Magnesium

Every chlorophyll molecule has magnesium at its centre, so magnesium is directly required for photosynthesis. Magnesium also activates many enzymes involved in energy transfer (including those that use ATP), protein synthesis, and the loading of sugars into the phloem for transport to fruit, grain, and storage organs.

Because it underpins both photosynthesis and sugar transport, adequate magnesium supports both leaf greenness and the movement of carbohydrates to developing yield components.

Magnesium shares the cation exchange capacity of soils with calcium and potassium, and the balance among these three cations affects how much magnesium is available for uptake. Light, sandy, and acidic soils generally hold less magnesium and are more prone to leaching losses and deficiency.

• Low cation exchange capacity (sandy or low-organic-matter soils) limits the magnesium reserve
• High potassium or calcium saturation can suppress magnesium uptake through cation competition
• Soil acidity reduces magnesium availability and is often addressed together with liming

Magnesium is mobile within the plant, so it is withdrawn from older leaves to support new growth when supply is short. The classic symptom is interveinal chlorosis on older leaves — the veins stay green while the tissue between them yellows, sometimes described as a marbled or mottled pattern — which can progress to necrosis in severe cases.

Magnesium is rarely directly toxic to plants at levels found in agricultural soils. The more common practical issue is the reverse relationship: heavy potassium fertilization, or naturally high soil potassium or calcium, can suppress magnesium uptake and induce deficiency through cation competition.

Magnesium, potassium, and calcium compete for uptake at the root and for exchange sites in the soil, so their balance matters as much as the absolute amount of any one nutrient. Magnesium is also functionally linked to nitrogen, since both are required for chlorophyll and photosynthetic capacity.