Soil Moisture Sensors

Soil moisture sensors are great tools to aid in irrigation management. They provide feedback on the movement and depth of moisture within the soil, providing the ability to identify the proper duration of irrigation. Proper use relies on a thorough understanding of the soil characteristics of the orchard, which include soil type, water holding capacity, and salinity level.

Sensors can be used to help schedule irrigation. Timing of irrigation usually occurs when moisture levels drop below certain trigger points at varying depths. These points are different for every soil and sensor type and require in-field calibration to help reduce unwanted plant stress. Calibration can occur by comparing sensors readings to plant stress responses (e.g. Pressure chamber readings) or to a “feel” test to determine how much water is still available to the plant.

Several factors need to be considered when planning to install the sensors. Sensor locations should be placed to account for varying soil types of the orchard. If only a few locations are planned, the predominant soil types should be selected. If possible, sensors should be installed at varying depths to provide moisture levels in the middle, bottom edge, and below the active rootzone. A common 3 sensor installation pattern is 12-18″, 30-42″, and 48-60.”

Work by the University of California has compared many soil moisture monitoring systems. Neutron probe data, dielectric, tensiometers, and electrical resistance blocks have all been found to respond to water applications similarly. Essentially, if sensors are properly installed and maintained, and time is taken to understand and interpret the data, they can provide similar information. The table below highlights some of the varying aspects of these systems. Each system has strengths and weaknesses. Please note that the sensors types are linked and when clicked will direct to further information.

“Feel” Tensiometers Dielectric Sensors Electrical Resistance Neutron Probes
Basic Operation Soil between fingers Measures the suction Measures dielectric constant Measures resistance Measures neutrons slowed by water
Requirement for Calibration Experience Minimal Yes, soil dependent Moderate Yes, soil dependent
Monitoring Frequency Manual, Once Manual or Automatic Automatic Automatic Manual, once
Zone of Measurement Size of Auger bucket 2″ off of sensor About 1” from outside edge 1″ off of sensor, less in heavy, wet soils 10″ diameter
Replacement, Maintenance None Annual (check of vacuum and gauges), some require removal Annual Maintenance Annual, replacement every 3-7 years Replace batteries, transport rules, annual radiation safety check
Affected by Salinity, Alkalinity None No Yes , but depends on sensor type Yes No
Soil Type Most Suitable All All Sand – Sandy Clay Loam (Non-cracking Soils) Sandy Loam – Clay All
Common Companies Hortau, Irrometers Decagon, Aquacheck, EnviroSCAN Watermarks Contracted Services
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