A Capacitive Probe to Control Your Irrigation with Precision

Capacitive and tensiometric probes are two very different technologies that don’t measure the same information:

The capacitive probe measures volumetric water content (%) as well as temperature (°C) at each horizon.

The tensiometer measures soil water availability (kpa) as well as temperature (°C)

Their measurement principle is also very different:

For the capacitive probe, the estimation of volumetric water content is done by measuring the soil’s dielectric permittivity. Electrodes create an electromagnetic field more or less modified by the medium. The more this field is disturbed, the more water there is in the soil.

For the tensiometer, the measurement of soil water availability is done by electrical resistivity following the water balance formed between the inside of the Watermark capsule and the soil.

There is no solution better than the other:

The capacitive probe is a very good solution for knowing precisely the water flow in your soil, as well as root consumption at different horizons.

The tensiometer allows you to know more about the actual water availability for the plant and the force the root must exert to extract water.

Both technologies work with all soil types, all crops and all irrigation systems.

You are free to choose one or the other according to your personal preferences.

The quality of a capacitive probe 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 that this contact is sustainable during the season (be careful for clay soils when they dry out, the probe can detach from the soil for example).

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There are different rules to guide the choice of capacitive sensor installation area, but no miracle answer and the installation of the capacitive probe must be reasoned according to the soil, the crop in place and the irrigation method.

Homogeneous soil type

Sensors must be installed in homogeneous soil. Triggering an irrigation or a modification in the irrigation schedule is mainly justified by a modification in the evolution of soil water content. However, the evolution of soil moisture depends directly on the soil type and its structure. In order to make a decision, the different measurement points must be distributed on the same soil type to be able to compare them 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 area of the plot; it may be the driest area for example.

Representative vegetation

Capacitive 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 could alter the representativeness of the measured area.

A representative irrigation area

The monitored area must be representative of the rest of the irrigated plots, however, several strategies can be applied:

In the case of the IRRINOV® method, it is imperative to install your capacitive probe on the first irrigated plot of the watering round.

In all cases, choose an average irrigation area, i.e., an area of average rainfall on the farm, or the area that will receive the average dose delivered by the installation.

Distribution of capacitive probes

It is preferable to equip a single plot considered representative and extrapolate the results to the rest of the farm based on the farmer’s knowledge of their operation.

Multiplying measurement points is important to have good representativeness of water content in the measured areas.

In the case of sprinkler irrigation, it is interesting to have measurements at two different depths minimum.

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

To guarantee the long-term accuracy of your capacitive probe, a few simple maintenance gestures are sufficient:

Check the proper positioning of the antenna: it must be vertical and properly screwed.

Check the integrity of the housing: it must be perfectly intact and properly closed.

The sensor must be properly inserted into the soil. A crack in the soil around the capacitive probe can cause measurement errors: it is 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 capacitive probe from the ground with care 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 PVC tubes.

To finish cleaning the probe, immerse it in water then remove soil residues using a sponge without using the abrasive side.