What Is a Soil Moisture Sensor? VWC, FDR, TDR and RH Explained

A soil moisture sensor is a sensor for continuous or fixed-point measurement of soil moisture status. It converts the electrical or electromagnetic properties associated with moisture in the soil surrounding the probe into readable values for use in irrigation management, greenhouses, orchards, cropland, water conservancy, scientific research, and long-term soil profile monitoring. The most common output indicator in agricultural monitoring is the volumetric moisture content VWC, not the relative humidity of the air RH.

What is a soil moisture sensor

Soil moisture sensors typically consist of inductive probes, signal processing circuits, and communication interfaces. After the probe is inserted or buried in the soil, it senses the soil within a certain range, and then the internal circuit converts the measurement results into digital or analog signals. In the automatic monitoring system, the RS485/Modbus type sensor can be directly connected to the data collector, RTU or PLC, forming continuous recording and remote monitoring.

The core problem it solves is not simply to judge whether the soil is dry or wet, but to quantify the moisture change in the root zone. In irrigation applications, when water needs to be irrigated, whether it reaches the root zone, and whether water is lost too quickly after irrigation can be judged by changes in moisture at different depths or at different locations.

What exactly does VWC stand for?

VWC (Volumetric Water Content) is the ratio of water volume to total soil volume. For example, 25% VWC means that approximately 25 volumetric units of water per 100 volumetric units of soil. It is two completely different physical quantities from the relative humidity of the air RH and cannot be directly converted.

Why do FDR and TDR measure moisture

The dielectric properties of water are very different from soil minerals and air, so changes in soil moisture content change the electromagnetic properties of the medium around the probe. FDR estimates the dielectric change through the frequency domain response, and TDR estimates the dielectric constant through the time domain pulse propagation characteristics. Both methods are indirect measurements, so soil texture, density, and salinity may affect the results.

Why different soils respond differently

The sandy soil has large pores and weak water retention; the specific surface area of the clay is large and the proportion of bound water is high. The dielectric response and volumetric moisture content may be different even if the “mass moisture content” of the two samples is the same. High-precision applications are better suited to establishing reference relationships with local soil samples rather than covering all soils with a common percentage.

How to choose NiuBoL soil moisture monitoring products

NiuBoL's soil moisture monitoring products cover single-probe, multi-depth profile monitoring monitoring monitoring monitoring monitoring monitoring, and mobile inspection schemes. The NBL-S-TM soil temperature and humidity sensor can measure soil moisture and soil temperature at the same time. The moisture measurement principle of FDR is adopted. The typical version supports DC 5-24V power supply and NBL-S-TM0/ Modbus communication, which is suitable for farmland irrigation, greenhouse, orchard, scientific research experiments and long-term monitoring of fixed monitoring points.

If you need to observe water infiltration in different soil layers, water storage in the root area, and deep leakage, you can choose the NBL-S-TMS multi-layer soil moisture monitoring program; if you need a rapid on-site inspection, you can choose USB/Type-C or handheld soil detection program. The product structure should be determined according to the monitoring depth, the number of distribution points, the power supply conditions and the data access method.

FAQ

Q1: Is the soil moisture %RH?

A1: No. Soil moisture is usually expressed as a percentage of soil moisture defined by VWC or equipment, and air moisture is used only for %RH.

Q2: Why is the sensor placed in water close to 100%, but only 40% in wet soil?

A2: The pure water test is mainly used to verify the upper limit of response and does not mean that the field soil should be close to 100% VWC. The saturated VWC of most forming soils is also well below 100%.

Q3: Which of FDR and TDR is more accurate?

A3: It is not possible to judge based on the principle name alone. Accuracy also depends on probe structure, calibration, soil type, mounting contact and signal processing.