Root-Zone pH

Definition

Root-Zone pH is the negative logarithm of the hydrogen-ion activity of the nutrient solution at the roots — a dimensionless 0–14 scale where 7 is neutral. It is sensed directly by glass-membrane combination electrodes on the PhytoSense Mesh in the supply line and at representative root zones, each temperature-compensated and referenced to two-point buffer calibration. pH is logarithmic, so each whole unit is a tenfold change in acidity; small drifts carry large consequences for what the plant can absorb.

Why it matters

pH is the gatekeeper of nutrient uptake. Even a perfectly balanced feed at the right EC (VC-MET-107) becomes unavailable if pH drifts: iron, manganese and phosphorus lock out above ~6.5, while calcium and magnesium become scarce below ~5.5. Off-band pH therefore produces deficiency symptoms with no fault in the nutrient recipe at all, quietly dragging on the Canopy Vitality Index (VC-MET-001). CHLORA trims acid or base into the loop to hold pH in band, and reads pH drift rate as an early indicator of biofilm or root-exudate load.

Formula

A directly measured signal. Reported pH is the electrode reading after temperature compensation and two-point slope calibration:

pH = 7 − ( E_meas − E_offset ) / S(T)

  E_meas    measured cell potential (mV)
  E_offset  zero offset from pH 7.00 buffer
  S(T)      Nernstian slope, temp-corrected
            S(T) = (2.303·R·(T+273.15)) / F  ≈ 59.2 mV/pH @25°C

Calibration:
  two-point with pH 4.01 / 7.00 buffers; slope must be
  92–102% of theoretical or the electrode is flagged.
Drift watch:
  dpH/dt beyond ±0.3 /h flags biofilm or reference fouling.

Inputs

Units & Scale

Reported on the dimensionless pH scale to one decimal place. Because pH is logarithmic it is never averaged across very different solutions; the supply-line value is the operational figure, with root-zone pH reported as a uptake-environment check. Confidence degrades to DEGRADED when an electrode slope falls outside 92–102 % of theoretical, when supply and root-zone pH diverge by > 0.6 units, or when the drift rate exceeds ±0.3 pH per hour (electrode or biofilm issue).

Sampling & Source

• Sampled every 5 minutes from the PhytoSense Mesh pH electrodes in supply and root zone.
• Nernst-compensated using the inline solution-temperature reading.
• Stale handling: an electrode silent > 15 min is dropped; loss of the supply electrode suspends pH dosing with a DEGRADED flag and holds the last valid set-point.
• Calibration: twice-weekly two-point buffer check (4.01 / 7.00); slope or offset out of tolerance raises a metrology work order.

Thresholds

Recent Trend

Zone-A supply-line pH, last 14 sampling roll-ups:

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — pH dosing & electrode-maintenance procedures.
• pH-excursion response & electrode replacement on CRIT — see Environment SOP set in the SOP Library.
• Acid/base dosing interlock & alarm escalation — Monitoring Systems.