Gardening glossary
Potassium
Potassium (the K in NPK, from the Latin *kalium*) is the third macronutrient but arguably the most behind-the-scenes. Unlike nitrogen and phosphorus, it does not build tissue directly — instead, it acts as the plant's electrical and hydraulic regulator. Potassium ions control the opening and closing of stomata, drive the movement of sugars from leaves to fruit, activate dozens of enzymes, and stiffen cell walls against pests and disease.
The deficiency symptoms reflect those jobs. Potassium-starved plants show:
- Yellowing and browning leaf margins (the classic "scorched" edge), usually on older leaves first - Weak, floppy stems - Poor fruit colour, size, and flavour - Reduced cold and drought tolerance
When to push potassium: as fruiting crops shift from foliage to fruit. Tomatoes, peppers, cucumbers, squash, strawberries, and apples all benefit from a potassium-heavy feed (a comfrey tea, kelp meal, or a 5-10-10 / 2-8-10 blend) once flowering begins. The same applies to root crops like potatoes and carrots — potassium drives starch and sugar storage.
Sources of potassium for the home garden:
- **Wood ash** (5–10% K by weight) — fast-acting and alkaline; use sparingly to avoid raising soil pH too far. - **Comfrey leaves** — chopped and either left as a mulch or steeped in water to make a potassium-rich liquid feed. - **Kelp meal and seaweed extract** — slow-release potassium plus a wide range of micronutrients. - **Sulphate of potash (0-0-50)** — concentrated mineral form when targeted dosing is required.
Most temperate soils are reasonably well-stocked with potassium, but sandy soils leach it quickly and intensively cropped vegetable beds run short by mid-season. A soil test every two or three years catches this before symptoms appear.