Measuring soil moisture helps schedule irrigations

Man's early attempts at irrigating plants took place almost 7,000 years ago in Mesopotamia. These early farmers must have realized quickly that if they didn't irrigate at the right time and in the right amount, plants died or yields were poor.

Modern irrigators face this same challenge. Maximizing profits by producing the highest yields at the lowest cost has driven farmers to find the optimum soil-moisture content for their crops. Landscape irrigation is, of course, not based on maximizing yields but rather on obtaining a desired aesthetic appearance. Nevertheless, the idea of an optimum soil-moisture content is just as valid.

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Too much water can be as damaging to many plants as too little. You need to remember that oxygen must move through the soil pores to replenish the soil atmosphere around the respiring roots. When the pore spaces fill with water instead of air, the diffusion of oxygen to the roots slows substantially. This lack of oxygen impairs the overall ability of the root system to function properly. High soil-moisture levels also can mean excessive drainage and leaching of soluble nutrients such as nitrates. Nitrate leaching can be a possible source of contamination to our surface and groundwater systems. High soil-moisture contents also encourage weeds to invade and increase the chances of plant disease. Perhaps the real driving force to developing a more efficient irrigation plan and implementing a tight water balance, however, is the cost of water. Water costs now represent the highest fraction of the operating budget for most golf courses, especially in areas such as Las Vegas. Many courses spend as much as $1 million dollars per year for municipal water.

As the cost of good-quality water increases, more landscape managers have begun considering the use of poorer-quality waters for irrigation. As such, irrigation scheduling is becoming more of a science. The use of weather stations and moisture and salinity sensors in these irrigated landscapes is now becoming common.

Differences in moisture levels Soil-water content can vary between extremes of saturation and very dry. When soil-moisture content is low enough, the remaining water may adhere so tightly to soil particles that it is unavailable for plant uptake. When soil moisture gets to this point, it is said to be at its permanent wilting point. Plants growing in such soil wilt and will not regain rigidity without the immediate application of additional water. The strength of the bond between water and the soil particles to which it adheres is measured in bars. One bar is equal in strength to the pressure of 1 atmosphere. In saturated soil, the strength of the water/soil bond is 0 bars, and the bond strength at the permanent wilting point is 15 bars.

Between the extremes of saturation and the permanent wilting point are soil-moisture contents optimum for plant growth. You often hear field capacity used to describe the amount of water in the soil profile shortly after irrigation. Field capacity is the maximum amount of water a soil can hold after excess water has drained off or percolated down and out of the root zone. Plants deplete a percentage of the available water (field capacity minus the unavailable water that's left at the permanent wilting point) between irrigations. How much depletion you should allow before irrigating again depends on the plant species, the physiological stage of the plant, the soil type, the quality of the irrigation water and the time of the year. However, a common recommendation is to allow a depletion of 50 percent of available water under non-saline conditions.

Many factors affect soil-water depletion rates. So, how do you know when a certain amount of depletion has occurred? The answer is to monitor the soil-moisture content or a parameter that infers soil-moisture content.

Monitoring the soil-moisture content provides you with feedback as to the correct irrigation frequency to set. As soil moisture varies in the root zone, so does the plant's level of stress. You expect a normal drop in soil-moisture content after each irrigation event as water drains and plants use water. However, extremely wide swings from wet to dry can damage the plant, particularly when soluble salts are present in the soil or irrigation water. Decreasing the soil moisture concentrates soil salinity, a problem that you can minimize through proper irrigation management.

Measuring soil-water content, however, only has practical significance if you know the relationship between the measured water content and landscape water needs. Differences in soil texture confound this relationship. As the clay content increases, water-holding capacity also increases. But the availability of water at a given water content is not the same for all soils. Thus, knowing the soil-water content only has meaning if you know how low you can let the water content get (before plant stress occurs) before applying the next irrigation.

