Magnesium deficiency in pastures

Over‐all responses of 10 per cent and 11 per cent to 2 and 4 cwt respectively of MgSO 4 7H 2 O per acre, were found in a pasture trial on a pumice soil, Whakarewarewa sandy loam, with an initial exchangeable Mg level of 0.14 me. per cent. Larger responses, over‐all 49 per cent and 46 per cent respectively, were found in a trial on another pumice soil, Te Rere sand, containing 0.07 to 0.11 me. per cent exchangeable Mg. Magnesium deficiency resulted in interveinal chlorosis in white and red clovers, often accompanied by strong red colours. Deficient cocksfoot on Te Rere sand was severely damaged by winter frosts. Increasing potassium chloride applications from 2 cwt to 4 cwt per acre had only small effects on magnesium levels in the herbage. Results indicate critical magnesium levels of about 0.12 per cent in dry matter of whole leaves of white clover and 0.14 per cent in red clover. dissected from mixed grass‐clover herbage cut at cow grazing or early silage height. Magnesium levels in leaves showing deficiency symptoms averaged 0.06 per cent in both white and red clovers. Recoveries indicate considerable leaching losses of magnesium from magnesium sulphate on these soils. Responses to magnesium fertilisers are likely in pastures when exchangeable magnesium levels in these soils fall below 0.15 me. per cent, or advisory test values below about 2.0 p.p.m.