Quick Guide: Soil Moisture Sensors

Most watering is done by habit or by the clock: every other morning, ten minutes a zone, more in July. A soil moisture sensor replaces the guess with a reading from the root zone. It shows when the soil has dried enough to need water, and, read after an irrigation, whether the water got down to where the roots are. Whatever the design, every sensor measures one of two things.

Two ways to measure soil water

Some sensors measure soil water tension, which is how tightly the soil holds on to its water and so how hard a root has to pull to get it. Others measure volumetric water content, the share of the soil’s volume that is water. Tension relates directly to what the plant feels; water content shows how much a watering added.

Tension sensors

Tensiometers

A tensiometer is a sealed tube of water with a porous ceramic tip at the bottom and a vacuum gauge at the top. Buried with the tip in the root zone, it lets water move out through the ceramic as the soil dries, and the gauge shows the pull in centibars. Zero means saturated soil. As the soil dries, the reading climbs.

University extension guides use roughly the same scale. Readings of 10 to 30 centibars mean the soil is moist, about where it sits a day or two after a thorough watering. Between 30 and 60, most soils are ready for water; heavy clays can go to 60 to 100 before plants feel it; past 100 the soil is getting dangerously dry. Sandy soils hold less water and should be watered at the low end of the range.

Tensiometers are cheap, need no batteries and read directly, a good fit for a vegetable plot or a small orchard. Their limits are maintenance and range. They must be kept topped up with water, especially in hot weather, and they stop working when the soil dries past about 80 centibars, when air breaks into the tube. That makes them better suited to sandy and loamy soils kept fairly moist than to clays allowed to dry deeply.

Gypsum blocks and granular matrix sensors

These are electrodes set in a small porous block, gypsum in the older type and a packed granular material in the newer one. The block takes up or gives back water until it matches the soil around it, and a handheld meter or logger reads the electrical resistance between the electrodes, which rises as the block dries. They are inexpensive and need little attention, and they work well into the dry range where tensiometers fail, so they suit clay soils and deep-rooted crops. They respond slowly, are less accurate in wet soil and in sand, and gypsum blocks gradually dissolve and need replacing every few seasons.

Water content sensors

Capacitance probes

Water has a dielectric constant of about 80, against roughly 3 to 5 for soil minerals and 1 for air, so the dielectric properties of soil change sharply as it wets and dries. A capacitance or frequency domain sensor measures that change and converts it to water content. These sensors read instantly, need no routine maintenance and can be stacked on one probe at several depths, which shows a watering front moving down through the root zone. Some also report soil temperature and salinity, and they are easy to connect to a logger or to an irrigation controller.

The catch is calibration and contact. Readings depend on soil type, so the numbers are most trustworthy after comparison with the same soil when it is known to be wet and dry. Any air gap around the sensor gives false readings, so installation has to be careful.

Time domain reflectometry and neutron probes

TDR sensors time an electrical pulse along metal rods pushed into the soil. They are accurate and usually need little soil-specific calibration, but they cost more and struggle in very salty soils. Neutron probes, the older research standard, are accurate over a large volume of soil but contain a radioactive source and require licensed operators, so they are no longer a practical choice outside research and large operations. The University of Florida’s comparison of field devices sets out the trade-offs, ranges and rough costs of each type.

Where to put a sensor

A sensor reports on a few inches of soil around it, so placement decides whether it tells the truth.

Pick a representative spot

Choose a place with the typical soil, sun and plants of its area, not the low corner where water collects or the high spot that dries first. If a garden has sandy beds and heavy beds, or sun and deep shade, each needs its own sensor.

Put it in the root zone

The sensor belongs where the active roots are. For a lawn, that means about 3 inches deep, toward the middle of a zone and away from sprinkler heads. For vegetables and shrubs, 6 to 12 inches is typical. For trees and vines, a pair of sensors works well: one about a third of the way down the root zone to say when to water, and one about two-thirds down to show whether the water got there. A deep sensor that never changes after a watering means the runs are too short; one that stays soaked means they are too long.

Mind the emitters

Under drip irrigation, only part of the soil is wetted. Put the sensor inside the wetted area near an emitter, where the roots concentrate, rather than in dry soil between emitters.

Install it snugly

Make a hole the size of the sensor, use a slurry of the same soil if needed, and press it firm so there are no gaps. Mark the spot.

Reading the trend

A single number matters less than the pattern over a week or a season. Readings should drop sharply after each watering and rise steadily between them. Watering when the reading reaches the chosen threshold, and checking the deeper sensor afterward, quickly shows how long each zone needs to run. Some irrigation controllers accept a soil moisture sensor directly and skip a scheduled cycle when the soil is still moist.