Magnesium deficiency in citrus grown in the gisborne district of New Zealand

Abstract

Magnesium (Mg) deficiency is widespread in citrus on the Gisborne plains. Soil-applied and foliar Mg fertilisers have done little to raise leaf Mg concentrations. Data analysis indicates that this deficiency is a result of high ratios of exchangeable potassium (K) and calcium (Ca) relative to Mg in Gisborne soils (0.5:1 and 7.4:1, respectively). Data from the literature suggests that K:Mg and Ca:Mg ratios greater than 0.4:1 and 7:1, respectively, can cause Mg deficiency. Significant responses to Mg by citrus, in terms of yield or fruit quality, are only likely in instances of severe Mg deficiency. Reductions in yield or fruit quality under conditions of mild Mg deficiency, such as are found in Gisborne, are only likely in some years. To increase our understanding of Mg deficiency in Gisborne citrus, future research should quantify the effect of mild Mg deficiency on fruit quality and yield and investigate the use of Mg:K and Mg:Ca ratios in soil samples deeper than the standard 0.15 cm for diagnosing Mg deficiency. further, the effects of soil moisture and rooting depth on the seasonal patterns of Mg, K, and Ca uptake and the effect of manipulating the shoot:root ratio on leaf Mg concentrations need to be understood. Nutrient management strategies for Mg must be developed following an evaluation of the effects of foliar sprays as well as long-term use of different forms of Mg besides kieserite, irrigation, mulching, and groundcover manipulation. Another option for addressing Mg deficiency is to select or breed citrus rootstocks for improved ability to obtain Mg from Gisborne soils. © 2008 Taylor & Francis Group, LLC.