Advice
Magnesium deficiency
Magnesium
is a plant nutrient found especially in leaves. It is involved
in photosynthesis, which is the process where plants use energy
from sunlight to produce food as carbohydrates. A deficiency
of magnesium reduces photosynthesis which affects the plant's
growth.
Plants affected
This disorder is quite common in fruit and vegetables (in particular tomatoes and apples) and rhododendrons.
Symptoms
Yellowing can be seen between leaf veins and around leaf margins, sometimes with red or orange tints. Discolouration begins on older leaves, then affects younger growth. If untreated the foliage will die.
Cause
Deficiency is most common on light sandy soils which can be low in available magnesium. Symptoms may also arise where poor root establishment has occurred.
Excessive use of potassium-containing fertilisers is another cause. When potassium is present in high concentrations in the soil, it is absorbed in preference to magnesium.
Treatment
Calcium-magnesium carbonate (Dolomite limestone) or magnesium sulphate (Epsom salts) applied to the soil in autumn or winter will help to improve deficient soils. Dolomite limestone also neutralises soil acidity, so it should not be applied to neutral or alkaline soils.
For immediate treatment apply Epsom salts as a foliar spray in summer.