Yellow Patches Between Green Veins — Is It Magnesium Deficiency?

When you see yellowing spreading across the leaf surface but the veins themselves remain green, this is called interveinal chlorosis, and magnesium deficiency is one of the most common causes. It tends to appear on older leaves first and can spread quickly through the plant if not addressed. The good news is that magnesium deficiency is one of the easier nutrient problems to fix.

Magnesium's Role in the Plant

Magnesium is at the centre of every chlorophyll molecule — it is literally what makes leaves green. It also activates many of the enzymes involved in photosynthesis and helps move phosphorus through the plant. Because it is mobile in plant tissues, when supplies fall short the plant pulls magnesium from older leaves and moves it to younger growth, so lower leaves show the deficiency before upper ones.

Recognising the Symptoms

The interveinal yellowing pattern is the key identifier. In mild cases, the areas between the main veins take on a pale or yellow-green hue while the veins stay dark green. In more severe cases the yellow patches can turn brown and the leaves may drop early. Tomatoes, roses, raspberries and apples are particularly prone to magnesium deficiency and will often show classic symptoms on the leaves around developing fruit.

Common Causes

Several factors can trigger magnesium deficiency even in well-managed gardens:

• Sandy or acidic soil: magnesium leaches quickly from light soils
• Excess potassium: high potassium levels compete with magnesium uptake — a common problem when gardeners over-apply tomato feed
• Excess calcium: heavy liming can also antagonise magnesium
• Container growing: pots run out of magnesium quickly and need supplementation
• Heavy cropping: tomatoes and peppers strip magnesium as they set fruit

How to Fix It Quickly

The fastest and most reliable remedy is Epsom salts (magnesium sulphate). Dissolve 20 g per litre of water and apply as a soil drench around the roots. For a faster response, use the same solution as a foliar spray directly on affected leaves — the plant absorbs it within a day or two. Repeat every two weeks until symptoms resolve. Epsom salts are inexpensive, widely available and safe to use on vegetables right up to harvest.

Long-Term Prevention

Dolomitic limestone contains magnesium as well as calcium and is a good soil amendment if your soil is also acidic. Incorporating compost regularly supplies a steady background level of magnesium. For container plants and fruiting crops, use a fertiliser that includes magnesium in its formulation — some tomato feeds include it for exactly this reason. Avoid repeatedly over-applying high-potassium feeds without balancing with magnesium.