So What Are We Actually Measuring
When properly calibrated, soil water sensors can give irrigators an accurate measurement of how much water is currently available, and how much has been depleted, in the soil for the crop. When used along with weather data and crop water use data, this accurate measurement of soil water can help irrigators make a more informed decision on when irrigation should start, how much to apply, and when to quit irrigating. When looking at sensors, be sure to have an understanding of soil water content and how they actually measure soil water.
Soil water content is described using many different terms, including saturation, field capacity, permanent wilting point, excess or gravitational water, unavailable water, plant available water holding capacity, commonly known as available water, and minimum allowable balance. For irrigated crop production in Nebraska, we usually focus on two of these terms, available water, and minimum allowable balance.
Available water is the volume of water stored between field capacity, which is the amount of water held by a soil after it has been saturated and allowed to drain for 24-48 hours, and permanent wilting point, which is the point where plants can no longer extract any water and will wilt and die. Therefore, available water is simply the amount of water a soil can supply to plants. The amount of available water in a soil is determined primarily by soil texture and organic matter content. Sandy soils, with large soil particles and low organic matter levels, hold less water than a silt or clay soil, which have smaller particles and higher organic matter levels. This difference in available water is the reason sandy spots show crop stress much sooner than surrounding soils.
Minimum allowable balance refers to the soil water content where plants usually start showing stress, which is typically 50 percent of the available water. With full irrigation, applications of water should be started before we get to this point, with 35% depletion often used as a trigger point. With deficit or limited irrigation, stress is tolerated at vegetative growth stages, when little or no yield loss is expected and late grain fill when stress impacts yeild at minimum levels.
So how do sensors actually measure soil water content? You would think that sensors measure water content directly, but that isn’t the case. Instead they measure soil water content indirectly by looking at other properties in the soil. By measuring a property and using a calibration, sensors estimate water content.
Understanding how each type of sensor achieves this estimate will help you when selecting a sensor for use in irrigation management. While there are many different types of sensors, for this article we are going to focus on the two main types being used for irrigation management in Nebraska; electrical resistance sensors and capacitance probes.
Electrical resistance sensors, also known as granular matrix sensors, estimate soil water tension by measuring the change in resistance between two electrodes (Figure 1).
Soil water tension is a measure of how hard plants have to work to extract water from the soil. Some advantages for resistance sensors are that they are relatively inexpensive and have been widely researched and calibrated for different soil types in Nebraska. Potential disadvantages are the importance of good soil contact which can be challenging in some soil types, and the lag time after installation and after wetting events.
Capacitance probes measure volumetric water content by emitting an electromagnetic field around the sensor to calculate the dielectric properties of soil (Figure 2).
Volumetric water content is the volume of water in a given volume of soil. Some advantages for capacitance probes are that they can be configured to measure multiple depths of soil with one sensor, continuous monitoring capability, and fast response times. Potential disadvantages are once again the importance of good soil contact and proper installation, higher price, as well as performance can be affected by soil salinity, temperature, and clay content.
For more information on these and other sensor types, refer to Nebraska Extension EC3002 Soil Water Sensors for Irrigation Management, available as a pdf from http://extensionpublications.unl.edu/assets/pdf/ec3002.pdf
This article was reviewed by Chuck Burr & Troy Ingram