Survey of magnesium content of soils and pastures and incidence of grass tetany in three selected areas of Taranaki

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Abstract
Results of a survey of the Mg content of representative yellow-brown loam soils in three selected areas of Taranaki are presented and discussed in relation to parent material and altitude. The areas investigated are in Inglewood, Stratford, and Waimate West counties. Factors affecting herbage Mg concentrations are examined and discussed in relation to the observed incidence of grass tetany (hypomagnesaemia) in dairy herds. The overall range found for exchangeable soil Mg contents v/as 0.36–3.35 me./100 g, with values in the Waimate West area generally higher than those in the lnglewood and Stratford areas. An overall trend of decreasing exchangeable soil Mg with increasing altitude was found. Similar ranges of Mg, K, Ca, and N concentrations in mixed pasture, ryegrass, and white clover were found within all three sampling areas. Statistical correlations between plant Mg concentrations and soil chemical properties were highest when the interactive effects of exchangeable soil K and Ca and soil pH were considered. For combined analyses over all sites the ratio index: showed promise as a means of predicting herbage Mg concentrations using soil data. Highest incidences of grass tetany in dairy cows (up to 5% of the herd) were associated with exchangeable soil Mg contents of < 1 me./100 g.