Nutrient Deficiencies, Lockout and Flushing: A Visual Triage Guide

Is it a nutrient deficiency or nutrient lockout? Often it is lockout: the feed is in the pot, but a wrong root-zone pH stops the plant taking it up. Yellowing on the old, lower leaves points to nitrogen; trouble on fresh top growth points to calcium or iron. Check pH first, read your runoff EC, and flush only if salts have built up.

When yellow leaves appear, don't panic-feed — reaching for more bottles usually makes it worse. Nutrient lockout is when a plant can't absorb nutrients that are physically present in the root zone, usually because the pH there has drifted out of the range that keeps those nutrients available. Work it in order:

1. Check pH of feed and runoff against target (pH, EC and water-quality guide).
2. Read runoff EC against feed EC.
3. Flush only if runoff EC is much higher.

Who this guide is for: UK home and hobby growers in soil, coco or hydro, staring at discoloured leaves and wanting a calm way to tell a deficiency from a pH problem — and fix it. Overwatering, pests and root rot: see our yellow leaves, brown spots and root rot guide.

How do you tell a nutrient deficiency from lockout? A symptom-to-cause triage table

Start by asking: where on the plant is the problem? Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) move inside the plant, so a shortage is pulled from the old, lower leaves to feed new growth: deficiencies show bottom-up. Immobile nutrients (calcium, iron, sulphur, boron, copper, manganese) can't move once laid down, so symptoms hit the newest top growth (Cornell University; University of Maryland Extension). See mobile and immobile nutrients.

Is it a deficiency or a lockout? Why pH is usually the real culprit

A deficiency (the nutrient isn't there) and a lockout (it's there but unavailable) look almost identical; you tell them apart by root-zone pH, not the leaf.

Nutrient availability is governed by pH: in soil, most nutrients are readily available between about pH 6.0 and 7.0, while iron and manganese drop out of reach above pH 7.0 (University of Maryland Extension). Off-range, the plant starves in a full larder, which is why our first move in store is a Bluelab pH Pen, not another bottle.

Target root-zone pH depends on your medium (see soil vs coco vs hydro for choosing one):

So the order is always symptom → pH → feed. If pH is spot-on and the symptom persists, treat it as a real deficiency; if pH is off, correct that first — the "deficiency" often reverses within a week. Keep Growth Technology pH Buffer to check your pen's calibration.

What else causes nutrient lockout besides pH?

If pH is right and the plant still struggles, look at these:

• Antagonism. Heavy potassium can compete with magnesium and calcium uptake.
• Reservoir precipitation. White sediment or cloudy solution means nutrients are dropping out; remix (reservoir management guide).
• Mixing order and add-ons. Mix base feed and Cal-Mag, then adjust pH. Stacking boosters on a full feed builds salt (bloom boosters and PK).
• Cold and thin air. A cold reservoir or root zone, roughly below 18 °C (64 °F), can slow uptake, phosphorus in particular. Dry winter air can also mimic a deficiency: see our VPD guide and winter grow room tips.

What does your runoff EC tell you?

Runoff drains from the pot after you water to excess. Measure its EC (electrical conductivity, a proxy for dissolved salt) and pH: it's the closest thing a home grower has to a soil test.

It's a comparison. If runoff EC is much higher than the feed, salt is accumulating — the setup for burn and lockout; if it's much lower, it's stripping feed fast and may want more. And if runoff pH sits well off target (say 5.3 or 7.3 when you feed at 6.2), you've confirmed a pH problem driving lockout (Ripper Seeds; Trym).

You'll need an EC/CF meter — a Bluelab EC Pen works to start, but a stick meter like the Bluelab Truncheon is the workhorse most UK growers use for daily checks. Calibrate pens with Growth Technology EC Standard 2.76.

How do you flush a plant properly — and when should you?

Flushing runs a large volume of clean, pH-corrected, very-low-EC water through the medium to wash out accumulated salts — a reset for lockout, not a "just in case" routine.

Flush when runoff EC climbs well above feed EC, when you've overfed, or to reset a locked-out medium. Use two to three times the container's volume in clean, pH-corrected water — about 22–33 L for an 11 L pot, 38–57 L for a 19 L pot, 54–81 L for a 27 L pot. As salts clear, runoff EC falls toward your flush-water EC, ideally near 0.2–0.5 mS/cm (roughly 100–250 ppm on the 500 scale) (Ripper Seeds). A flushing agent such as T.A. Flash Clean dissolves built-up fertiliser salts; follow its label.

