Expectation
- Install sensors.
- Define optimal moisture thresholds.
- Start irrigation when readings fall below the low threshold.
- Stop irrigation when readings exceed the high threshold.
- Irrigation becomes automatic and optimal.
Reality
- Different sensors show different values in the same soil.
- Readings often look noisy or chaotic.
- Results depend on installation quality, soil type, depth, placement, and calibration.
- Proper installation requires expert-level knowledge.
- Over time, people stop trusting the numbers and start overriding the system.
The reasons
The problem is not the sensor itself. VWC sensors measure capacitance, not soil moisture directly.
This leads to several fundamental limitations:
- It is not possible to derive true soil moisture from capacitance alone, because different soil types have different dielectric properties. 20% VWC in clay soil and 20% VWC in sandy soil mean completely different things.
- To interpret readings correctly, the system must know the soil type — but soil composition often changes even within a single farm or field.
- Water content is not the same as plant-available water. The same VWC can result in very different water availability for roots.
- Low-cost FDR sensors typically have limited accuracy and stability.
More precise TDR sensors exist, but their cost makes large-scale deployment impractical.
Soil moisture sensors can provide useful trends and local signals, but expecting them to deliver reliable, absolute irrigation decisions on their own is unrealistic.