Short answer

Flowering cannabis needs less nitrogen and more phosphorus and potassium than it did in veg. A workable schedule: weeks 1 and 2 at half bloom strength, weeks 3 through 6 at full bloom strength, weeks 7 and 8 at half strength, then plain pH-adjusted water for the last 7 to 14 days. Target pH 6.0 to 6.5 in soil and 5.8 to 6.2 in coco or hydro. Target EC 1.6 to 2.4 mS/cm, which reads 800 to 1200 PPM on the 500 scale.

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What changes when flowering starts

Nitrogen drives leaves and stems. During veg the plant wants a lot of it. After the light cycle flips to 12/12, demand for nitrogen drops and demand for phosphorus and potassium rises. Phosphorus supports bud site formation and root energy. Potassium moves sugars and water, and it affects flower density and resin. Calcium and magnesium stay in the mix at the same rate. Sulfur and the micronutrients come from the base nutrient line.

cannabis nutrient deficiency schedule signs

Most bloom bottles list an NPK ratio near 2-8-4 or 5-15-14. That is roughly half the nitrogen and two to three times the phosphorus of a veg formula. You do not need to buy a separate bloom line if your base nutrient has a two or three part system, because the part ratios shift on their own.

cannabis nutrients schedule ph and ec

Week-by-week schedule

  1. Weeks 1 and 2 (transition): Feed half the bloom dose. Keep some veg nutrient in the mix if your line allows it. Stretch happens here and the plant still uses nitrogen.
  2. Weeks 3 and 4 (early flower): Move to full bloom dose. Bud sites set. Add a phosphorus and potassium booster if you use one.
  3. Weeks 5 and 6 (peak flower): Hold at full dose. This is the heaviest feeding window. Add magnesium if leaves show interveinal yellowing.
  4. Weeks 7 and 8 (ripening): Cut to half dose. Drop nitrogen first. Keep potassium.
  5. Final 7 to 14 days: Water only, pH adjusted. No nutrients.

Autoflowers do not follow the same clock. Start counting from the day pistils show, not from seed. A 10 week auto may only have a 3 week peak window.

cannabis nutrient deficiency schedule signs

How much to feed

Batch mix in a reservoir and check EC before you pour. If runoff EC is more than 0.4 above input, you are feeding too high. In soil, feed every other watering. In coco, feed every watering with 10 to 20 percent runoff. In deep water culture, change the reservoir every 7 to 10 days. Keep water between 65 and 70 degrees F. Warm water holds less oxygen and invites root rot.

pH and EC targets

Calibrate your pH pen every two weeks. A pen that reads 0.3 high will lock out phosphorus at the root zone and the leaves will not show it for a week.

Deficiency and burn signs

Phosphorus shortage shows as dark green leaves with purple stems and slow bud growth. Potassium shortage shows as brown edges on upper fan leaves. Magnesium shortage shows as yellow between the veins on middle leaves. Nitrogen excess shows as dark waxy leaves, clawing tips, and delayed ripening. Tip burn, crisp leaf edges, and a white salt crust on the pot mean the dose is too high. Flush with plain water at three times the pot volume and resume at 60 percent strength.

Flush and harvest

Stop all nutrients 7 to 14 days before chop. Soil holds salts longer than coco, so go 10 to 14 days in soil and 7 to 10 in coco or hydro. Judge the harvest window by trichomes, not by the schedule. Clear or cloudy heads with a few amber is the common target. A fixed calendar week count is a starting point, not a rule.