Magnesium deficiency causes yellowing of older leaves.  The symptom begins between the major veins, which retain a narrow green border.  A light tan burn will develop in the yellow regions if the deficiency is severe.  Fruit yields are reduced. 

Incorporate magnetite (300 kg/ha) or dolomite (800 kg/ha) into deficient soils before planting.  Fortnightly foliar sprays of MgSO4 (2 kg/100 L) at high volume (500-1000 L/ha).