Root-Zone Temperature

Definition

Root-Zone Temperature is the temperature of the growing medium (or recirculating nutrient solution, in hydroponic loops) at root depth, in degrees Celsius. It is sensed directly by sealed thermistor or RTD probes on the PhytoSense Mesh, buried at mid-root depth in representative containers and inline in the hydro return. Because the root zone has high thermal mass and lags the air, it is tracked separately from air temperature (VC-MET-100) rather than inferred from it.

Why it matters

Roots run on temperature. Below ~15 °C, water-uptake kinetics and nutrient transport slow until the canopy wilts in fully moist substrate — a chilling-induced drought — and membrane function degrades. Above ~28 °C, dissolved oxygen falls (warm water holds less), respiration outpaces supply, and the warm-wet root zone becomes ideal for Pythium and Phytophthora. Root-zone temperature therefore couples tightly to dissolved oxygen (VC-MET-110) and substrate moisture, and a warm excursion is one of CHLORA's leading root-rot precursors.

Formula

A directly measured signal. Zone value is the representative-probe trimmed mean with node calibration applied:

Trz_zone = trimmed_mean( { Trz_i · g_i + b_i } )

  Trz_i   raw probe reading (°C), node i
  g_i,b_i span/offset from annual bath calibration

Coupling watch (advisory, not in the band):
  DO_sat(T) falls ~2% per °C of warming → cross-check
  Dissolved Oxygen (VC-MET-110) on any warm excursion.
Rot-risk flag:
  Trz > 26 °C AND VWC > 45% vol for ≥ 30 min raises a
  warm-wet root-rot precursor advisory.

Inputs

Units & Scale

Reported in degrees Celsius to one decimal place. Root-zone temperature is intensive and not summed; zone roll-up is the representative-probe trimmed mean, with the hydro-loop RTD as an advisory cross-check. Confidence degrades to DEGRADED when fewer than two probes report or when substrate and hydro-loop temperatures diverge by > 3 °C (a stuck probe or poor solution mixing).

Sampling & Source

• Sampled every 5 minutes from the PhytoSense Mesh buried-probe set and hydro-loop RTD.
• Tracked independently of air temperature owing to the root zone's thermal lag.
• Stale handling: a probe silent > 15 min is dropped; loss of the buried set fails over to the hydro-loop RTD with a DEGRADED flag.
• Calibration: annual bath check; drift > 0.3 °C raises a metrology work order.

Thresholds

Recent Trend

Zone-A buried-probe root-zone temperature, last 14 sampling roll-ups (°C):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — root-zone conditioning & rot-prevention procedures.
• Warm-wet root-rot response on CRIT — see Environment SOP set in the SOP Library.
• Reservoir-chiller interlock & alarm escalation — Monitoring Systems.