How to Calculate Pore Water Electrical Conductivity from Bulk Soil EC

Are you a grower looking to understand how to calculate pore water electrical conductivity from bulk soil EC? In this guide, we explain the Hilhorst model, walk through the math, and show how wireless LoRaWAN sensors like the SenseCAP S-2108 automate this calculation in real time.

What Is Pore Water Electrical Conductivity?

Pore water electrical conductivity (pwEC) is a measure of the ability of water in the soil or substrate pores to conduct an electrical current. It reflects the concentration of dissolved nutrients and salts available to plant roots — making it a more precise indicator of plant-available nutrition than bulk EC alone.

The term "pore water" refers to water that fills the voids between substrate particles and is not bound to the solid matrix. This water conducts electricity at a different level than the bulk substrate, which includes solid particles and air pockets.

To measure pwEC, sensors first determine bulk EC (σb) and volumetric water content (VWC), then apply the Hilhorst equation to calculate the pore water value. Modern LoRaWAN sensors like the SenseCAP S-2108 perform this calculation automatically and transmit results wirelessly to a cloud dashboard.

Why Measure Pore Water EC Instead of Bulk EC?

Bulk EC measures the total electrical conductivity of the substrate — including solid particles, air, and water. It gives a general picture of salinity but does not accurately reflect the nutrient concentration available to roots, especially at varying moisture levels.

Pore water EC isolates the conductivity of the water phase, giving a direct reading of the soluble salt and nutrient concentration that roots actually experience. This distinction is critical in soilless systems (rockwool, coco coir, peat) where substrate moisture fluctuates significantly between irrigation events.

High pwEC can indicate salt accumulation or over-fertilization. Low pwEC may signal nutrient deficiency or excessive dilution from irrigation. Neither condition is reliably detected from bulk EC alone.

Factors Affecting Pore Water EC Measurements

Several factors influence the accuracy of pwEC readings:

• Cation and anion exchange capacity — substrates with high exchange capacity (e.g., peat, clay) can buffer ions, affecting the relationship between bulk EC and pwEC
• Sensor placement — sensors must be positioned in the active root zone at representative moisture levels; placement in dry zones will skew results
• Volumetric water content — the Hilhorst equation is not reliable below 10% VWC (0.10 m³/m³); avoid interpreting pwEC data at very low moisture levels
• Temperature — dielectric permittivity of water is temperature-dependent; the Hilhorst equation includes a temperature correction term

Calculating Pore Water EC Using the Hilhorst Equation

Using the method described by Hilhorst (2000), pore water conductivity (σw) is calculated from:

σw = (εw × σb) / (εb − εb=0)   (eq. 1)

• σw — pore water electrical conductivity (dS/m)
• εw — real portion of the dielectric permittivity of pore water (unitless)
• σb — bulk electrical conductivity (dS/m) measured by the sensor
• εb — real portion of the dielectric permittivity of the bulk substrate (unitless)
• εb=0 — dielectric permittivity of the dry substrate when σb = 0 (offset constant; Hilhorst recommends 4.1 as a generic value)

The dielectric permittivity of water (εw) is corrected for temperature using:

εw = 80.3 − 0.37 × (T_soil − 20)   (eq. 2)

Where T_soil is the substrate temperature in °C measured by the sensor.

This method has been validated to ±20% accuracy in moist substrates. It should not be used when VWC is below 0.10 m³/m³ (10%).

Pore Water EC Calculator (Google Spreadsheet)

Use our free spreadsheet to convert bulk EC readings to pore water EC using the Hilhorst equation:

Field Measurement with the SenseCAP S-2108

The SenseCAP S-2108 is a wireless LoRaWAN sensor that simultaneously measures VWC, substrate temperature, and bulk EC, then calculates and transmits pore water EC automatically using the Hilhorst method. No manual calculation required.

Key specifications relevant to pwEC measurement:

• VWC range: 0–100% (m³/m³), ±3% accuracy
• Bulk EC range: 0–20 dS/m
• Temperature range: -40 to 80°C, ±0.5°C accuracy
• Compatible substrates: rockwool, coco coir, peat, soil
• Wireless range: 2 km urban / 10 km line-of-sight (LoRaWAN)

Pair the S-2108 with a SenseCAP Outdoor Gateway or SenseCAP M2 Indoor Gateway to connect sensors to the SenseCAP cloud dashboard or your own server via open API.

Interpreting Pore Water EC Data for Crop Steering

Once you have continuous pwEC data, the primary application is crop steering — adjusting irrigation and fertigation to push plants toward vegetative or generative growth.

General pwEC target ranges by crop (adjust based on growth stage and cultivar):

• Lettuce / leafy greens: 0.8–1.5 dS/m
• Tomato / cucumber / pepper: 2.5–4.5 dS/m (generative steering at higher end)
• Cannabis (vegetative): 1.5–2.5 dS/m
• Cannabis (flower): 2.5–4.0 dS/m
• Strawberry / berry: 1.5–2.5 dS/m

If pwEC falls below target, increase fertilizer concentration or reduce irrigation volume. If pwEC exceeds target, flush with lower-EC solution or increase irrigation frequency to leach excess salts.

pwEC data also reveals salinity trends over time — gradual increases between irrigation events indicate salt accumulation and signal the need for a leaching event before plant stress occurs.

Summary

Pore water EC is a more accurate and actionable metric than bulk EC for managing substrate nutrition in commercial greenhouse and hydroponic production. The Hilhorst equation provides a reliable method for calculating pwEC from bulk EC and VWC, and modern wireless sensors automate this process continuously.

Browse SenseCAP sensors and gateways at Custom Hydro, or contact us for a sensor configuration recommendation for your operation.