Practical Use of Soil Moisture Sensors and Their Data for Irrigation Scheduling

Practical Use of Soil Moisture Sensors and Their Data for Irrigation Scheduling

FS083E
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R. Peters, PE, Ph.D., Extension Specialist, Kefyalew Desta, Ph.D., Soil and Crop Management, Leigh Nelson, PE, WSU Prosser Irrigated Agriculture Research & Extension Center
Good irrigation water management will increase yields, improve crop quality, conserve water, save energy, decrease fertilizer requirements, and reduce non-point source pollution. Using soil moisture measurements is one of the best and simplest ways to get feedback to help make improved water management decisions. However, the installation, calibration, and interpretation of the data from these instruments is often overwhelming or not deemed valuable for most busy growers. Here’s an attempt to provide practical recommendations for using these sensors.
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Good management of irrigation water will increase crop yields, improve crop quality, conserve water, save energy, decrease fertilizer requirements, and reduce nonpoint source pollution. All of these are positive benefits and help contribute to profitable crop production. Using soil moisture measurements is one of the best and simplest ways to get feedback to help make improved water man­agement decisions. However, the system installation and calibration, plus interpretation of the data from soil water sensors is often confusing or overwhelming to most busy growers. This paper doesn’t attempt to compare sensors but does provide practical recommendations for using these instruments and interpreting their measurements for more profitable crop production.

Those who understand the basics of soil water interactions and the major differences between soil water sensors can skip these first sections and go directly to the Placement of Sensors, and the Soil Water Content and Tension-Based Sensor sections.

Soil Water Basics

Soil water fills about 25% of the space in the soil. This water is held in the pore space, or the cracks and empty spaces between soil particles. When all of the pore space is completely filled, the soil is said to be saturated. Excess water will drain out over time to a point where the soil will hold a certain amount of water indefinitely against the downward pull of gravity. This soil water content is called field capacity. As a plant’s roots remove water from the soil, the soil will dry out to a point where the suction or pull of the soil on the water exceeds the plant’s ability to absorb water. At this point, the plant will wilt and die. This soil water content is referred to as permanent wilting point. The difference between field capacity and permanent wilting point is the available water holding capacity (AWC) of the soil (Figure 1).

Different soils have different available water holding capac­ities (Table 1). Sands can’t hold very much water compared to silts and clays. A plant’s rooting depth is also an impor­tant consideration. A plant with deeper roots has access to a much larger volume of soil and, consequently, to a larger reservoir of soil water to draw upon before it runs out, compared to a shallow-rooted plant (such as onions or potatoes). Applying more water than a soil can hold simply results in deep percolation: water that is lost below the root zone of the plant, along with essential plant nutrients and other soluble compounds.

At first, as the soil water is depleted from field capacity (100%

of available water) down towards permanent wilting point (0% of the available water) plant production is gener­ally not affected, but there is a point at which production drops off (Figure 2). This point is commonly chosen as a management allowable deficit (MAD). This point and the shape of this curve are different for different plants. Soil water depletion below this MAD point will result in signifi­cant yield losses.

Soil Moisture Sensors

The major types of soil moisture sensors are listed in Table 2 and grouped according to the technology used to measure soil moisture. There are a variety of good publica­tions that describe the various sensors’ comparative advan­tages and disadvantages and their installation procedures (Enciso et al., 2012; Shock et al., 2001; Paige and Keefer 2008; and Evett et al., 2003, 2007, and 2011). There is additional information on the installation and use of these sensors on the various manufacturers’ websites.

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Copyright 2013 Washington State University

Published June, 2013

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