Blossom End Rot and Tip Burn: Calcium Deficiency in Your Garden

A dark, sunken patch at the base of tomatoes, peppers or courgettes. Brown crispy edges on the inner leaves of a cabbage or lettuce. Distorted new growth on celery. These are all symptoms of calcium deficiency — and the frustrating thing is that most soils actually contain plenty of calcium. The problem is usually delivery, not supply.

Why Calcium Matters

Calcium is a structural nutrient. It builds and reinforces cell walls, which is why calcium-deficient tissue collapses and rots rather than simply discolouring. Calcium is also involved in root cell function and the movement of other nutrients through the plant. Once calcium is incorporated into plant tissue it cannot be moved to other parts — it is an immobile nutrient — so new, rapidly growing tissue is always at most risk when calcium supply falters.

The Real Cause: Water Stress, Not Soil Shortage

Calcium travels through plants dissolved in water — it moves with the transpiration stream, being drawn up through roots and deposited in leaves and other tissues as water evaporates from the leaf surface. Fruits and the inner growing points of leafy crops have almost no transpiration, so they rely on a continuous flow of calcium-rich water. When that flow is interrupted, those parts are the first to suffer.

The triggers for calcium delivery failure include:

• Irregular or erratic watering (wet-dry-wet cycles)
• High humidity that reduces transpiration
• Very high nitrogen or potassium feeding that competes with calcium uptake
• Compacted or waterlogged roots that cannot function properly

How to Treat It

For an immediate response, apply calcium foliar sprays directly to developing fruits and inner leaves. Calcium nitrate solution or a commercial blossom end rot spray works quickly. However, the more important fix is usually watering: establish a consistent, regular watering routine so the plant never dries out and then gets a large drink. Mulching helps maintain even soil moisture.

If your soil is genuinely low in calcium (acidic soils in high-rainfall areas can be), garden lime or calcitic limestone will correct both the calcium and pH situation. A soil test will tell you if this is needed.

Prevention in Containers

Container plants are especially prone to calcium deficiency because pots dry out unevenly and heavy feeding exhausts nutrients quickly. Water little and often rather than large infrequent doses. Use a balanced fertiliser rather than one very high in nitrogen or potassium alone, and refresh compost each season.