Short answer. Feed by EC, not by the label. Start at a quarter to half the label rate: about EC 0.4–0.8 mS/cm for seedlings, 1.0–1.8 in veg and 1.6–2.2 in flower in coco and hydro, lower in soil. Raise it only while leaf tips stay clean. Feed right through to harvest. Don't flush: it starves the plant while it packs on its final weight and resin. In late flower, taper only nitrogen and keep P, K, Ca, Mg, S and the micros fed.
House position: taper nitrogen, don't flush. Pre-harvest flushing is a Myth, and an actively harmful one. Cannabis is a bioaccumulator: what it takes up is stored in tissue, and plain water in the final days doesn't "flush it out". Flushing is for toilets. Starving the plant in its most critical phase, when it is packing on most of its final weight, trichomes and resin, does far more harm than good. The GenetixBay approach is to cut nitrogen back in late flower while keeping everything else fed. The one valid flush treats the medium, not the plant: a root-zone flush for salt build-up, at any stage. Full evidence: the flush debate.
What do you need to feed properly?
Feeding properly needs a complete base nutrient and two meters. Everything else is optional.
- [ ] An EC meter reading in mS/cm, and a 1.413 mS/cm calibration solution (meter calibration)
- [ ] A pH meter with pH 4 and 7 buffers, plus pH up and down (pH made simple)
- [ ] A complete base nutrient made for your medium: a one-part or A+B mineral feed for coco and hydro, or a soil feed (mineral or organic)
- [ ] Cal-Mag only if your water is RO or very soft, or your coco is unbuffered (Cal-Mag)
- [ ] Measuring syringes (1, 5 and 10 mL) and a jug or bucket marked in litres
- [ ] A cup to catch runoff for EC checks (runoff testing)
- [ ] Your grow journal for input EC, runoff EC and what you saw
- [ ] Gloves and eye protection for concentrated nutrients and pH adjusters
You don't need a cabinet of additives, and you don't need a bloom booster.
How do you mix and feed, step by step?
Mix and feed in nine steps. The order matters: nutrients first, pH last.
- Start with your water. Measure its EC. Tap water is often 0.2–0.8 mS/cm; RO is near 0. Note it; your target is the final EC of the mix.
- Add the base nutrient at a quarter to half the label rate. Add each part to the water separately and stir between parts. Never mix concentrates together undiluted (A and B parts react).
- Add Cal-Mag first, if you need it. On RO water or unbuffered coco, add Cal-Mag before the base so the water carries about 0.3–0.4 mS/cm of it.
- Measure the EC. Compare with the stage target (table below). Add nutrient or water to adjust.
- Adjust pH last. Soil 6.2–6.8; coco 5.8–6.2; hydro 5.6–6.0. Nutrients shift pH, so always adjust after mixing.
- Feed. Coco and hydro: every watering. Soil: feed every second or third watering, or every watering at a lower EC, once the mix's starter charge runs out (usually after 2–4 weeks).
- Check runoff EC weekly. Use the pour-through method [9]. If runoff is more than about 1.0 mS/cm above input, salts are building up. If it is far below input, the plant is hungry.
- Change one thing at a time. Raise or lower EC in steps of about 0.2 mS/cm, then wait 3–5 days and judge on new growth.
- Feed through to harvest. Keep the stage strength until the chop. If you mix single salts or use per-element dosing, start the nitrogen taper about 2–3 weeks before your expected harvest (below).
What EC should you feed at each stage?
Feed EC depends on the stage and the medium. These are starting ranges for a mineral feed; soil and organic feeds run lower because the mix holds nutrients. Cannabis research shows no benefit from pushing overall strength higher [3][4].
| Stage | Coco / hydro EC (mS/cm) | Soil EC (mS/cm) | ppm500 (coco) | Notes |
|---|---|---|---|---|
| Seedling (first true leaves) | 0.4–0.8 | Water only | 200–400 | Most bagged soils need nothing yet |
| Early veg | 0.8–1.2 | 0.6–1.0 | 400–600 | Start at ¼–½ label |
| Late veg | 1.2–1.8 | 0.8–1.4 | 600–900 | |
| Transition / stretch | 1.4–1.8 | 1.0–1.4 | 700–900 | Hungry phase; watch new growth |
| Mid flower | 1.6–2.2 | 1.2–1.6 | 800–1,100 | Peak demand |
| Late flower (last 2–3 weeks) | 1.6–2.2, N tapering | 1.2–1.6 | 800–1,100 | Feed through; no flush |
| Runoff EC | Within ~0.5 of input | Within ~0.5 of input | +1.0 or more = salt build-up |
Convert carefully: 1.0 mS/cm reads 500 ppm on a 500-scale meter and 700 on a 700-scale meter (EC and ppm scales). Always talk in EC. In organic coir veg, a cannabis trial found an optimal fertiliser rate rather than "more is better" [7].
