Potassium deficiency

also called Potash deficiency

Scorched, curling leaf edges on the oldest leaves, weak stems and fruit that will not color up properly. Common on sandy soil and in long-cropped containers.

Disorder Serious Doesn't spread

What you'll see

Leaf margins yellowing then browning and dying, in a band around the edge of the oldest leaves.
Leaf edges curling downward and the leaf becoming brittle.
Yellow mottling and brown spots between the veins on older leaves as it goes on.
Tomatoes ripening unevenly with hard yellow-green shoulders, and fruit small and poorly flavored.
Weak, floppy stems, poor flowering, and plants that fall over in wind and take frost badly.

Why it happens

Potassium runs the plant water regulation, moves sugars around and hardens tissue, and the plant will strip it out of the old leaves to supply the new ones — which is why the damage always starts at the bottom. It leaches readily from light soils, and heavy cropping and high magnesium or calcium levels both make it harder to take up.

Sandy and chalky soils that leach, containers cropped all season on a nitrogen-heavy feed, and heavy-cropping plants — tomatoes, potatoes, currants and apples — in a wet season.

Treating it

Feed with sulfate of potash, or with a high-potassium tomato feed for a faster effect.
Use comfrey liquid or wood ash, both genuinely high in potassium, though wood ash also raises pH so use it sparingly.
Foliar-feed for a quick correction on a fruiting plant that needs help this month.
Stop the high-nitrogen feed, which pushes leaf growth the plant cannot supply with potassium.
Mulch and keep watering even, since potassium moves to the root in soil water.

Keeping it away

Test the soil and correct before the season, rather than diagnosing it from the leaves in August.
Add compost and well-rotted manure annually, which supplies potassium and holds it against leaching.
Feed fruiting crops and container plants with a high-potassium feed from first flower.
Do not over-lime; excess calcium and magnesium compete directly with potassium uptake.
Grow a cover crop over winter on light soil to hold nutrients that would otherwise wash out.

Who gets it

Tomatoes, potatoes, beans, currants, apples and any heavy-cropping plant in a container.