Magnesium, Magnesium Deficiency, Magnesium Deficient, Nutrient Deficiency, Mineral Deficiency
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Plants need magnesium because it's one of the main constituents making up the plant pigment chlorophyll. As well as giving plants their beautiful green colour, chlorophyll also allows plants to absorb solar energy from the sun and conduct photosynthesis. Plants most likely to be affected are those planted in acidic, sandy soils which are quite often lacking in other nutrients, typically calcium. It's also easily leached so that symptoms can be intensified during the rainy season. Unfortunately, a magnesium deficiency can be hard to deduce because symptoms depend on the plant in question. Magnesium is a mobile nutrient, so is often transported up the plant during shortages, hence why symptoms will appear in the oldest leaves first.
Areas between the leaf veins turn yellow, producing a marbling effect.
Areas which surround the veins (including the veins themselves) stay green.
On peas and beans, leaves show marbling in the centre.
Plants pigmented with red, such as beetroot, can turn purplish.
Green plants can begin to turn purple-ish as symptoms progress.
Symptoms always appear on the oldest leaves first.
Apples may lose leaves at an alarming rate.
Magnesium deficiencies are commonly associated with calcium shortages, so an option would be to use a magnesian limestone for liming should help restore the nutrient balance. Dolomite limestone would be another alternative. Limestone shouldn't be applied near ericaceous plants, or if the soil is already alkaline. Epsom salts diluted with water (20g:1L) with a dash of liquid detergent applied fortnightly as a foliar feed can be beneficial. Be sure to restrict treating plants during duller periods of the day to avoid leaves getting burnt.
A magnesium deficiency will not affect yields unless calcium levels are also extremely low. Most of the time damage is just aesthetic. Always be mindful when applying fertilisers, taking care to follow manufacturers instructions. Too much potassium can interfere with other nutrients, such as magnesium levels.