) (at all harvests), total DM yield, and residual exchangeable Mg were all significantly correlated with initial exchangeable Mg but not with reserve Mg.