A Tool for Smarter Irrigation
Measuring Soil Moisture with a Tensiometer
2013 WInterby Joe Meek
Helena-area Botanist & Small-scale Hay Producer
In a hypothetical summer irrigation scenario, you finished the second of four irrigation sets over your hay field a week ago as you head into the first cutting. It has been a confusing spring for irrigation with rain early and then a short, hot, dry spell, but last night brought a welcome all-night drizzle and sharply cooler temperatures and your rain gauge reads just over an inch. You planned to start your third irrigation set in a day or two, but now wonder if you should hold off. What did last night’s rain do and how did the recent hot, dry spell impact overall soil moisture?
Many irrigators ponder these questions but often start their next irrigation set without fully understanding changing soil moisture conditions one or two feet down in the soil profile, because getting behind with irrigation can be detrimental to hay production. Soil moisture is highly dependent on many variables, such as precipitation, temperature, wind, humidity, soil type, plant type, and growth stage, which makes it difficult to predict soil moisture levels.
There are tools available to help assess soil moisture conditions and plan irrigation sets accordingly. The first is the Paul Brown soil moisture probe. This tool was discussed in the Winter 2008 issue of Big Sky Small Acres and can be found at www.msuextension.org/BSSA/Articles/2008/Winter2008/Files/Understanding%20soil%20moisture%20a%20small%20acreage%20issue.pdf.
The second tool is called a soil tensiometer, pictured above. Gauge and reservoir remain above ground while the tube and ceramic tip extend into the soil. “A soil tensi-what,” you ask? Simply put, a soil tensiometer (pronounced ?ten(t)-se-'ä-m -t r) is a device that measures how tightly the soil is holding onto water. When soil moisture is high, the soil gives up water rather freely, but when soil starts to dry out, water gets harder to pull away from soil particles. A tensiometer actually measures how hard it is to pull soil water out of the soil, and displays it as a measure of centibars on a vacuum gauge (see How It Works on the following page). It is a great tool to help make decisions on when and how much irrigation water to apply.
In my own sprinkler-irrigated five-acre hayfield (70% grass/30% alfalfa mix), I use two tensiometers. One measures soil moisture six inches below the surface (the most important zone for grasses) and the other measures at 18 inches (important to the alfalfa). I install the units in the spring, as soon as the soil thaws, and remove them in the fall before it freezes. When I first started using them, I checked them daily and graphed the results. Very quickly the graphed results began to show me when I would hit the trigger point for next irrigation. I was able to eliminate two irrigation sets in the first year, resulting in an electricity cost savings that paid for one of the tensiometers.
After several years of use, I have a good feel for managing soil moisture in the field. While I don’t graph the results anymore, I do check the gauges every few days to keep track of changing conditions. I now adjust how long I run the sprinklers for each set based on comparisons between the shallow and deep soil moisture, since soil moisture depletion changes as the crop grows, days get hotter, and rainfall diminishes. I irrigate almost exclusively at night to minimize evaporative loss and my better understanding of managing soil moisture allows me to do so. Importantly, I pay close attention to deep soil moisture and reduce the length of time my sprinklers run after applying fertilizer in order to minimize leaching. I apply enough water to move the fertilizer into the eighteen-inch root zone, but not enough to saturate that zone to keep the fertilizer from leaching out of the zone and becoming unavailable to the plants. In all cases, continuing to pay for electricity to pump irrigation water beyond the soil saturation point is wasting money and, in fact, may be leaching precious nutrients out of the root zone.
Installing a tensiometer is relatively easy. To make the hole that is slightly smaller in diameter than the tensiometer, I drive a ¾-inch steel pipe into the ground just short of the intended depth of the ceramic tip. Add a little water to the hole then insert the tensiometer and push it firmly into and just past the bottom of the pre-made hole. This ensures good contact with the soil that is needed to make the connection between soil water and the tensiometer through the porous ceramic tip. Shortly after good contact is made, the fluid in the tensiometer becomes connected to soil water.
Tensiometers require very little maintenance. During the growing season, make sure the vegetation immediately surrounding the tensiometers is representative of the hay in the field. For example, when the hay is cut, trim the vegetation directly around the units to mimic the remainder of the field so soils around the units are subject to moisture depletion via evaporation and transpiration at a rate similar to the rest of the field. Also, the small water reservoir on the top of each unit must be refilled about twice per season. The units should be removed from the field before freeze-up and stored in a cool, dry location. Some units have replaceable ceramic tips that may eventually lose porosity and may need to be replaced.
Tensiometers can be purchased from a number of suppliers with costs typically starting at $50 per unit, ranging up to around $200, depending on the length of the unit. To learn more about tensiometer units and to find a list of dealers, review these manufacturer websites: www.irrometer.com or www.soilmoisture.com.
SIDEBAR
IRROMETER Tensiometer: How It Works
The tensiometer is a direct measurement system, which means that it actually reads the physical forces at work in the soil. Tensiometers act like a dummy root, allowing the soil moisture to interact with the instrument through the ceramic tip. Soil water tension outside of the instrument tries to remove the water from it, which creates a measurable tension inside the column. This tension is read with a mechanical gauge attached to the instrument. While this is the most accurate and proven method available, there is some maintenance required periodically to keep them full of water, and they must be removed from the field during the winter months to avoid freezing. (Reprinted with permission from Irrometer, Inc.)