Magnesium (Mg) is one of the secondary macronutrients, needed by plants in relatively large amounts. It is important in leaf growth, photosynthesis (the process plants use to harness the sun’s energy to produce sugars and oxygen) and is present in chlorophyll (the light-absorbing plant pigment), so is important for deep green leaf colour.
Magnesium deficiency normally starts on the older, lower leaves and then spreads to the younger leaves.
Magnesium deficiency is caused when there are insufficient suitable magnesium-containing materials in the growing medium (soil or compost). It can also be caused when plants are growing in dry soil, which prevents the roots taking up the magnesium from the soil.
Magnesium may be washed out of the soil or compost by excessive rainfall or overwatering, leading to a deficiency. Shortages are worse on light, sandy and chalky soils that drain quickly and contain little organic matter to hold on to the magnesium.
Excessive use of high potash fertilisers (including tomato fertilisers) may lead to magnesium deficiency, since potassium is taken up by the roots in preference to magnesium.