After a flush, let the medium dry back before feeding again — waterlogged roots can't recover — then feed at half strength, building back up with a stage-by-stage feeding schedule. Don't flush a plant that's simply hungry; you'll wash out the little it has.

What causes leaf tip burn, and is it different from a deficiency?

Yes — confusing the two makes things worse. Leaf tip burn is an excess problem: too much mineral salt overwhelms the roots and scorches tissue by osmotic stress, so tips go yellow then crispy brown (Royal Queen Seeds; Atlas Scientific). Its cousin clawing, leaves curling down at the top, signals nitrogen toxicity from overfeeding. The fix for both: cut feed strength, check runoff EC, and flush if salts have built up. Crispy tips and pale lower leaves together can mean lockout. If the leaves show spots, pests or mould instead, see pest and disease prevention or Disease Control.

Do Superthrive or Fish Mix actually help a plant recover?

Rescue tonics play a modest role: they support a stressed plant while you fix the pH or salt problem; they don't fix it for you.

Superthrive is a long-established vitamin-and-hormone tonic. Its label lists the plant hormone 1-Naphthyl Acetic Acid at 0.048% plus vitamin B1 (thiamine), to ease transplant shock and support rooting rather than feed the plant (manufacturer label). Treat it as a support act, not a cure.

BioBizz Fish Mix is a different tool: an organic fish-emulsion feed with an NPK of roughly 5-1-4, the strongest nitrogen source in the BioBizz range (BioBizz). A nitrogen shortage shows bottom-up, so a gentle organic feed like Fish Mix is a sensible recovery step once pH is confirmed in range, feeding soil microbes rather than dumping raw salt. See choosing a nutrient line for the base feed. For coco or RO-water growers, BioBizz Calmag or Canna Calmag Agent covers the calcium/magnesium side. To rebuild a tired medium's biology (see root-zone health), insect-frass like Ecothrive Charge adds beneficial microbes alongside a gentle, balanced NPK of roughly 3.5-2.5-2 (Ecothrive).

FAQ

What is nutrient lockout in plants, and how is it different from a deficiency? Nutrient lockout is when a plant can't absorb nutrients physically present in the root zone, usually because the pH has drifted out of range — the food is there, but the plant can't take it up. A true deficiency means the nutrient isn't there. They look identical on the leaf, so you tell them apart by checking pH and runoff EC, not the leaf alone.

Can you fix nutrient lockout without flushing, and how long does recovery take? Often, yes. If the lockout is purely pH-driven and salts haven't built up, correcting your feed and water pH restores uptake on its own. Pause feeding, give plain pH-corrected water, then reintroduce feed at half strength. Judge recovery by new growth; damaged leaves rarely turn fully green again.

How do I flush a plant, and how much water do I use? Run clean, pH-corrected, low-EC water — two to three times the container's volume, so roughly 22–33 L for an 11 L pot — until runoff EC falls toward your flush-water EC. Then dry back and feed again at half strength.

Why are my plant's leaves purple? Purple or dark older leaves can mean a phosphorus shortage, but cold roots, stress and plant genetics can also produce the colour (Cornell University). Check root-zone temperature and pH first, and add phosphorus only if both are fine.

Does Superthrive fix nutrient deficiencies? No. Superthrive is a vitamin-and-hormone tonic for easing stress and supporting roots, not a fertiliser that corrects a shortage. It can help a plant hold on while you fix the real cause.

Recommended kit

Gear we'd point a UK grower to when leaves start turning:

• Bluelab Truncheon: a stick meter for daily EC checks.
• BioBizz Fish Mix: gentle organic nitrogen once pH is in range.
• Shogun Cal-Mag: for calcium and magnesium issues in coco or RO water.
• Ecothrive Charge: insect-frass conditioner after flushing.
• Disease Control: for pest or pathogen damage, not feeding.

Most relevant collection: if you click one link, make it pH and EC testers.

Why trust The Grow Superstore

We've helped UK indoor growers since 2009, online and from our physical store at 6 Wulfric Square, Bretton, Peterborough PE3 8RF. "Yellow leaves — what do I feed?" is one of the questions we field most, and the answer is always the same: check pH and runoff before reaching for another bottle. Pop in with a leaf photo and runoff sample for a free consultation, and we'll help you tell deficiency from lockout.