Most deficiencies are not shortages

A grower sees yellow leaves and adds iron. Nothing changes. Adds more. Still nothing. The pot was full of iron the whole time. The pH was 7.5, and at that pH the plant simply cannot take it up.

This is the most common story in growing. The nutrient is there, but it is locked out.

So this decoder works like a good mechanic. Check the basics before you replace parts. First pH. Then where on the plant the damage sits. Then the pattern on the leaf. Most of the time, those three steps name the problem before you open a single bottle.

Step one is always pH

Every nutrient has a pH range where roots can take it up. Outside it, feeding more does nothing.

Check the pH of the water going in, and of the runoff coming out. A big gap means the root zone has drifted. Fix that first.

The classic false alarm

Bright lemon yellow new leaves with a fine green net of veins, and deep green leaves below. That stark top to bottom split is the look of iron lockout.

Above about pH 7, iron falls out of solution and the plant cannot reach it. Adding iron at pH 7.5 does nothing at all. Bring the pH back into range and new growth comes in green.

Top or bottom? The fastest clue

Some nutrients can travel inside the plant. When supply runs short, the plant pulls them out of old leaves to feed new ones. These are the mobile nutrients: nitrogen, phosphorus, potassium and magnesium. Their damage shows at the bottom first.

Others are stuck where they land. These are the immobile nutrients: calcium, iron, sulfur, zinc, manganese and boron. The old leaves keep their share, so the damage shows on new growth at the top.

Just noticing which end of the plant is affected cuts your list in half before you read anything else.

The bottom four: mobile

Look at the oldest, lowest leaves first.

The top six: immobile

Look at the newest growth at the crown.

Nitrogen

The oldest leaves fade to an even pale yellow, veins and all, then droop and drop. No spots, no pattern. The fading climbs upward over time.

Check pH first. If it is genuine, a fast nitrogen source like fish hydrolysate or blood meal helps in veg.

In late flower, leave it alone. Lower leaves fading around week 8 is the plant moving nitrogen into the flowers. That is normal and healthy.

Phosphorus

Lower leaves turn unusually dark, a dull matte blue green, and their stems flush deep purple red. Bronze blotches can follow.

Before you feed, feel the root zone. Phosphorus uptake falls off sharply below about 60 F, so a cold floor or cold water can cause this on its own.

Also ask whether it is the strain. Purple stems on an otherwise healthy plant are often just genetics.

If it is real, a soluble source works within the week. Bone meal is too slow to rescue anything mid flower.

Potassium

A continuous brown, scorched band follows the whole serrated edge of the lower leaves, while the center stays green. The edges curl up.

Look at your calcium and magnesium before you add potassium. These three compete at the root, and heavy Cal-Mag use often crowds potassium out.

Sulfate of potash or langbeinite corrects a genuine shortage. A balance of roughly 3 to 5 parts each of potassium and calcium to 1 part magnesium keeps them from fighting.

Magnesium

Yellow spreads between the veins on lower and middle leaves, while the veins stay bright green. It looks like a green skeleton laid on a yellow leaf. Rusty spots follow.

Often the real cause is too much potassium, not too little magnesium. Check that first, because adding magnesium on top of high potassium makes it worse.

In coco, unbuffered coir strips calcium and magnesium out of your feed. Use buffered coir. If it is truly magnesium, Epsom salt at a quarter to a third of a teaspoon per gallon works within days.

Calcium

New leaves come in small, twisted and misshapen, with hooked tips and rusty brown spots. Growing points can deform. Old leaves stay fine.

Calcium travels with water as the plant transpires. Drought then flood watering causes this even when there is plenty in the pot, so water consistently.

Coco and reverse osmosis water supply no calcium at all. On either, a Cal-Mag supplement is not optional. Gypsum adds calcium without moving pH. Lime adds it and raises pH, which only helps if pH is low.

Iron and manganese

Both show on new growth as yellow between green veins. Both are usually a pH problem. The difference is in the texture of the yellow.

Iron chlorosis is clean, bright and almost lemon colored. Manganese adds small dark dead flecks scattered through the yellow, like pinpricks.

Fix pH first for either one. If you do need iron, match the chelate to your pH: EDTA below 6.5, DTPA up to 7.5, EDDHA above that. Iron sulfate does nothing in alkaline conditions.

Sulfur, zinc and boron

The rare three. All show at the top. Confirm before you dose.

• Sulfur: newest leaves turn evenly pale, veins included, sometimes with pink stems. It looks like nitrogen at the wrong end. Gypsum or Epsom salt fixes it.
• Zinc: a spacing problem. New leaves come small and bunched in a tight rosette. Excess phosphorus from bloom boosters often causes it, so cut the phosphorus first.
• Boron: the growing tip browns, goes corky and stops. Usually erratic watering. Never dose boron on a guess. The safe margin is tiny, and the damage does not wash out. Get a soil test.

Lookalikes and the one tell

The decoder, step by step

• 1. Check pH going in and coming out. Out of range? Fix that first and wait for new growth.
• 2. Check water. Lift the pot. Heavy and droopy, or light and droopy, is a watering problem, not a nutrient one.
• 3. Check temperature at the roots. Below about 60 F, suspect cold before phosphorus.
• 4. Top or bottom? Bottom points to N, P, K or Mg. Top points to Ca, Fe, S, Zn, Mn or B.
• 5. Read the pattern. Whole leaf, between veins, edge rim, tips only or flecks.
• 6. Check the lookalike table, change one thing, and judge by new growth.

Keep a record of what you tried

Fixes take a week or more to show, and it is easy to forget what you changed and when. If you would like a place to keep it, Canna GLAP lets you log each feed, add a note with your pH reading, and photograph the leaf.

Cannie can walk you through the same steps in this decoder when a leaf looks wrong, and help you tell a lookalike from the real thing.

Your journal and photos are encrypted on your phone before they are stored, and photos have their location removed. We cannot read any of it.

See Canna GLAP