The Cultivation CanonTabatabaei Advising
The Water & Fertigation Room

SOP — Feed Mixing & EC/pH Calibration

The step-by-step flagship procedure for mixing a repeatable, precipitate-free batch and hitting your target EC and pH every time.

5 min read

This is the flagship procedure for the water room. Run it the same way every batch and your feed becomes a controlled variable instead of a source of noise. The order of operations is not a suggestion — it is the chemistry that keeps calcium in solution.

Standard Operating Procedure
Feed Mixing & EC/pH Calibration SOP
Goal: Mix a repeatable, precipitate-free nutrient batch and hit target EC/pH.Cadence: Every batch
  1. 1Test source water first: record source EC and pH. Feed EC targets assume RO/soft (source <0.4 mS/cm); if hard, subtract source EC from your target and reduce dose.
  2. 2Calibrate meters against fresh standards (EC 1.413 mS/cm; pH 4.0 & 7.0) — an uncalibrated meter is the silent cause of 'mystery' lockout.
  3. 3Fill the mixing tank to ~90% with water and start agitation.
  4. 4If you run SILICA, add it FIRST of all, alone, to the raw water (very high pH) and let it fully disperse.
  5. 5Add PART A (the calcium / nitrate side) and dissolve fully.
  6. 6Add Mg / Epsom next if it is a separate supplement, and dissolve fully.
  7. 7Add PART B (the phosphate / sulfate side) LAST. Never combine concentrated calcium with concentrated phosphates/sulfates — you will drop insoluble gypsum.
  8. 8Top off to final volume and let the mix fully homogenize. Printed grams/mL are only a starting point — you dial to measured EC, not to a fixed weight.
  9. 9Measure EC and adjust to the stage target (veg ~1.4–2.0; flower 2.0–2.8). Add water to lower, salts to raise; re-mix and re-read.
  10. 10Adjust pH LAST, after EC is set (adding acid/base shifts EC slightly). Target 5.8–6.2 coco / 5.5–6.0 rockwool.
  11. 11Log the batch: date, source EC, final EC, pH, volume, recipe — your crop-steering audit trail and first stop when diagnosing drift.
  12. 12Manage by runoff, not just feed: check runoff EC each irrigation. Runoff should sit ~feed EC + 0.3 mS/cm; much higher means salts are stacking — increase runoff.

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