Reading a Cannabis Feeding Chart: Avoid Overfeeding, Lockout and Nutrient Problems

Why most feeding charts are used wrong

Open any manufacturer's chart — BioBizz, Canna, Plagron, Advanced Nutrients, House & Garden — and you see the same thing: a grid of weeks, milliliters per liter, and four shades of green. Most growers treat it like a cooking recipe. Week 3, line 4, 3 ml/L. Done.

The problem: plants don't grow to a calendar. A chart cannot know your genetics, your light intensity, your VPD, your pot size, your substrate, your water quality, or how much your plant transpired yesterday. It's a starting point calibrated for a healthy, fully developed reference plant under manufacturer-defined conditions. Yours is almost never that plant.

How plants actually take up nutrients

Almost every beginner guide skips this — and that's why lockouts feel like mysteries. Nutrients don't jump from the bottle into the leaves. They travel a chain, and every link has to work.

This is why pH can matter more than adding more fertilizer. If pH is out of range, ions are physically present in the substrate but chemically unavailable. Pumping in more nutrients only raises EC further and makes things worse. And this is why a plant that transpires poorly (cold room, high humidity, small canopy) will show 'deficiencies' at perfectly normal EC values — the delivery truck isn't running.

The three levers that decide uptake

• pH — decides which ions are chemically available at the root surface.
• EC — decides how concentrated the buffet is and how hard osmosis has to work.
• Transpiration — decides how fast the buffet is actually delivered to the leaves (driven by VPD, light, temperature, CO₂).

How to read a feeding chart in five steps

1. Find your medium column. Soil, coco and hydro need very different EC ranges — using the soil column on a coco plant is a guaranteed lockout.
2. Locate the current week relative to the 12/12 photoperiod switch (vegetative weeks first, then flower weeks 1–8 counted from the flip).
3. Read the ml-per-liter values for each bottle: base A/B, bloom booster, CalMag, enzymes, microbes.
4. Multiply by your real water volume. Then apply the 50 % rule on the first feed of a new week.
5. Measure EC and pH of the mixed solution. Compare to the target range for your medium and stage — that's your reality check.

What each column on a chart really means

Why plants rarely feed exactly by the schedule

A chart is a single-variable plan for a many-variable system. Everything on the list below shifts the actual nutrient demand up or down — often by 30–50 %.

• Genetics — a heavy indica may run 20 % lower EC than a sativa hybrid at the same week.
• Light intensity (PPFD) — more usable light = more photosynthesis = more nutrient demand.
• Light source (LED vs HPS) — HPS runs hotter, drives more transpiration, often more CalMag.
• VPD — a plant at 1.2 kPa transpires faster than one at 0.7 kPa and will feel 'hungrier'.
• CO₂ enrichment — >800 ppm raises upper EC tolerance meaningfully.
• Pot size and root mass — a rootbound plant drains a pot in half a day and shows salt buildup faster.
• Substrate age — reused coco or soil already contains residual salts.
• Water quality — RO vs 0.6 mS/cm tap water means very different starting points.
• Canopy training — a topped, LST'd plant with 20 tops eats more than an untrained single-cola plant.

EC, pH and lockout — the three-way relationship

What is EC?

EC (Electrical Conductivity, mS/cm) measures how much dissolved salt is in the water. Every nutrient — nitrate, potassium, calcium, magnesium — is a salt in solution. More nutrients = higher EC. It's the single most useful number in cultivation because it lets you compare BioBizz, Canna and Plagron on the same scale. Two brands might print completely different ml/L values that land on the exact same EC.

EC target ranges by medium and stage

These ranges assume RO or low-mineral tap water with a background EC under 0.3. If your tap water already reads 0.5–0.7 mS/cm before any nutrients, subtract that from the target — otherwise you're feeding 0.5 mS/cm of unknown ions on top of your recipe.

What is pH — and why it decides everything

pH measures how acidic or alkaline the root zone is. Every nutrient ion has a pH window in which it stays dissolved and root-available. Move outside that window and the ion falls out of solution or binds to something else — the plant can't get it, no matter how much you added.

What is a nutrient lockout?

A lockout is when nutrients are physically present in the substrate but the plant can no longer absorb them. It looks exactly like a deficiency. The fatal mistake: adding more nutrients to fix it — which raises EC further, drives pH further off, and deepens the lockout.

