The most well known role of magnesium in plants is its occurrence at the centre of the chlorophyll molecule. In addition to this function inadequate levels of magnesium can inhibit CO2 assimilation during photosynthesis. Consequently most of the concerns related to magnesium deficiency are related to aspects of tree growth and cropping.
Although there is little evidence for direct effects of either deficient or excess magnesium on fruit quality in North America, higher levels of magnesium in Cox confer increased resistance to flesh breakdown. Sprays containing MgSO4 (Epsom salts) reduced flesh breakdown in stored Discovery and Cox apples. However, it is known that a high ratio of magnesium to calcium increases wastage due to bitter pit.
It is advised to supplement magnesium where leaf analysis indicates that levels are sub-optimal or where deficiency symptoms are evident on the foliage. Sprays containing Epsom salts will rectify deficiency more rapidly than soil applied forms of magnesium such as magnesian limestone, kieserite or calcined magnesite (MAFF, 2000). Two to five applications of magnesium sulphate (20 kg 1000 l-1 ha-1) may be necessary, applied at 14-day intervals.
It is important to follow the magnesium sprays with a comprehensive programme of calcium sprays to offset any increased bitter pit potential. Proprietary formulations containing magnesium may be substituted for Epsom salts as appropriate. More recently, liquid formulated magnesium products such as Mantrac Pro (Yara) have been developed to provide a concentrated form of foliar applied magnesium.