Scorched Leaf Edges on My Vegetables — Is It Potassium Deficiency?

Brown or yellowing margins on leaves that look as though they have been lightly scorched by heat — sometimes called leaf scorch or marginal necrosis — are the classic symptom of potassium deficiency. Potassium is the third major plant nutrient, essential for water regulation, disease resistance, fruit quality, and overall stress tolerance. Without adequate potassium, crops suffer through drought, resist disease poorly, and produce fruit that looks poor and ripens unevenly. Identifying and correcting this deficiency is important for fruiting and root crops in particular.

Recognising the Symptoms

Potassium, like nitrogen and phosphorus, is mobile within the plant — it moves from old tissues to new when supply is limited. Deficiency symptoms therefore appear first on older, lower leaves. The characteristic scorch begins at the leaf margin and tips, progressing inward. Leaves may also cup or curl. Plants show reduced vigour, poor resistance to drought, and increased susceptibility to fungal diseases. Tomatoes, potatoes, peppers, courgettes, and soft fruit are the crops most visibly affected. Root crops like beetroot and carrot produce poor-quality, poorly flavoured roots when potassium is short.

Soils Most at Risk

Sandy soils are most vulnerable to potassium deficiency because potassium — a positively charged ion — is held on soil particle surfaces, and sandy soils have far fewer of these than clay. Heavy rainfall leaches it through the profile. Soils with high magnesium levels (from repeated dolomitic lime applications, for example) can develop potassium deficiency through competition — excess magnesium blocks potassium uptake even when potassium is present. High-cropping gardens that remove large quantities of biomass without returning organic matter or compost also gradually deplete potassium.

Correcting Potassium Deficiency

The fastest organic correction is sulphate of potash (potassium sulphate), which is approved for organic use and water-soluble — it acts within a week or two when watered in around affected plants at 30 to 50 g per square metre. Wood ash is rich in potassium carbonate and provides an immediate boost; apply at 100 to 150 g per square metre and water in. Note that wood ash also raises pH, so avoid it on already-alkaline soil or around acid-loving plants. Tomato feeds, seaweed-based liquid fertilisers, and comfrey leaf tea are all high in potassium and useful for regular supplementary feeding of fruiting crops through the season.

Long-Term Potassium Management

Greensand (glauconite) is a slow-release natural mineral that improves potassium levels over multiple seasons while also improving soil texture. Granite dust is another slow-release option. Adding comfrey to the composting pile or using comfrey as a mulch brings up deep potassium from the subsoil — comfrey roots penetrate deep into the ground and accumulate potassium in their leaves, which release it rapidly when they rot. Growing comfrey in a corner of the garden and cutting it three or four times per year to use as mulch or liquid feed is a self-sustaining potassium strategy that costs nothing once established.

Distinguishing Potassium Deficiency from Other Causes

Leaf scorch can also be caused by wind damage, salt spray, chemical burns, magnesium deficiency, or boron deficiency. Potassium deficiency specifically starts at the leaf tips and margins on older leaves and progresses uniformly inward. Magnesium deficiency shows as yellowing between the veins (interveinal chlorosis) rather than marginal scorch. A soil test that measures exchangeable potassium confirms the diagnosis and guides the amendment rate. If in doubt, a trial application of sulphate of potash on a few affected plants should show improvement within two to three weeks if potassium is the limiting factor.