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Case File No. 28-NU01
Subject: pH Stabilization
Investigating Officer: Matty Ridge, Lead Lockout Detective
1. The Hook & Incident Log — “The Swing that Broke the Grow”
09:30 — Grower emails: “Runoff is 5.4. I fixed it with pH Up!”
18:00 — Grower emails: “Runoff is now 7.2. Why are the stems still purple?”
Conclusion: It wasn’t the nutrients, mate. It was the bloody pH rollercoaster. The constant swing shattered the root zone’s ability to absorb anything, leading to a total system crash.
Nutrients are expensive. Lights are expensive. Your whole system works—the engine runs, the pump pushes, the lights shine—until pH doesn’t. pH is the unseen gatekeeper. If the gate’s locked, it doesn’t matter how much high-grade fertiliser you shove at the plant. It starves.
2. The Core Science — pH and the Nutrient Availability Tax
The pH scale is a logarithmic ruler for hydrogen ion activity. For every single whole number you drop (e.g., 7 → 6), the solution is ten times more acidic. Go from 7 to 5? That’s a 100× increase in acidity. Respect the logarithmic scale, or your roots will pay the price.
The “Sweet Spot” Lie
The central truth about pH is that it’s not a single perfect number—it’s a moving availability window. Each major nutrient (N, P, K) and every micronutrient (Fe, Mn, B) has a different pH range where it is chemically available for the roots to absorb.
[[infographic:ph-availability-window]]
Foreman’s Insight:
“Stop chasing 6.3 like it’s a jackpot. Your plants don’t want perfection—they want consistency. Oscillate the pH within the ideal range so all nutrients get their turn.”
3. The Command Chain: Target pH by Media
The optimal range varies widely depending on what you’re growing in. Soil has natural biological buffers; hydro/coco has none, demanding tighter control.
| Medium | Target pH Range | The Foreman’s Logic |
|---|---|---|
| Hydro / Coco | 5.6–6.2 (oscillate 5.7–6.1) | Keeps P and micros available; a gentle swing prevents micronutrient lock-out. |
| Soil / Peat | 6.2–6.8 | Buffered chemistry + biology; supports Ca/Mg while keeping Fe/Mn accessible. |
| Living Soil / Organic | 6.2–7.0 | Microbes mediate availability—avoid drastic pH correction; steer with inputs, not acids. |
Foreman’s Warning:
“Flat-lining your pH kills rhythm. Let it breathe within the range. If you hold 6.0 steady, you will lock out micronutrients. Oscillate or suffer.”
4. Field Report: When pH Fails — Diagnosing Lock-Out Through Colour
Your plants are a biological dashboard. Their colour tells you exactly what pH mistake you made.
The Purple Problem: When pH (or Cold) Blocks Phosphorus
Purple stems and leaf anthocyanin can be genetics—but in production rooms they’re often a stress signal. In hydro/coco, phosphorus availability drops sharply below ~5.5 pH; if your input sits at 5.3–5.5 for days, expect P-deficiency traits. Cold root zones or low Leaf Temperature (LT) (sub-20 °C media) also suppress P mobility and can trigger the same purple look.
Action: Keep hydro/coco in the 5.6–6.2 window, maintain substrate 20–22 °C, and verify that EC isn’t sky-high (salt lock can mimic pH issues).
[[photo:purple-stems-ph-54-lt-19c]]
| Symptom (Colour) | Likely pH Error | Nutrient Lock-Out |
|---|---|---|
| Purple Stems / Dark Leaves | Too Low (< 5.8) or Cold LT/Root Zone | Phosphorus (P) |
| Top Yellowing (Interveinal Chlorosis) | Too High (> 6.8) | Iron (Fe), Manganese (Mn) |
| Rust Spots / Crisp Margins | Varies / Wild Swings | Calcium (Ca), Magnesium (Mg) |
5. Advanced Protocols: The Foreman’s pH Correction Kit
- pH Down: Use Nitric Acid in vegetative growth (adds N) and switch to Phosphoric Acid in flower (adds P for bud set). Avoid sulfuric acid in horticulture mixes.
- pH Up: Use Potassium Silicate (preferred) or Potassium Hydroxide. Silicate adds K and Si, strengthening tissues while you correct pH.
Safety Protocol (Acids & Bases):
Always add acid to water, never water to acid. Wear gloves/eye protection. Never add pH Up and pH Down back-to-back—mix thoroughly between adjustments. Ensure ventilation; acid fumes corrode metal and sensors.
Titration Protocol:
Titrate in 0.1 increments. Test the water, add one drop, mix, wait 10 minutes, then re-test. Patience beats panic. Drastic corrections send shockwaves through the root zone.
RO Water Mandate: Defeating Alkalinity
Most growers measure pH but ignore alkalinity. Alkalinity is the water’s ability to resist pH change (the carbonate buffer, CaCO3). If your tap water is hard (pH 8.0 and high EC), you are fighting a losing battle.
Targets: Source water alkalinity ≤ 60 ppm as CaCO3. If higher, use RO and re-mineralise to ~0.3 EC before nutrients.
6. Case Closure Log: The Tap Water Trap (Real-Life Example)
This is the most common crime scene in the grow room:
Grower starts with hard tap water (pH 8.0) and measures the pH of their nutrient solution at 6.0. Success! However, the water’s underlying Calcium Carbonate buffer is enormous. Overnight, that buffer aggressively consumes the acid, pulling the pH back up to 7.0 or higher.
This daily swing locks out Iron and Manganese, leading to chronic upper yellowing. The grower increases feed strength, worsening the problem. You can’t out-pH carbonate.
Matty’s Final Fix: Measure EC and alkalinity, not just pH. Filter your water source with RO to eliminate the carbonate buffer, giving you the control required to hit the targets in the Command Chain. Control the source, control the grow.
Final Verdict: pH is the foundation. Master the window, understand the buffers, and you eliminate 90% of nutrient problems instantly.
Filed and signed,
Officer Ridge — Dept. of Bud & Order
Methodology: Ranges validated across multiple 2×4 and 4×4 test benches, quality LEDs, ambient CO₂. Hydro/coco data assumes daily pH checks; soil assumes buffered media.