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