Yellowing Between the Veins on Older Leaves — Could It Be Magnesium Deficiency?

Magnesium deficiency produces one of the most recognisable patterns in plant nutrition problems: interveinal chlorosis that starts on older, lower leaves and progresses upward through the plant. Unlike iron deficiency, which hits the youngest growth, magnesium is mobile in the plant and is redirected from old tissue to young. When supply is low, the older leaves are sacrificed first, turning yellow between still-green veins in a striking, distinctive pattern.

Why Magnesium Matters to Plants

Magnesium sits at the centre of every chlorophyll molecule — it is literally the atom that makes photosynthesis possible. Without adequate magnesium, plants cannot make chlorophyll and the characteristic green colour fades. Magnesium is also essential for phosphorus uptake and energy transfer within the plant. Deficiency is not just an aesthetic problem: affected plants photosynthesize less efficiently, grow more slowly, and produce lower yields. Tomatoes, peppers, apples, and roses are among the crops most commonly and visibly affected.

What Causes Magnesium Deficiency

Sandy and acidic soils are most at risk because magnesium leaches readily in these conditions. High potassium levels compete with magnesium for uptake — plants absorb both through the same transport mechanisms, and when potassium is abundant (from heavy wood ash or potassium fertiliser applications), magnesium absorption is suppressed. Repeated use of calcitic lime, which adds calcium without magnesium, can create a relative magnesium deficit even when absolute magnesium levels appear adequate. Heavy fruit crops draw heavily on magnesium reserves, and deficiency in tomatoes often appears in the second half of the season when demand is highest.

Correcting It: Epsom Salts

Epsom salts (magnesium sulphate) are the fastest, most widely available correction for magnesium deficiency. Dissolve 20 g per litre of water and apply as a foliar spray, or use at 30 to 50 g per square metre dissolved in water as a soil drench. Foliar application works fastest — symptoms begin to improve within seven to fourteen days. Repeat after three to four weeks if symptoms persist. For tomatoes and peppers in containers or growing bags, an Epsom salt feed every three to four weeks from midsummer onwards is a sensible preventive measure. Avoid applying to soil that already has high magnesium levels as excess magnesium can damage soil structure in clay soils.

Long-Term Management

Where magnesium deficiency recurs season after season, dolomitic lime is the preferred long-term solution if pH correction is also needed. Dolomitic lime contains both calcium carbonate and magnesium carbonate, correcting both pH and magnesium simultaneously. If pH is already correct, magnesium limestone — calcium magnesium carbonate — provides magnesium without pushing pH too high. Annual compost additions help buffer magnesium levels, as organic matter improves the soil's cation exchange capacity and its ability to hold magnesium against leaching. On sandy soils, split-feeding magnesium — applying a small amount two or three times per season — is more effective than one large application that leaches away.

Confirming the Diagnosis

Magnesium deficiency is confirmed by a soil test that measures exchangeable magnesium. Lab tests typically report in mg per litre or ppm; values below 50 mg/l in sandy soil or below 70 mg/l in loam or clay are considered deficient. A simpler field test: treat a few affected plants with Epsom salt solution and check for improvement after two weeks. If the yellowing stops progressing and new growth is greener, magnesium was the limiting factor. If there is no response, consider iron deficiency or other causes.