Keywords: magnesium, crop nutrition, magnesium fertilisation, foliar fertilisation, crop quality, magnesium deficiency, shoot:root ratio, stress alleviation.
The essential requirement for magnesium (Mg) and its basic biochemical and physiological functions in plant metabolism have been understood for many years. Despite this understanding, however, in comparison with the other major nutrients not a great deal of attention has been given to Mg in crop nutrition. With some justification therefore Mg has been described as the almost forgotten mineral nutrient. There is now increasing evidence of the occurrence of Mg deficient symptoms in crop plants and the consequent need for Mg fertilisation for the improvement of yields and crop quality. Magnesium uptake by plants can be low particularly on acid sandy soils poor in Mg as a consequence of the competitive effect of other cations including potassium (K+), ammonium (NH4+) and aluminium (Al3+). Plants poorly supplied in Mg have low root to shoot ratios making them less well able to acquire nutrients and water. The need for Mg in the phloem loading process means that in inadequately supplied plants the transport of photosynthates can be impaired and thus too the quality of grains, fruits and tubers. Adequate amounts of Mg are therefore particularly required during the period of reproductive growth. This is particularly important under conditions when Mg acquisition for the soil can be impaired as under drought, or high K supply or on low pH soils. A lack of Mg also disturbs nitrogen (N) metabolism so that crops low in Mg are unable to make optimal use of N fertilisers, which is of significance both to crop production and the environment. More evidence is forthcoming of the mitigating effect of higher Mg concentrations in the plant on various environmental stresses including light, heat and drought, stresses which are likely to take on even more importance with the increasing weather extremes caused by global climate change. From the physiological principles involved, six areas of research in field crop production are considered in relation to the strategic use of Mg fertilisers for the benefit of farmers.
Volker Römheld, Institute of Plant Nutrition (330), University of Hohenheim, 70593 Stuttgart, Germany, and
Ernest A Kirkby, Institute of Integrative and Comparative Biology, Faculty of Biology, University of Leeds, Leeds LS2 9JT, UK.
23 pages, 6 figures, 2 tables, 62 references.