Soil Moisture Sensing Comparing the Technologies

By Steven R. Evett, PhD

Soil moisture monitoring can be useful as an irrigation management tool for both landscapes and agriculture, sometimes replacing an evapotranspiration-based approach or as a useful check on ET-based approaches since the latter tend to drift off-target over time.

All moisture sensors, also known as soil water sensors, measure a response to a soil property that is related to water content and then use a calibration equation to convert the measurement into a soil moisture estimate. Often the calibration is internal to the sensor or sensing system. Although individual sensors are available, the market is demanding moisture sensing systems that feed data into a central point using either wired or wireless telemetry. Sensing systems may be tied into a decision support system that further processes the data to provide an irrigation recommendation or even control an irrigation valve or system directly. All sensing technologies are affected by the quality of sensor installation and location in the field.

There are four main types of sensing technology: neutron probes, resistance sensors, capacitance sensors and time domain sensors. Except for the neutron probe, sensed soil volumes are small, with the greatest response (90 percent or more) being to soil water within 1.5 inches or less of the sensor.

Neutron Probe Technology

The neutron probe counts neutrons slowed by collision with hydrogen atoms, most of which are in the soil water. The source of neutrons is slightly radioactive and is regulated such that safety training and licensing to possess a neutron probe are required and costly. Also, the neutron method is a manual method since the probe cannot be left

14 Irrigation TODAY | January 2017

unattended. Due to its accuracy when calibrated, large volume of measurement, lack of interference from salinity and temperature, and easy measurement at multiple depths in the soil, the neutron probe is still used by some consultants, particularly for high-value crops and where the needed access tubes can be left in for multiple seasons.

Resistance Sensors

Granular matrix sensors and their cousins, gypsum blocks, are buried in contact with the soil at whatever depth(s) are of interest (e.g., root zone). The sensor body comes into equilibrium with soil moisture so that it dries as the soil dries and becomes wetter as the soil is wetted. An alternating voltage is passed through two wires embedded in the sensor but separated by part of the sensor body. The electrical resistance between these wires increases as the sensor dries and decreases as the sensor becomes wetter. The resistance readings can be manual or automated. The resistance is better related to the soil matric potential than to the soil water content. Because plant water uptake is in direct correspondence to the matric potential, these “resistance” sensors are fairly direct indicators of plant water need. But since the water content is not sensed, the data cannot be directly used to indicate how much to irrigate. Resistance sensors have been widely adopted in horticultural and landscape irrigation. Issues with loss of soil contact can occur in some soils.

Capacitance Sensors

The other two major types of soil water sensors respond to soil electrical properties, including the apparent permittivity (i.e., ability of an electromagnetic field, or EMF, to move through the soil) and the bulk electrical

conductivity (known as BEC). Both the permittivity and the BEC increase with water content, although the permittivity increase is nonlinear. The most common of these “electromagnetic” (known as EM) sensors operates by imposing an oscillating EMF on the soil and measuring a frequency response. These “capacitance” sensors essentially couple a capacitive element with the soil and monitor the frequency changes driven by changes in the soil electrical capacitance. Capacitance is calculated from the frequency measurements. The capacitance is influenced by both the permittivity and the BEC, as well as by the shape and extent of the EMF that permeates the soil around the sensor element.

The shape and extent factors are known as the “geometric factor” in the electrical engineering equation for capacitance, and they affect the value of the capacitance and the sensed frequency (see fig. 1). Except for uniform sands, agricultural soils generally have enough small-scale (inch scale) variability in structure, water content and BEC to substantially influence the geometric factor beyond the effect solely of bulk water content, resulting in biased readings. Also, changes in soil temperature directly influence BEC, as do changes in salinity that may occur during an irrigation season or even in response to a single irrigation. They provide useful information on general trends in water content, and when deployed at multiple depths, they can indicate the depth of wetting due to irrigation and precipitation. Irrigation managers may find these kinds of information invaluable as aids to timing and general management of irrigation operations. However, the capacitance sensors generally are not sufficiently accurate to allow successful irrigation management following the management allowed depletion paradigm (see fig. 2).

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