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 range
of Mg fertilisers applied to a pumice soil (Typic Udivitrand).
The rate of fertiliser dissolution was little influenced by whether the
fertiliser was applied directly on to the soil surface (litter removed) or on
to the litter layer in a Pinus radiata plantation.
Twenty-seven months since fertiliser application the mean (with and without
litter) percentage of Mg dissolved was in the sequence: Epsom salts >
calcined magnesite 1–2 mm > granmag (a partially acidulated and
granulated calmag product) > calcined magnesite 2–4 mm > forestry
grade dolomite. The specific dissolution rate constants
(mg/cm2 .day of fertiliser) for the slowly soluble
Mg fertilisers calculated using an elemental sulfur oxidation cubic model were
587 for calcined 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 elemental 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 fertilisers for
P. radiata.
