Chlorosis or yellowing of a plant’s leaves is often a symptom of nutritional deficiency or viral attack but can also reflect the presence of other cultural or weather influences and even natural ageing. In the absence of these factors yellowing between the veins of older leaves on a plant is an indicator of magnesium deficiency.

Magnesium is an essential macronutrient constituting 0.2-0.4% of plants' dry matter and is necessary for normal plant growth. Magnesium deficiency is a detrimental plant disorder that occurs most often in strongly acidic, light, sandy soils, where magnesium can easily leach away. This can also occur with plants grown in containers if additional fertilisers are not administered. Some fertilisers contain little or no magnesium and being overgenerous with fertilisers containing potassium can further aggravate the stress from magnesium deficiency, as can aluminium toxicity.

Magnesium has an important role in photosynthesis because it forms the central atom of chlorophylls. Chlorophylls are the green pigments present in plant cells which capture the energy from sunlight. Without sufficient amounts of magnesium, plants begin to degrade the chlorophyll in older leaves, which causes the main symptom, interveinal chlorosis, or yellowing between leaf veins which stay green, giving the leaves a marbled appearance. Due to magnesium's mobile nature, a plant deprived of magnesium will first break down chlorophyll in older leaves and transport the liberated magnesium to younger leaves which have greater photosynthetic needs. Therefore, the first sign of magnesium deficiency is the chlorosis of old leaves which progresses to the young leaves as the deficiency progresses.

Magnesium also acts as an activator for many critical enzymes in carbon fixation. Thus, low amounts of Mg lead to a decrease in photosynthetic and enzymatic activity within the plants. Magnesium is also crucial in stabilizing ribosome structures; hence, a lack of magnesium causes depolymerization of ribosomes leading to premature aging of the plant. After prolonged magnesium deficiency, necrosis (death of plant tissue) and dropping of older leaves occurs. Plants deficient in magnesium also produce smaller, woodier fruits.

An organic treatment is to apply compost mulch, which can prevent leaching during excessive rainfall and provide plants with sufficient amounts of nutrients, including magnesium or a feed of seaweed fertiliser. Adding Epsom salts (as a solution of 25 grams per liter or 4 oz per gal) or crushed dolomitic limestone to the soil can also rectify magnesium deficiencies.