Serpentine-superphosphate and dolomite as magnesium sources on Whakarewarewa sandy loam

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Abstract
Serpentine-superphosphate at 4 cwt per acre per annum and superphosphate plus dolomite at equivalent total Mg and P rates were compared with normal superphosphate (equal total P) in a field experiment on a pumice soil deficient in P and marginally deficient in Mg for pasture growth. Both Mg sources appreciably raised exchangeable Mg levels in the soil and maintained pasture Mg levels above 0.20% for most of each year over a period of 4 years. Yield responses to Mg increased from nil in 1961–2 to 20% in 1964–5, under the total removal system used. Increases in herbage Mg concentrations (22 to 73%) were closely related to the square roots of the ratios of exchangeable Mg levels in the plus-Mg and no-Mg treatments. Four cwt serpentine-superphosphate was equivalent to approximately 1.3 cwt dolomite in its effect on soil and plant Mg levels. Potassium chloride at 1 and 3 cwt per acre significantly increased yields in the plots receiving P and S fertiliser. It significantly reduced exchangeable Mg in the last two samplings and Mg concentrations in the herbage in all cuts in the third and fourth years, but did not reduce Mg uptake (yield × concentration). Superphosphate significantly reduced exchangeable Mg levels in the soil in 1964–5 and herbage Mg concentrations in many of the cuts analysed, but appreciably increased uptake of Mg.

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