Soil moisture sensors help guide agricultural irrigation. Good irrigation management leads to better crops, uses less input and improves profitability. Soil moisture sensors help irrigators understand what is happening at the root of crops.

Scheduling irrigation

To be used effectively, soil moisture sensors must be:

used in an irrigation shift that delivers water evenly

installed correctly and placed in an area which is representative of the crop being grown

used in combination with other irrigation management information (soil moisture sensors only measure a tiny area of an irrigation shift):

evaporation-based scheduling

soil moisture monitoring

grower observation.

Sensor types

There are basically two groups of sensors:

water potential sensors, such as tensiometers and granular matrix sensors
soil moisture sensors that give a percentage or relative content of soil moisture.

water potential sensors, such as tensiometers and granular matrix sensors
soil moisture sensors that give a percentage or relative content of soil moisture.

Water potential sensors

These sensors measure how hard it is to remove water from the soil, providing the best indication of available water for plants. Soil type and water content influence the suction pressure required to remove water from the soil, but a monitored sensor, which is recorded and graphed, will show the sharp fall that indicates water has become hard for a plant to access.

Questions when choosing a water potential sensor:

Do they accurately read in the desired range for the crop in which they are used?

Do they react quickly enough to be useful for the crop being monitored?
The two most common types of water potential sensors are tensiometers and granular matrix sensors, such as gypsum block and watermark sensors.

Tensiometer sensors

Tensiometers (Figure 1) are the most responsive water potential sensor, and they require the most care and maintenance. There are two types of tensiometer tip: one is used in sands, and the other in clays and loams. Use the appropriate tip to see quick reactions to changes in water status.

Tensiometer in apple crop

Tensiometer being used to manage irrigation in an apple orchard
Tensiometers work by measuring suction pressure at the tensiometer’s porous tip. Water is drawn out of or into the tip, depending on water availability. This creates a suction pressure representing the suction force required for a plant to obtain water from the soil. Measurements can be done by manually reading a vacuum gauge, or automatically, using a logging pressure transducer.

To maintain tensiometers, check for bubbles and refill the fluid used to create the vacuum within the tensiometer.