Potassium Deficiency in Plants, Leaf Scorch and the Fix (2026)

Last updated: 30 September 2026

> Quick Answer: Potassium deficiency in plants shows as yellow, then brown, crisp edges on the older lower leaves, while the leaf centre stays green. Flowers and fruit come fewer and smaller. The organic fix is a small dose of wood ash, about 3.5 to 5 g for a 30 cm pot.

On a terrace, potassium runs short fastest in pots of flowering and fruiting plants, because every watering carries a little of it out through the drainage hole. The first sign is on the oldest leaves, at the rim, not in the middle.

Where it shows is the clue. Potassium moves inside the plant, so a hungry plant strips it from old leaves to feed new ones, and the old leaves scorch first.

What does potassium deficiency look like on a pot plant?

It starts as yellowing along the edges of the lower, older leaves, which then turns brown, dry and papery while the centre and the veins stay green. New leaves at the top usually look normal until the shortage becomes severe. Stems can weaken and flowering drops off.

Iowa State University Extension describes the same progression: yellowing along the leaf margins on older leaves that turns brown and scorched. Look for these together:

• Yellow then brown, crisp leaf edges on the lowest leaves
• A green midrib and centre on the same leaves
• Healthy-looking new growth at the tip, at first
• Fewer flowers, small fruit, or fruit that ripens unevenly
• Weak, floppy stems on otherwise leafy plants

Which pot plants show it first?

Heavy flowering and fruiting plants in pots show it first, because they use the most potassium and have the least soil to draw from. On Indian terraces that usually means tomato, chilli, brinjal, rose and hibiscus, and fruiting plants such as lemon kept in containers.

Leafy greens like spinach and methi? They rarely show it before something else runs out first. For a plant-by-plant view of what each pot needs through the year, see which fertiliser suits each plant.

Potassium, nitrogen or magnesium, how do you tell the three apart?

All three show on older leaves first, so look at the pattern. Potassium scorches the edges and leaves the centre green. Nitrogen yellows the whole leaf evenly, starting from the tip along the midrib. Magnesium yellows between the veins while the veins themselves stay green.

Iowa State's guide makes the same split: magnesium shows as more distinct yellowing between the veins, and nitrogen as more uniform yellowing of the whole older leaf rather than a scorched margin.

How much wood ash per pot, and how often?

About 3.5 to 5 g of wood ash for a 30 cm pot, roughly a level teaspoon to a heaped one, mixed into the top layer and watered in. That scales down the RHS rate of 50 to 70 g per square metre. Wait four to six weeks and watch the new leaves before adding more.

The RHS guide to wood ash puts its potassium at about 3%, Iowa State University Extension at 5% to 8% potash, and Virginia's Piedmont Master Gardeners at 3% to 8%, varying with the wood burnt. Nobody publishes a pot dose, so start small.

SatRas wood ash fertiliser costs ₹229 for a 900 g pack. At 5 g a pot a month, 20 pots use about 100 g, so a pack lasts around nine months at roughly ₹25 a month. The pack prints no potassium analysis, so rely on the published ranges above.

To apply it without scorching roots:

1. Water the pot lightly the day before, so the soil is moist.
2. Weigh or measure the ash; do not pour from the pack.
3. Sprinkle it in a ring a few centimetres from the stem.
4. Scratch it into the top 2 to 3 cm of soil.
5. Water gently until a little drains out, then note the date.

If the same pots also need nitrogen and pest protection, the plant care combo pack bundles wood ash with mustard cake, neem seed powder and a growth promoter at ₹859 for four 900 g packs.

When should you not use wood ash?

Skip it on acid-loving plants, on seedlings, and in any pot where the soil is already alkaline. Wood ash is strongly alkaline and raises soil pH quickly, which can lock up iron and other nutrients. The limits below come from the extension guidance.

• Acid-loving plants such as azalea, gardenia and blueberry; Iowa State advises against ash around them.
• Seedlings and fresh transplants, which fresh ash can scorch before it reacts with moist soil.
• Soil already above pH 7, where Iowa State advises against ash altogether.
• Alongside ammonium-based nitrogen fertilisers, since alkaline ash can drive off ammonia.

Iowa State notes ash has 25% to 59% of the liming power of ground limestone and changes pH faster than lime. That is why it is a small, occasional dose, never a routine top-up; for acid-loving pots, pick another input from the plant care range.

How long until the plant recovers?

Scorched leaves won't turn green again. Judge recovery on the new growth instead. Healthy new leaves and better flowering usually show over the next few weeks, depending on season and how actively the plant is growing. Trim badly scorched leaves once the new ones are established.

Pots in heavy monsoon rain lose potassium faster, because water running through the drainage hole carries it away. Expect to see scorch return sooner after a wet spell, and check again once the rains ease rather than dosing during them.

If new leaves still scorch after one dose and six weeks, check drainage and watering first. Waterlogged roots struggle to take up potassium even when it is present, and in a pot that is a more common problem than an empty soil.

What should you avoid when fixing potassium deficiency?

Avoid adding ash on a guess, and avoid adding it often, however healthy the plant looks afterwards. On a hot terrace, plenty of scorched edges turn out to be heat, salt or watering stress rather than potassium. These are the mistakes that cost gardeners the most plants.

• Treating every brown edge as potassium; sun scorch and salty water look similar.
• Heaping ash on the surface in large amounts, which raises pH fast.
• Repeating the dose weekly instead of waiting for new growth.
• Using ash from charcoal briquettes, painted wood or plastic, which can carry contaminants.
• Using ash on acid-loving plants.

Frequently asked questions

Is wood ash a potassium fertiliser?

Yes, a mild one. Extension services put wood ash at roughly 3% to 8% potassium, alongside calcium and small amounts of phosphorus and trace elements. Its content varies with the wood burnt, and young wood such as prunings tends to carry more. It also acts as a liming agent, so use it sparingly.

Can too much potassium harm plants?

Yes. Excess potassium can interfere with the plant's uptake of magnesium and calcium, which then show their own deficiency signs. In pots the bigger risk from overusing wood ash is usually the pH rise rather than the potassium itself. Small, spaced doses avoid both.

Does banana peel fix potassium deficiency?

Slowly, and only a little. Banana peel does contain potassium, but it has to break down in the soil before plants can use it, which takes weeks. Chopped peel buried in the top layer or composted first is fine as a supplement. For a visible shortage, a measured dose of wood ash acts faster.

Can wood ash be used on a vegetable bed in the ground?

Yes, at the RHS rate of 50 to 70 g per square metre, spread on bare soil in late winter and worked in before planting. That gives the ash time to react with moist soil. Avoid it on beds for potatoes, where higher pH is linked to more potato scab, and on soils that already test above pH 7.

How do I check whether my pot soil is alkaline?

Use a simple soil pH kit or meter from a garden shop and test a moistened sample from a few centimetres down. Most vegetables and flowers prefer slightly acidic to neutral soil, around pH 6 to 7. If the reading is already above 7, choose a potassium source other than wood ash.

Does sulphate of potash work faster than wood ash?

Usually, yes. Sulphate of potash is a concentrated mineral source that dissolves readily and does not raise pH, so it suits plants that dislike alkaline soil. It is not an organic input in the traditional sense. Wood ash is slower and milder, and its liming effect is the trade-off for its low cost.