Soils can vary tremendously in both physical and chemical properties at the same site. As the variability in soil texture and structure increases, so does the variability in soil-water content. Thus, you'll need to take a large number of samples or measurements to accurately assess the mean soil-water content within 10 percent. Research has demonstrated that you need at least nine samples per field site (sites varied in size from 37 to 370 acres) to estimate the average soil-water content near field capacity, and you need 23 samples to estimate the soil-water content near the permanent wilting point (sites varied in size from 1 to 210 acres).

Your tools for measuring You can assess soil-moisture content in many ways. To most accurately measure the soil-water content, you must take a sample of a known volume of soil from a given depth, weigh it, oven-dry it at 105oC for 48 hours and then re-weigh it. The grams of water (fresh weight - oven dried weight) divided by the soil volume (cubic centimeters) equals the volumetric water content. Although this technique gives the definitive answer, the technique is not quick or easy if you need immediate answers. Quicker, indirect ways of estimating soil-water content exist. These include using tensiometers, gypsum blocks, a neutron probe, time-domain reflectometry, frequency-domain probes and even remote sensing.

* Tensiometers. For years, irrigators in agriculture have used tensiometers to set irrigation frequencies and to assess the depth of water penetration. Tensiometers provide a simple but accurate means of measuring soil-moisture tension, or the force at which water sticks to soil particles. You may also have heard this referred to as soil-matric potential. Tensiometers operate best near field capacity (0 to 0.8 bars). You must service them on a regular basis, and they need protection during freezing weather. You'll find a direct correlation between soil-moisture tension or matric potential and soil-moisture content. This relationship is known as a soil-moisture release curve. Such curves vary considerably based on soil texture. Once you determine a soil-moisture release curve for a soil, you can use soil-moisture tension to estimate soil-water content. At a given soil tension, higher water contents are associated with higher clay-content soils.

* Gypsum blocks. Irrigation managers have used these products for some time. You place these blocks in the soil at root-zone depths. The blocks absorb moisture from the soil. You then use electrodes in the blocks to measure the electrical conductance, which increases as moisture in the blocks increases. You use the measured values to estimate soil-matric potential and soil-moisture content. Gypsum blocks deteriorate with time, and you shouldn't use them in saline soils.

* Neutron probes. Scientists have used neutron probes for more than 30 years to estimate soil-water content. A probe that you lower into the soil releases fast-moving neutrons. These neutrons collide with atoms of similar size and mass, which include hydrogen found in water molecules. The higher the soil-water content, the greater the number of collisions and the greater the number of neutrons returning to the detector. You then can correlate these return counts with the soil-moisture content. The downside: using this technique requires a radiation permit, and the probe is fairly expensive.

* Time-domain reflectometry (TDR) is a fairly new technique you can use to estimate soil-water content. You direct an electromagnetic pulse down parallel wave guides in the soil. You then use an instrument to measure the speed at which this pulse travels to the end of the wave guides and returns. This technique is based on the dielectric constant of water, which is much higher than that of mineral soils. Salinity can influence the results, although researchers are developing new techniques to overcome this problem. Its major drawback at this time is cost.

* Frequency-domain probes also respond to changes in the apparent dielectric constant. However, these probes continually produce a high-output signal and measure soil water in voltage-ratio terms, which provide greater measurement stability. The cost is less than that of TDR equipment.

* Remote sensing. In the future, we may base irrigation scheduling (timing and volume) on remote sensing (digital-spectral data collected every 3 days with a resolution of 3 to 6 feet). You can estimate evapotranspiration (ET), canopy temperature and moisture content (soil and tissue) with this technique. However, we still need to develop response curves for each plant species and region of the country, and the cost will need to be reasonable if it is to become an integral part of irrigation management.

Although we still have room for improvement, no doubt the early irrigators of Mesopotamia would be impressed with the technological advancements we've made in the area of soil-moisture monitoring and irrigation scheduling.

Dr. Dale A. Devitt is an associate professor of soil and water at the University of Nevada - Reno but is housed in the Biology Department at the University of Nevada - Las Vegas. Bob Morris is a cooperative-extension agent with the University of Nevada - Reno.

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