Application of Mg fertilisers has been suggested as a means of reducing the
incidence of Mg deficiency of forest trees in New Zealand and Europe. The
objective of this study was to determine the rates of dissolution of a rang
e of Mg fertilisers applied to a pumice soil (Typic Udivitrand).
The rate of fertiliser dissolution was little influenced by whether the fer
tiliser was applied directly on to the soil surface (litter removed) or on
to the litter layer in a Pinus radiata plantation. Twenty-seven months sinc
e fertiliser application the mean (with and without litter) percentage of M
g dissolved was in the sequence: Epsom salts > calcined magnesite 1-2 mm >
granmag (a partially acidulated and granulated calmag product) > calcined m
agnesite 2-4 mm > forestry grade dolomite. The specific dissolution rate co
nstants ( g/cm(2).day of fertiliser) for the slowly soluble Mg fertilisers
calculated using an elemental sulfur oxidation cubic model were 587 for cal
cined magnesite 1-2 mm, 426 for calcined magnesite 2-4 mm, 385 for granmag,
and 18 for forestry grade dolomite. In a laboratory incubation study the e
lemental sulfur oxidation cubic model described the rate of dissolution of
Mg fertilisers within narrow fertiliser particle size ranges. The specific
fertiliser dissolution rate constants, however, increased with decreases in
particle size, suggesting that the rate of dissolution depends on factors
other than surface area when particle sizes varied widely.
Slowly soluble, alkaline Mg fertilisers had a significant liming effect on
the soil. They were more effective in increasing soil exchangeable Mg than
soluble Mg salts over a long-period and therefore, they are better fertilis
ers for P. radiata.