Common lockout causes: pH out of range for the medium; excess salts in the root zone (high runoff EC); antagonistic ion ratios (too much potassium blocks calcium and magnesium); a dead microbiome in organic soil after over-sterilizing.

Recognizing overfeeding — beyond burned tips

Most guides only mention burned leaf tips. That's the last symptom, not the first. If you can read the earlier signs, you save the plant before any leaf gets damaged.

The overfeeding vs deficiency vs lockout decision tree

Practical feeding strategy by stage

Seedling (week 1–2)

Do almost nothing. Seedlings live off their cotyledons and any starter nutrients in the substrate for 10–14 days. Overfeeding at this stage is the #1 killer. Plain pH-corrected water is usually enough until the first true leaf pair has 3–5 fingers.

Vegetative growth

Ramp up gradually. Start at 0.6 mS/cm, add 0.1–0.2 mS/cm per feed if growth is vigorous and leaves stay healthy. Nitrogen dominates. Watch for the first sign of tip burn — that's the ceiling; back off 0.2 mS/cm and stay there for the rest of veg.

Pre-flower / stretch (flower week 1–3)

Nitrogen demand still high, potassium demand rising. Don't jump to a full bloom booster on day one of 12/12 — the plant is still building structure. Transition gradually over 7–10 days.

Peak flower (flower week 4–7)

Highest nutrient demand of the entire grow. This is where most growers can safely follow (or slightly reduce) the chart. Potassium and phosphorus dominate. CalMag becomes critical in coco and hydro because bud tissue draws heavily on calcium.

Late flower / ripen

Demand drops as the plant finishes filling flowers. Lower EC over 7–14 days. Whether to fully flush is style-dependent (see next section).

Organic vs mineral — different mindsets, not better or worse

Neither is 'better'. Organic is more forgiving but slower to correct. Mineral is more responsive but punishes overdosing faster. Choose the mindset that matches how you like to grow.

Soil vs coco vs hydro — feeding mindset

• Soil: low feed, high pH buffer. Let the substrate work. Feed every 2–3 waterings, plain pH water in between.
• Coco: feed every watering, never let it dry out completely, CalMag every feed, watch runoff EC daily.
• Hydro / DWC: weekly reservoir change, daily top-off, EC and pH every 24 h, reservoir temperature 18–20 °C.

Common mistakes and myths

The eight most common mistakes

• Feeding by the calendar instead of by the plant.
• Ignoring runoff EC — flying blind on salt accumulation.
• Ignoring pH — perfect EC at pH 4.8 is still a lockout.
• Stacking every booster the brand sells 'because it's on the chart'.
• Believing more nutrients = more yield.
• Never measuring input EC or pH before feeding.
• Watering too often, never letting the substrate breathe.
• Chasing single measurements instead of trends.

Myths worth killing

• ❌ 'Burned tips always mean overfeeding' — can also be low humidity, heat, or wind burn from a fan.
• ❌ 'Organic soil doesn't need pH management' — it needs less, not none.
• ❌ 'CalMag helps everything' — only helps when Ca or Mg is actually missing. Otherwise it just raises EC.
• ❌ 'Flushing at the end always improves smoke' — no reliable evidence. The main reason to flush late is to correct salt buildup.
• ❌ 'Higher EC = bigger yield' — only true up to the plant's genetic ceiling. Beyond that, EC only causes stress.
• ❌ 'Runoff pH doesn't matter in coco' — it's the fastest early-warning signal you have.

What an experienced grower actually does

Two plants of the same strain in the same tent, fed the exact same solution, will drink different amounts of water and produce different runoff EC. There is no chart that predicts this. What an experienced grower does is not memorize numbers — it's build a feedback loop.

1. Mix nutrients to the chart × 0.5 for the first feed of a new week. Measure input EC and pH. Log both.
2. pH-correct to the window for your medium. Log the correction.
3. Water until ~20 % runoff. Measure runoff EC and pH. Log both.
4. Look at the plant. Leaf color, tip condition, new growth rate, water uptake in the last 24 h.
5. If runoff EC > input EC + 0.3 → reduce next feed by 0.2 mS/cm.
6. If runoff pH drifts out of the window two feeds in a row → flush with pH-corrected water.
7. Only change one thing at a time. Wait 3–5 days before evaluating.
8. Trust the trend, not the single reading.

“Plants don't die from too few nutrients. They die from too many, wrong ratios, or a pH that won't let them eat.”

