Why Do My Lemon Leaves Have Yellow Patches?

Irregular yellow patches on lemon tree leaves — typically between the veins while the veins themselves remain green — are a classic sign of magnesium deficiency. This is one of the most common nutritional problems in container-grown citrus and is also frequently seen in garden-grown trees in sandy or acidic soils. Identifying the pattern correctly is important because the appearance of magnesium deficiency is different from other yellowing problems such as overwatering or nitrogen deficiency.

Why magnesium deficiency happens

Magnesium is a mobile nutrient in plants — when supply is short, the plant moves magnesium from older leaves to younger growing tips. This is why symptoms appear first on the older, lower leaves while younger upper leaves may look normal. The deficiency itself can arise from genuinely low magnesium levels in the soil or compost, from high pH that locks up magnesium, from overwatering that leaches it from the root zone, or from excessive potassium fertilising (potassium and magnesium compete for uptake). Container-grown citrus is particularly prone to magnesium deficiency because regular watering leaches nutrients from the compost over time.

Treating magnesium deficiency

The fastest treatment is a foliar spray of Epsom salts (magnesium sulphate): dissolve 20 g per litre of water and spray the foliage in the early morning or evening, covering the leaves thoroughly including the undersides. The magnesium is absorbed through the leaves and improvement is typically visible within two to three weeks. Repeat the spray two or three times at fortnightly intervals for best results. For a soil drench, dissolve Epsom salts at 30 g per litre and water thoroughly around the root zone.

Long-term prevention

For container citrus, repot into fresh citrus compost every two to three years and use a citrus-specific fertiliser that includes magnesium and trace elements throughout the growing season. General-purpose fertilisers often lack adequate magnesium for citrus. For garden-grown trees, apply Epsom salts as a preventative top-dressing at 35 g per square metre each spring, and check that the soil pH is in the range of 6.0–7.0 — high pH makes magnesium unavailable even when it is present in the soil.

Distinguishing from iron deficiency

Iron deficiency looks similar but the pattern is slightly different: iron deficiency tends to affect the youngest leaves at the shoot tips first (because iron is less mobile than magnesium), and the yellowing is often more uniform across the leaf rather than patchy. Magnesium deficiency affects older leaves first. In practice, both can occur together in chalky soils or with high-pH compost — treating both with a liquid seaweed feed (which contains trace elements) alongside the Epsom salts treatment is a reasonable approach when you are uncertain.