Soil
Three probes buried in pots. For the first time, we can see what’s happening below the surface — and correlate it with the atmospheric conditions above.
Six readings from three probes: moisture, temperature, and EC for south probe 1; moisture and temperature for south probe 2 and the west probe. The south canna lilies hold 60–70% moisture (well-watered by the daily wall drip). The west pot sits lower at ~50% — may need attention depending on what’s planted there.
Moisture Trends
All three probes on one chart. Watch for the sawtooth pattern: the 6 AM wall drip cycle spikes moisture, then it gradually declines through the day as plants transpire and soil evaporates. The decline rate tells you how hard the plants are working — steep drops on hot, high-VPD days when atmospheric demand pulls moisture from every surface. Flat holds on cool, humid days when transpiration is minimal.
Temperature and EC
Left: soil temperature tracks air temperature with a lag — the soil acts as a small thermal buffer. South canna lily soil runs 5–10°F cooler than air (evaporative cooling from wet soil). The west pot tracks closer to air temp (less thermal mass in a smaller pot). Right: electrical conductivity from the SEN0601 on south probe 1. Currently 140–150 µS/cm — low to moderate. Canna lilies are moderate feeders. Low EC suggests the fertilizer line hasn’t been used recently. When fertigation runs, EC should spike then gradually decline as the plant consumes nutrients.
The Correlation Story
Soil moisture vs air VPD — this is where it gets interesting. When VPD spikes (dry air aggressively pulling moisture from leaves), plants draw more water from the soil. If soil moisture drops while VPD is high, the plants are under dual stress — atmospheric AND root zone. This chart is the early warning system: diverging lines mean trouble before the plants show visible symptoms.
At 5,000 feet with 14% outdoor RH on spring afternoons, atmospheric demand is extreme. The soil probes finally let us see whether the plants can keep up.
Probe Inventory
| Probe | Model | Zone | Modbus Addr | Placement | Measures |
|---|---|---|---|---|---|
| South 1 | DFRobot SEN0601 | South | 7 | Canna lily pot #1 | Moisture, Temp, EC |
| South 2 | DFRobot SEN0600 | South | 8 | Canna lily pot #2 | Moisture, Temp |
| West | DFRobot SEN0600 | West | 9 | Random plant pot | Moisture, Temp |
All probes: IP68, 316 stainless steel, designed for permanent soil burial. RS485 Modbus RTU, 9600 baud, daisy-chained on the same bus as the 6 air climate probes. 30-second update interval. Installed 2026-03-28.
Future: Demand-Based Irrigation
These probes enable condition-based irrigation instead of time-based. The current wall drip runs daily at 6 AM regardless of whether the soil is wet or dry. With moisture data, the system can:
- Skip irrigation when moisture is above threshold (save water)
- Extend irrigation when moisture drops below threshold (prevent wilt)
- Predict demand by correlating moisture decline rate with VPD and ET₀
- Schedule fertigation using EC trends — fertilize when nutrient concentration drops
The SEN0601’s EC measurement on south probe 1 enables fertigation monitoring — tracking nutrient concentration in the root zone over time and correlating with plant growth stage and water delivery.
→ See Irrigation for the water delivery systems these probes will eventually control. → See Humidity for the atmospheric VPD that drives root zone water demand.