Potassium Deficiency

Potassium deficiency is the plant running out of the element that regulates water, stomata and sugar transport. It shows up as burn around the outside edges of older leaves, which is why it gets confused with nutrient burn constantly.

What you are seeing#

Older, lower and mid-canopy leaves first, since potassium is mobile and gets relocated to the flowers.

The leaf edge yellows, then goes brown and crisp, working inward from the outer margin and from the tips of each blade. The very edge curls upward slightly. The center of the leaf stays green longer than the rim, which produces a leaf with a green core and a scorched outline.

Rust colored or brown speckles appear between the veins in the still-green area. Leaf tips can look burnt while the leaf is otherwise healthy. Stems may get weak and the plant can look floppy despite adequate water.

In flower, buds stop swelling and ripen unevenly.

Why it happens#

Potassium demand climbs sharply once flowering starts, roughly doubling from week two to week six. A grow-formula feed that was fine in veg simply does not carry enough.

Beginner causes:

• Staying on veg nutrients into flower.
• High sodium in tap water, which competes directly with potassium at the root.
• Excess calcium or magnesium from heavy Cal-Mag dosing, which blocks potassium uptake.
• pH above 7.0 in soil or above 6.5 in coco and hydro.
• Underwatering. Dry pockets in the medium leave potassium physically unreachable.

How to fix it#

1. Read your runoff pH. Target 6.2 to 6.8 in soil and 5.8 to 6.2 in coco and hydro. Above that band potassium uptake falls off fast.
2. Switch to a bloom nutrient with a high third number. Feed at three quarter strength, around EC 1.4 to 1.8 in flower.
3. Cut your Cal-Mag dose in half if you were adding it on top of a complete feed. Over-supplementing calcium is a real and very common cause of this.
4. Water to at least 15 to 20 percent runoff each time so the whole root ball is wetted, not just the center channel.
5. Over the next week, if you are on well water or softened water, switch to reverse osmosis or filtered water. Softened water is high in sodium and will keep causing this.

How to tell it apart#

Versus nutrient burn. This is the important one. Nutrient burn hits the newest, fastest growing top leaves first and the tips look uniformly crisped and often bent. Potassium deficiency hits the oldest lower leaves first and burns the whole outer margin, not just the tip, with rust spots inside the green. Position on the plant is the discriminator: bottom up means deficiency, top down means excess.

Versus light burn. Light burn is strictly positional. It affects only the leaves closest to the lamp, usually the top canopy directly under the center of the fixture, and leaves in the shade a few inches away are perfectly fine. Potassium deficiency ignores where the light is.

Versus pH lockout. Same story as every deficiency on this site. Potassium is one of the first elements to become unavailable above pH 7.0, so measure before you feed. In soil, most apparent potassium deficiency is a pH problem.

How long until it looks better#

The scorched margins are dead tissue and they do not grow back. Do not strip those leaves off unless more than half the blade is brown, because the green part is still working for you.

With a corrected feed at correct pH, the progression should stop within five to seven days. New leaves and the upper canopy should stay clean. In flower, bud swelling picking back up is the signal you want, and that takes seven to ten days to be obvious. If more lower leaves keep scorching after two weeks, you have a pH or sodium problem that a new bottle will not fix.