Ecowitt's commitment to measurement transparency

Ecowitt values the trust customers place in every measurement. We therefore provide not only readings, but also a clear explanation of the measurement principle, applicable conditions, and known limitations. This FAQ is based on WH52 test records and relevant technical literature. Its purpose is to help customers interpret moisture and EC behavior under low-moisture conditions and obtain stable, repeatable results.

1. Measurement Principle

1.1 How does WH52 measure soil moisture and EC?

WH52 uses a capacitive sensor to measure soil moisture and a two-electrode method to measure bulk electrical conductivity (EC). Because these parameters are based on different physical mechanisms, they may not respond at the same rate or in the same direction after watering, drying, soil disturbance, or a change of substrate.

EC = Kcell / R

Kcell is the cell constant determined by the electrode geometry, and R is the total resistance measured by the two-electrode system. In practical soil measurement, the total resistance can be approximated as:

R = RS + RC + RE ≈ RS + RC

• · RS: bulk-soil resistance, representing conduction through the soil water phase and mobile ions.
• · RC: electrode-to-soil contact resistance, affected by electrode surface condition, air gaps, interface polarization, and soil contact.
• · RE: resistance of the metal electrodes, normally much smaller than the other components and therefore negligible.

A two-electrode system measures the series combination of these resistance components and cannot completely separate RS from RC in a single reading. Under very dry or poorly contacted conditions, RC therefore contributes directly to the EC result. This is a physical characteristic of the measurement method rather than simply a display or software issue [1-2].

2. Readings at Low Moisture Levels

2.1 Why can EC read low, zero, or fluctuate in dry soil?

EC measurement depends on ions moving through continuous water pathways in the pore space. As soil moisture decreases, water films around soil particles become thinner and discontinuous, the proportion of air in the pores increases, and ionic pathways are reduced. At the same time, the effective contact area between the electrodes and soil decreases, causing RC to rise significantly. Because EC is inversely proportional to total resistance, the calculated EC may decrease, remain at zero, or fluctuate [1-2].

This behavior does not automatically indicate probe failure. Ecowitt recommends checking soil moisture, probe contact, and stabilization time before evaluating the device condition.

2.2 At what moisture level is EC generally more stable?

Under the default calibration, existing tests indicate that WH52 EC readings are generally more stable at approximately 15% moisture or above in sandy soil, 14% or above in potting or nutrient soil, and 16% or above in loam. These values are test-based references, not universal guaranteed thresholds. Particle-size distribution, bulk density, salinity, compaction, organic matter, and installation conditions can all affect the result. As a practical secondary check, the soil should normally be moist enough to form a clump that does not immediately fall apart; however, trend observation under the actual soil condition remains essential.

2.3 Why can the moisture reading temporarily decrease after adding a small amount of water?

In air-dried soil, a small addition of water may initially become bound to soil-particle surfaces. Bound water has a different dielectric response from free water and may increase dielectric damping. If the change in interference temporarily exceeds the useful signal produced by the added water, a capacitive sensor may show a short-term decrease even though water has been added [5]. Add water evenly, allow sufficient redistribution time, and evaluate a continuous trend rather than a single instantaneous value.

3. Watering and Substrate Effects

3.1 Why should readings be allowed to stabilize after watering or disturbing the soil?

After watering, moisture must redistribute through the pore network. Disturbing the soil or reinserting the probe also changes air gaps, compaction, and interface contact around the sensor. Stabilization time varies with soil type, water volume, temperature, and compaction, so one fixed waiting period is not appropriate for every application. Consider the reading stable when several consecutive measurements show only small changes.

3.2 Why can EC continue to increase after watering potting soil?

As water fills the pore space, the substrate develops more continuous conductive pathways. Watering can also change the ion-exchange equilibrium between substrate colloids and the pore solution, releasing ions such as calcium, magnesium, potassium, and ammonium into solution. EC in potting soil or soilless substrate may therefore continue changing after watering. This reflects the combined effects of substrate properties, moisture, and soluble salts and should not, by itself, be interpreted as sensor malfunction [4].

4. Obtaining Reliable Measurements

4.1 How can measurement stability and repeatability be improved?

• Select a representative location and avoid stones, large roots, and obvious voids.
• Maintain consistent insertion depth, orientation, and soil compaction, and ensure uniform contact between the probe and soil.
• Do not make decisions from a single reading immediately after watering, soil disturbance, or probe reinstallation.
• Compare trends in the same soil type under similar moisture and calibration conditions; do not directly compare absolute EC values across different substrates.
• Keep the probe surface clean. If abnormal readings persist after contact is improved and sufficient stabilization time is allowed, contact Ecowitt Technical Support.

4.2 Does low-moisture EC error affect normal agricultural use?

Extremely low moisture already causes soil water films to break, reduces nutrient mass flow and diffusion, and limits root water and nutrient uptake [3]. WH52 EC data is therefore most suitable for trend monitoring when the root zone has continuous moisture pathways and the reading has stabilized. Instantaneous EC values in dry soil should be interpreted cautiously and should not be used as the sole basis for irrigation or fertilization decisions.

References

The following publications support the discussion of two-electrode contact resistance, moisture-dependent EC behavior, drought-related nutrient transport limitations, and bound-water effects in dielectric measurement.

• 1. Li, Y., Zhu, C. Q., & Meng, Q. S. (2023). Contact Resistance between Calcareous Sand and Electrodes Based on the Two-Electrode Method. Advances in Materials Science and Engineering, 2023, 1027533.
• 2. Orozco-López, S., et al. (2012). Laboratory measurement of soil electrical resistivity: Two-electrode vs. four-electrode methods. Geotechnical Testing Journal, 35(4), 567-576.
• 3. Gao, Y., et al. (2026). Carbon-Nitrogen Interplay Under Drought Stress: Mechanisms of Dual Limitation. Journal of Plant Ecology. https://doi.org/10.1093/jpe/rtag022
• 4. Scoggins, H. L., & van Iersel, M. W. (2006). In situ probes for measurement of electrical conductivity of soilless substrates: Effects of temperature and substrate moisture content. HortScience, 41(1), 210-214.
• 5. Dyck, M., Miyamoto, T., Iwata, Y., & Kameyama, K. (2019). Bound Water, Phase Configuration, and Dielectric Damping Effects on TDR-Measured Apparent Permittivity. Vadose Zone Journal, 18(1), 190027.