FAQ — 20 questions on feeding, EC, pH and lockout

How often should I feed my cannabis plant?

Depends on the medium. Soil: every 2–3 waterings, with plain pH water in between. Coco: every watering (light feed). Hydro: continuously via the reservoir. The plant, not the calendar, decides frequency — go by dry-back and EC trends.

Should I feed at every watering?

In coco and hydro, yes — at low EC. In soil, no — plain water in between prevents salt buildup and lets the microbiome work.

How do I measure EC correctly?

Calibrate the meter monthly with 1.413 mS/cm calibration solution. Rinse the probe with distilled water between samples. Measure at solution temperature 20–22 °C — cold water reads low.

When do I need CalMag?

Always in coco. Always with RO or soft water. Sometimes in hydro. Rarely in soil with hard tap water. Symptom: rust-brown spots on middle leaves + pale new growth = classic Ca/Mg deficiency.

Do I need bloom boosters?

No. A quality two- or three-part base line already contains the P and K needed for flowering. Boosters can push slightly higher yields but are the #1 cause of tip burn. Add them last, at half dose, only if the plant looks under-fed at week 4 of flower.

What's the fastest way to fix a lockout?

Measure runoff EC and pH. Flush with pH-corrected water at 3× pot volume. Skip the next feed. Resume at 50 % EC. Most lockouts clear in 4–7 days.

How do I flush correctly?

Use plain water pH-corrected to the medium's target (soil 6.5, coco 5.8). Pour slowly, in stages, until you've passed 3× the pot volume through the substrate. Let the pot drain fully. Do not feed for one full dry-back cycle.

Why do two identical plants show different needs?

Genetic variation, root architecture, and microclimate inside the canopy. Even clones drift. This is why per-plant observation always beats a shared feeding schedule.

What pH should my water be?

Soil 6.2–6.8. Coco 5.8–6.2. Hydro 5.5–6.2. Correct after mixing nutrients (nutrients change the pH), not before.

My runoff EC is higher than my input EC — is that bad?

It means salts are accumulating in the substrate faster than the plant uses them. If the gap is >0.3 mS/cm, reduce the next feed by 0.2 mS/cm. If the gap is >0.8 mS/cm, flush.

Can I mix all my nutrients together?

Add them one at a time to water, in the order the manufacturer specifies (usually CalMag first, then base A, base B, then boosters, then microbes last). Never mix concentrates directly — they will precipitate.

Does temperature affect EC?

Yes. Modern meters correct automatically to 25 °C. If yours doesn't, add ~2 % per °C below 25 °C mentally. Reservoir temperature should stay 18–22 °C — warmer breeds pathogens.

Should I flush before harvest?

Optional and debated. Reducing EC over the last 7–14 days is universally sensible. A hard flush (only plain water for two weeks) is a style choice, not a proven quality boost.

My leaves are dark green and glossy — is that good?

No. That's nitrogen excess. Healthy cannabis is a medium, matte, vibrant green. Dark and shiny = back off nitrogen at the next feed.

What if I follow the chart exactly and my plant still burns?

The chart was calibrated for a different plant. Drop to 60 % of the printed values and rebuild from there. Your plant is telling you the truth — the chart isn't.

Do autoflowers need less fertilizer?

Generally yes — about 60–70 % of what photoperiods take. Their compressed lifecycle and smaller root mass make them more sensitive to overfeeding.

Are enzymes and microbes worth it?

Enzymes: yes in reused substrate. Microbes: yes in organic soil, less critical in mineral hydro. Both are hard to overdose and cheap insurance.

How do I know if I'm underfeeding?

Uniform pale green across the whole plant, slow growth, small new leaves, older leaves yellowing from the tip inward. Bump EC by 0.2 mS/cm and re-evaluate in 3 days.

Can bad water quality cause lockout?

Absolutely. Hard tap water can add 0.5+ mS/cm of calcium bicarbonate that pushes pH upward and antagonizes potassium and magnesium. Get a water report or use RO.

What's the single most important habit?

Log every feed. Input EC, input pH, runoff EC, runoff pH, and one sentence of plant observation. Trends over 10 feeds tell you more than any chart ever will.

Bottom line

A feeding chart is a map. It tells you roughly where the road goes. But you're the one driving, and you're the only one who can see the plant in front of you. Learn to read the plant — leaf color, tip condition, growth rate, water uptake, runoff EC and pH — and the chart becomes what it was always meant to be: a starting point, not a destination.