What is the nitrogen taper, and how do you do it?
The nitrogen taper cuts nitrogen back over the last 2–3 weeks of flower, towards zero near the finish, while keeping phosphorus, potassium, calcium, magnesium, sulfur and the micronutrients fed. It is what GenetixBay does instead of flushing.
Why it works: in a controlled study, lower nitrogen raised cannabinoid and terpene concentration while higher nitrogen raised total inflorescence yield [1]. It is a trade-off you set on purpose, not a reason to starve the plant. Late-flower N demand is lower anyway; the optimum N in flower was moderate in soilless trials [2].
How to do it:
- About 3 weeks before expected harvest, reduce nitrogen by roughly a third.
- About 2 weeks out, reduce it to about a third of normal.
- The last 5–7 days, nitrogen near zero; P, K, Ca, Mg, S and micros unchanged, so EC drops only a little.
A bottled 2-part can't drop nitrogen without dropping everything else. That is why the taper needs single salts or per-element dosing, the reason Jardin doses per element. On bottled nutrients, simply feed the flower formula at stage strength right to the end. Don't swap to plain water. Lower fan leaves yellowing slowly in the last 2–3 weeks is normal.
Why don't you flush before harvest?
Don't flush before harvest because it costs yield and resin and gives nothing back. Controlled trials found that pre-harvest nutrient withdrawal didn't improve flower quality, and longer withdrawal tended to cut dry mass and cannabinoid yield [5][6]. Taste and clean ash come from a healthy plant, a slow 7–14 day dry and a proper cure, not from starving it. See drying and curing.
The one valid flush is a root-zone flush for salt build-up, at any stage: runoff EC about 1.0 mS/cm or more above input, salt crust, or spreading burnt tips. Run 2–3 times the pot volume through slowly, using plain pH-adjusted water (pH 6.3–6.8) in soil or a light EC 0.4–0.8 feed with Ca and Mg at pH 5.8–6.0 in coco, then go straight back to normal feeding. Steps are in the flush debate.
What about Cal-Mag and bloom boosters?
Cal-Mag under LEDs is a Myth. LEDs don't create a Cal-Mag need. Calcium and magnesium demand is driven mainly by the cultivar, the water source (RO vs tap), the medium (coco's cation exchange grabs Ca and Mg) and root-zone pH [8]. Add Cal-Mag for RO or very soft water and unbuffered coco, not for the light (Cal-Mag).
Bloom boosters hit a plateau. Past moderate phosphorus, more P didn't raise yield or cannabinoids in controlled trials; it mostly ended up in the runoff [3][4]. See bloom boosters.
How will you know it's working?
You'll know your feeding is right when the plant looks evenly green and your EC numbers stay stable:
- Leaf colour: mid to deep green, not pale (hungry) and not glossy dark with clawing tips (too much nitrogen).
- Leaf tips: clean. A few millimetres of tan tip on a few leaves means you're at the ceiling; back off 0.2 mS/cm.
- New growth: even and steady, a new node every 2–4 days in veg.
- Runoff EC: within about 0.5 mS/cm of input, week after week.
- Late flower: lower fan leaves fade slowly from the bottom over the last 2–3 weeks, while sugar leaves and upper fans stay green.
- Reservoir (hydro): EC falls slightly and steadily day to day, meaning the plant is drinking and eating.
Deficiency pictures are in mobile vs immobile nutrients; check pH and the root zone before blaming a nutrient (how to diagnose any problem).
What mistakes do beginners make with feeding?
Beginners mostly overfeed, and then flush.
- Full label strength from week one. Labels are often written for the maximum.
- Feeding seedlings in hot soil, which already carries enough nutrients.
- Mixing by ppm without knowing the scale, which is ambiguous by 40 % (EC and ppm).
- Adjusting pH before adding nutrients.
- Treating every yellow leaf with more food instead of checking pH and watering first.
- Adding Cal-Mag "because LED" on hard tap water with a complete base feed.
- Stacking bloom boosters and additives past the P plateau.
- Flushing for the last 1–2 weeks. It starves the plant in its most critical phase.
- Reading fading fan leaves as proof a flush "worked" or as a reason to feed more in the last week.
- Doing a root-zone flush with plain water in coco, which strips Ca and Mg.
Evidence
- Nitrogen trade-off between yield and cannabinoid concentration: Supported [1][2].
- No yield gain from high P or high overall strength: Supported by controlled trials [3][4].
- Pre-harvest flushing: Myth [5][6]; house position, stated strongly.
- Optimal organic rate in coir veg: Supported [7].
- Cal-Mag under LED: Myth; Ca/Mg disorders by cultivar, water and medium: Supported [8].
- Runoff EC monitoring: Established extension method [9]. Start-low feeding and EC steps: Grower practice.
