A tensiometric probe to irrigate your crops when they really need it

A tensiometer is a tool for measuring soil matric water potential.

Unlike volumetric humidity (percentage of water in soil) measured with a capacitive sensor, matric water potential translates the force with which water is retained in the soil and therefore the force that must be applied to a volume of water to extract a certain quantity of free water. It’s generally expressed in pressure units (kPa, cbar, or hydraulic head) and is commonly called suction.

Today, tensiometric probes used in agriculture are electric; old tensiometers were composed of a manometer to read a measurement directly in the field. They’re no longer really used because they were more tedious to install, more sensitive, and required going to the field to read the measurement. However, the operating principle remains the same.

A tensiometric probe is composed of a porous sensor permeable to water; the latter will more or less fill with water until a pressure equilibrium is created between the soil and the tensiometric probe. The quantity of water absorbed by the tensiometer will represent the tension (or suction) measurement. For the Watermark probe (used in Weenat tensiometers), two electrodes measure the resistance that translates this water quantity.

The quality of a tensiometer installation is essential to obtain a good measurement. The installation objective is to have good contact between the soil and the probe, and to ensure this contact is durable during the season (be careful for clay soils when they dry out; the probe can detach from the soil, for example).

The explanations below are summarized in this video.

Different rules exist to guide the choice of tensiometer installation area, but there’s no miracle answer, and tensiometer installation must be reasoned according to soil, crop in place, and irrigation method.

Homogeneous soil type
Sensors must be installed in homogeneous soil. Triggering irrigation or modifying irrigation frequency is mainly justified by a change in tension evolution (water availability decreases less rapidly than expected, for example). However, tension evolution depends directly on soil type and structure. To be able to make a decision and not distort the median, different measurement points must be distributed over the same soil type to be able to compare it and determine an irrigation strategy.

This homogeneous soil zone may be only a small part of the plot and is not necessarily the most representative zone of the plot; it may be the most drying zone, for example.

Representative vegetation
Tensiometric probes must be installed in an area where the crop is homogeneous and representative of the plot: species, vegetation state, crop stage, density, health status… Be careful during installation not to disturb the crop in place, which would risk altering the representativeness of the measured area.

A representative irrigation area
The monitored area must be representative of the rest of the irrigated plots. Choose an average irrigation area, that is, an area of average rainfall on the farm, or the area that will receive the average dose delivered by the installation.

Tensiometer distribution
It’s preferable to equip a single plot deemed representative and extrapolate the results to the rest of the farm based on the operator’s knowledge of their operation.

Multiplying measurement points is important to have good representativeness of water availability in measured areas. It’s generally accepted that 6 measurement points, distributed in 6, 3, or 2 sites (depending on irrigation method) allow good irrigation monitoring.

In the case of sprinkler irrigation, it’s interesting to have measurements at two different depths minimum. Tensiometric probes can be distributed over 3 measurement sites with two different depths or two measurement sites with three different depths.

With drip irrigation, it’s not necessary to measure at two different depths. It’s mandatory to have at least 3 measurement sites because with 1 or 2 measurement sites, it’s impossible to decide in case of divergence.

The installation depth of tensiometric probes depends on the crop, but also on soil depth, and depending on irrigation type, it may be necessary to measure several horizons.

Installing tensiometers when drip irrigating
For drip irrigation, it’s not necessary to measure different horizons and you can be satisfied with installing tensiometers at a single depth. The objective is to measure tension at the periphery of the dripper’s influence zone (also called wet bulb) in the root system and ensure this tension remains more or less stable during the season. Depending on soil type, it may be important to install an agricultural tensiometer deeper to ensure water diffuses properly.

Installing tensiometers for sprinkler irrigation
In the case of sprinkler irrigation, the objective is to ensure that the applied dose recharges the soil with water for the duration of the watering round, so it’s necessary to measure different depths to ensure the soil recharges sufficiently with water during irrigation and that this applied water dose is properly used.

Weenat tensiometers come in two different sizes: 15-30 cm and 30-60 cm. The advantage of these dimensions is to be able to install devices both in the roots between 15 cm and 30 cm as well as deeper probes to monitor the evolution of water availability in the following horizons up to 60 cm.

To guarantee the precision of your tensiometric probe over the long term, a few simple maintenance gestures are enough:

• Check the proper positioning of the antenna: it must be vertical and properly screwed.
• Check the housing integrity: it must be perfectly intact and properly closed.
• The sensor must be properly inserted into the soil (use the marks present on the rod). A crack in the soil around the tensiometric probe can generate measurement errors: it’s then necessary to remove it from the soil before reinstalling it nearby.

The lifespan of your sensors can also be extended by winterizing them when the irrigation season is over.

• Remove the tensiometers from the ground carefully when the soil is moist: just after the last irrigation of the season or after intensive watering around the sensor.
• Using a sponge or damp cloth, remove the soil present on the soil temperature and tensiometry probes.
• To finish cleaning the tensiometric probe, immerse it in water then remove soil residue using a sponge without using the abrasive side.
• Water may have accumulated in the sensor rod: empty it before storage.
• Store your sensors horizontally in a dry place sheltered from frost and dust.