The principal role for magnesium in plants is as an essential component of chlorophyll. However, only approximately 15 to 20% of the plant magnesium content is present in chlorophyll with the remainder in either the ionic state or bound in complexes with organic constituents. Additionally, magnesium has a role in a range of enzyme-regulated physiological processes including phosphorylation, assimilation of carbon dioxide and protein synthesis. In the majority of soil types, soil magnesium reserves are gradually depleted unless magnesium is applied at some stage in the crop rotation.
An absolute shortage of magnesium is most likely to occur on sandy soils with low cation exchange capacity, especially where the latter is dominated by other cations and magnesium is subject to leaching loss. On heavier soils, weathering of soil minerals can be expected to be sufficient to maintain a satisfactory level of exchangeable magnesium and solution concentrations are relatively high. Magnesium deficiency symptoms are often associated with nitrogen deficiency, but a crop response to applied N is more likely than to magnesium. On soils of adequate magnesium status, transient deficiency symptoms may often occur, coinciding with periods of rapid growth as magnesium is transported from older leaves to younger, expanding leaves, or may be induced by drought.
Symptoms of Mg deficiency, where they occur in arable crops, including cereals and oilseed rape, are much more likely to have been induced by poor soil conditions or other factors which restrict root development and Mg uptake, rather than an absolute shortage in the soil.
