Soil Moisture Sensors: Selecting, Installing, and Using Them Effectively on your Farm
Accurate soil moisture measurement is essential for effective irrigation scheduling.
Soil moisture can be measured using:
- Feel and appearance, a traditional method that lacks precision and is unsuitable for modern irrigation practices.
- Direct measurement, which involves soil sampling, weighing, and oven-drying; while accurate, it is costly, labor-intensive, and unsuitable for automation.
- Indirect measurement using sensors, which provide continuous, real-time data without repeated soil disturbance.
Sensors are the most suitable option for advanced irrigation practices and automation. They are non-destructive, require minimal soil disturbance, enable frequent monitoring for optimal irrigation timing, and offer long-term cost-effectiveness.
Sensor Selection
Sensor selection is the first key step in effective soil moisture monitoring and selecting the right soil moisture sensor is critical. Factors to consider for your farm conditions are: sensor type, cost, accuracy, ease of use, and durability.
Soil moisture sensors measure either volumetric water content or soil water potential and can be categorized into five types:
- Neutron sensors (also known as neutron probes)
- Capacitance sensors, which are widely available on the market
- Travel-time sensors (such as time-domain reflectometry, or TDR)
- Resistance sensors (such as gypsum blocks and granular matrix sensors)
- Tensiometers
Neutron probes are the most accurate sensors but require a license to operate and cannot be automated.
Capacitance sensors offer fast response times, provide usable trends, and are suitable for automation. However, they are sensitive to soil salinity, temperature, and clay content, requiring field calibration for accuracy.
Travel-time sensors (such as TDR) provide rapid responses, reliable trends, and compatibility with automation. They are less affected by soil salinity, temperature, and clay content than capacitance sensors.
Resistance sensors (such as gypsum blocks and granular matrix sensors) provide usable trends and are easy to log data. However, they do not respond quickly and require time to equilibrate with soil moisture. They are also affected by soil salinity and may need recalibration over time. Granular matrix sensors are more durable than gypsum blocks.
Tensiometers directly measure soil water tension rather than estimating it like other sensors. They can be read manually using a pressure gauge or automatically with a pressure transducer that converts readings into electronic signals for data logging. Tensiometers are widely used and unaffected by soil salinity but require regular maintenance, including refilling with water and purging air bubbles.
Installation
After selecting the appropriate sensor, proper installation is the next critical step.
- Follow the manufacturer’s guidelines for proper sensor installation and orientation.
- Choose representative locations that reflect distinct soil types. Use the soil database on the Growers Irrigation Academy website to see the soil types of your farm.
- Install sensors at appropriate depths based on the effective rooting depth of the crop.
- Install additional sensors at the bottom of the root zone to assess irrigation sufficiency and detect potential leaching.
- Minimize soil disturbance during installation to help the soil return to its natural state more quickly.
- Ensure good soil contact and prevent preferential flow or air gaps, which can compromise data accuracy.
Effective Utilization
Depending on the farm setup, data can be collected manually—either by direct reading or handheld readers—or automatically using data loggers.
- Manual readings are practical for smaller operations or periodic spot-checks. However, they can be time-consuming, especially for large fields or frequent monitoring.
- Automated systems are ideal for continuous monitoring and remote access, improving efficiency and decision-making.
Software tools or spreadsheets can help calculate averages and identify outliers. When analyzing soil moisture data, focus on patterns and trends to determine optimal irrigation timing and water application rates. The goal is to ensure efficient irrigation by identifying when and how much water should be applied. Soil moisture sensors allow farmers to do just that.