Blog · Agriculture
Soil moisture sensors explained
Soil moisture sensors come in two families. Volumetric sensors tell you how much water is in the soil, and tension sensors tell you how hard a plant has to work to take that water up.
Both are useful, and they answer different questions.
Two ways to describe soil water
Volumetric water content is the share of the soil's volume that is water, usually given as a percentage. A reading of 30 per cent means that 30 per cent of that block of soil, by volume, is water. The rest is mineral particles, organic matter and air.
Soil water tension, also called matric potential or suction, describes how tightly the soil holds its water. It is measured in units of pressure, commonly kilopascals or centibars, which are the same size. Wet soil holds water loosely, so tension is low. As the soil dries, the remaining water clings to particles in ever thinner films, and tension rises.
The link between the two depends on soil texture. Clay has many fine pores and holds a lot of water tightly. Sand has large pores, holds less water and releases it easily. The same volumetric reading can mean a comfortable crop in a sandy soil and a stressed crop in a clay.
Field capacity and wilting point
After heavy rain or irrigation, soil is saturated. Water drains out of the largest pores under gravity over the next day or few days. The water content at which this free drainage has largely stopped is called field capacity. It is the practical upper limit of water the soil can store for the crop. It is often taken to correspond to a tension of around 33 kPa, with lower figures of around 10 kPa used for sandy soils.
As plants draw water, the soil dries until the remaining water is held too tightly for roots to extract. At that point plants wilt and do not recover overnight. This is the permanent wilting point, conventionally set at a tension of 1,500 kPa.
The water held between those two points is the plant available water. Crops begin to suffer well before wilting point, so irrigation guidance is usually expressed as a share of the available water that can be used before refilling. That share depends on the crop and its stage and is a question for your agronomist or extension service.
Volumetric sensors
Most electronic volumetric sensors rely on one physical fact. Water has a much higher dielectric permittivity, at about 80, than dry soil minerals, at about 3 to 5, or air, at 1. Measure the permittivity of the soil around the sensor and you can estimate its water content.
- Capacitance sensors, sometimes described as frequency domain sensors, use the soil as part of an electrical circuit and measure how the circuit responds. They are relatively inexpensive, use little power, and are common in probes that measure at several depths.
- Time domain reflectometry (TDR) sends an electromagnetic pulse along metal rods and measures how long it takes to travel. The wetter the soil, the slower the pulse. TDR is generally regarded as accurate across a range of soils and is widely used in research. The equipment tends to cost more.
- Neutron probes are an older method that measures water over a larger volume of soil. They contain a radioactive source, so their use is regulated and they are mostly found in research and specialist services.
Points to watch with dielectric sensors:
- They sense only a small volume of soil around the sensor.
- Air gaps between sensor and soil cause low readings, so careful installation matters a great deal.
- Salinity, temperature and high clay content can affect readings, especially for sensors working at lower frequencies.
- Factory calibrations are general. A calibration for your own soil improves absolute accuracy. Without one, trends are more dependable than absolute numbers.
Tension sensors
- Tensiometers are water-filled tubes with a porous ceramic tip at the bottom and a vacuum gauge or pressure sensor at the top. As the soil dries, it pulls water out through the tip and creates a suction that the gauge reads directly. They are accurate in wet to moderately dry soil but stop working above roughly 80 kPa, when air enters the system. They need refilling and servicing, and protection from freezing.
- Granular matrix sensors contain electrodes set in a granular material that comes into balance with the surrounding soil. Electrical resistance between the electrodes changes with moisture and is converted to tension. They cover a wider range than tensiometers, need little maintenance and are low in cost. They respond more slowly, are less precise in very wet soil, and need a correction for soil temperature. Gypsum blocks work on a similar principle.
The strength of tension readings is that they mean roughly the same thing to a plant in any soil.
| Volumetric | Tension | |
|---|---|---|
| Answers | How much water is there? | How available is it to the plant? |
| Units | Per cent by volume | kPa or centibars |
| Examples | Capacitance, TDR | Tensiometer, granular matrix |
| Good for | Working out how much water to apply, tracking depth of wetting | Judging when the crop will begin to struggle |
| Main caution | Needs soil-specific interpretation | Limited range or slower response |
Installation and placement
A sensor reports on a fistful of soil, and you are using it to represent a field.
- Choose a spot that is typical of the area it represents in soil type, slope and crop stand. Avoid headlands, wheelings and wet hollows unless those are what you want to monitor.
- Place sensors in the active root zone, and ideally at two or more depths. A shallow sensor shows when the crop is using water. A deep one shows whether water is passing below the roots.
- Ensure firm contact between sensor and undisturbed soil.
- Allow time for the soil to settle before trusting the readings.
Reading the graph
With volumetric sensors, the pattern often teaches more than the number. After a thorough wetting, the line rises sharply, falls quickly while free drainage takes place, then flattens. That shoulder is a working estimate of field capacity for that spot. In the growing season, the line then falls in daily steps, steep by day and flat at night, as the crop draws water. When the steps become shallower in warm weather, the crop is finding water harder to extract.
Sensors are one source among several, and a spade still adds to the picture. Local weather has a large effect, as our guide to microclimate and weather stations explains.
Where Prism fits
Prism Microclimate, built with FarmGrid, covers 961k fields across one whole state and includes field-level soil moisture alongside weather, frost, spray and field-work windows, growing degree days and a morning brief per field. See our agriculture page, or read what a digital twin of a farm looks like for how sensor data fits with other farm records.