Response of field-grown maize to applied magnesium in acidic soils in north-eastern Australia

Split-plot field experiments, with main plots consisting of various rates of calcitic lime and single rates of dolomite, gypsum, and calcium silicate, were conducted at each of 4 sites to determine the effect of band-applied magnesium (Mg) on maize yield. The sites were acidic with pH values of 4.5, 4.9, 5.0, and 6.1 and exchangeable Mg levels of 0.16, 0.10, 6.0, and 2.0 cmol(+)/kg, respectively. Magnesium significantly ( P < 0.05) increased grain yield at the 2 low-Mg sites, both of which were strongly acidic and responsive to lime application, but the nature of the Mg × lime interaction was different at each of the 2 responsive sites. The absence of a response to Mg at lime rates =1 t/ha at one responsive site was attributed to the presence of small amounts of Mg in the calcitic lime and/or an improved root environment enabling better exploitation of the soil Mg. Supplying a readily soluble source of Mg in the fertiliser band also resulted in increased grain yield in the gypsum, dolomite, and calcium silicate treatments at the 2 Mg-responsive sites. When the initial soil pH was strongly acidic, exchangeable Mg levels increased with increasing lime rate, suggesting that the small quantities of Mg that occur in the majority of liming materials may be of importance with respect to Mg nutrition. In contrast, gypsum application exacerbated the Mg deficiency at one site. The relationship between grain yield response and soil Mg level across all sites indicated that above an exchangeable Mg level of 0.27 cmol(+)/kg there would be little likelihood of a response to applied Mg.