Scorched Edges Aren't Always Burn — Meet Potassium

Potassium deficiency: burnt, curling margins on older leaves.

There’s a particular sinking feeling when the edges of your leaves go brown and crispy, because everyone knows what that means: you overfed her. Except sometimes you didn’t. I once cut my nutrients in half over scorched edges, watched it get worse for two weeks, and only then learned I’d been starving the plant of the exact thing she was asking for. The plant was short on potassium, and I put her on a diet.

The short version:

• Yellow then brown creeping in from the EDGES of the older, lower leaves — margins scorched, sometimes curling, middle of the leaf still green
• Looks like nutrient burn, but burn hits the TIPS from overfeeding; this comes in from the edges from going short
• Check pH first — high pH locks potassium out even when the feed has plenty
• Fix is a proper bloom feed with real potassium in it; judge recovery on new growth

Want the full breakdown? Keep scrolling.

Why are my leaf edges turning brown and crispy?

Edges first: yellowing, then the rusty ‘burnt crisp’ margin.

Potassium is the plant’s plumbing manager — it runs the valves that control water movement in and out of every cell. The leaf margins are the end of that supply line, so when potassium runs short, the edges dry out and scorch first while the centre of the leaf, closer to the supply, stays green. It starts on the older lower leaves because potassium is mobile — the plant strips it from old growth to feed the new.

So the picture is: older leaves, edges yellowing then browning then crisping, working inward, sometimes with the leaf curling. Middle still green. New growth still okay — for now.

Is it potassium deficiency or nutrient burn?

This is the call that matters, because the fixes point in opposite directions, and getting it backwards — as I can personally report — makes everything worse.

• Nutrient burn comes from overfeeding and hits the leaf TIPS — the very points go brown and crispy first, often across the whole plant, often soon after you’ve upped the feed. The nutrient burn guide has the full picture.
• Potassium deficiency comes in from the side EDGES of the older leaves, the margins scorching while the tips of younger leaves stay clean.

Tips = too much. Edges = too little. Six words that would have saved me a fortnight.

How do I fix potassium deficiency in cannabis?

• pH first, always. High pH locks potassium out — and then there’s no point feeding more of it. Soil wants 6.2–6.5; coco and hydro want 5.8–6.2. Test what’s going in, correct it, hold it there.
• Feed her properly. A real bloom nutrient carries the potassium she needs — it’s the K in NPK and bloom feeds are built around it. CANNA’s bloom range covers it, and DIG stock the lot. If you’re deep into flower and the deficiency has a hold, a PK booster closes the gap faster.
• Don’t expect the old leaves back. Scorched margins are scorched for good. The plant’s done with those leaves; you should be too. Watch the new growth come in clean — that’s the fix confirmed.
• One change at a time. Correct pH, feed properly, then hands off for a week. She’ll tell you if it worked.

Potassium demand peaks in flower — she’s building buds and the plumbing is working overtime — so this one loves to appear right when the stakes feel highest. It’s fixable. Edges, pH, bloom feed, patience.

FAQ

Can potassium deficiency look like nutrient burn? Constantly — it’s the most common mix-up in the colour-trouble family. Burn = tips, from overfeeding. Potassium = edges of older leaves, from running short or locked out. Check which part of the leaf browned first.

What pH locks out potassium? It gets progressively less available as pH climbs above the target window — above about 6.5 in soil, above about 6.2 in coco/hydro. Correct the pH before adding anything.

Do I need a PK booster? Not as a rule. A proper bloom feed at full rate covers potassium for most grows. Boosters earn their place late in flower or when correcting an established deficiency — not as a permanent extra.

Will the scorched leaves recover? No — margin scorch is permanent on that leaf. Recovery shows in the new growth coming through without it.

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Not sure this is your problem? Run her through the Diagnosis Buddy — five questions and you’ll know.