Using Watermark® Soil Moisture Sensors

A very popular method of scheduling an irrigation is by estimating the evapotranspiration, ET, of the crop. ET is the total amount of water used by the crop and that lost by evaporation, too. However, in recent years research has shown that using soil moisture, in addition to ET, can save a grower from 10 to 20% on irrigation cost.

Methods of measuring soil moisture

Traditionally, growers have used tensiometers, and electrical resistance sensors for measuring soil moisture. While tensiometers are fast in responding to changes in moisture, they're labor intensive to operate, expensive and, have a limited moisture range. This drove many growers to use electrical resistance soil moisture sensors.

The first electrical resistance soil moisture sensors that growers used were gypsum blocks. Gypsum blocks are cheap, easy to use devices, but they have a flat response in the wetter regions of soil moisture, slow to respond, and degrade rather quickly in low pH soil. Even though tensiometers cost much more, and are a pain to operate, many growers switched to them because of the problems with gypsum blocks. This is not to say that there isn't a place for gypsum blocks. They will do a good job. Especially, if you use them in fields that are worked up yearly. You can find information on using and making gypsum blocks here.

Granular matrix sensor

The Watermark®, granular matrix sensor, (GMS), solves many of the problems associated with gypsum blocks. They respond fairly fast, have more response in the wet soil range, cost much less than tensiometers, and can remain in the soil of years. For disadvantages, the GMS has a slower response than tensiometers, a flat response from 0-10 centi-bar, and you'll need a special meter to read them.

We've been using GMS in our work for 10 years. In that time we found them to be reliable, sturdy, easy to maintain, and easy to read. We use those manufactured by Irrometer, with the brand name, Watermark®. In 2004 GMS's are available for $30-35 each.

How the GMS Works

Figure 1 shows a Watermark® GMS with a five foot lead wire. Both ends of the sensor are sealed, and inactive. The active portion is the area along the sides of the cylinder with the perforated stainless steel screen. Inside are two electrodes at a fixed distance from each other. The lead wires connect to these electrodes.

Around the electrodes is a fine granulated substance mixed with gypsum. The gypsum buffers the soil solution so your reading is not affected by fertilizer, and naturally occurring ions. All of this is wrapped in a permeable membrane.

As soil water enters the sensor it reduces the electrical resistance between the electrodes. A special ohmmeter is used to excite and read the resistance. The manufacturer has calibrated the electrical resistance, to soil moisture tension, and provided formulas for our use.

Figure 1 A Watermark® GMS with a five foot lead wire.

Figure 2 Chart showing soil moisture tension at resistance of GMS. This data collected at 68 °F.

Figure 2 shows a chart of the soil moisture tension vs. GMS resistance. The resistance must be read with an alternating current, AC, ohmmeter. If you use a direct current, DC, ohmmeter certain ions will migrate to the electrodes and cause a small ground current to flow that will mess up the reading. DC also causes electrolysis to form oxides on the electrodes that will increase their resistance, and degrade them.

To use this chart for irrigation scheduling follow this rule of thumb: for row crops irrigate when tension is above 40 cbar, about 6000 ohms; for grasses, irrigate when tension is above 60 cbar, about 8000 ohms.

Installing a GMS Sensor

First: soak the sensor overnight before installing it in the field. In fact, the manufacturer recommends soaking overnight and completely drying, three times before using the Watermark® sensor. We found that this did not make a difference in our trials in the Delta Junction, AK area. Just soaking overnight once and installing worked for us. We did carry them to the field in a jar of water.

The easiest way to install a GMS is to use a one inch diameter soil sample probe to make the hole. Make the hole to the appropriate depth for your crop. If you don't have access to a soil probe, or want to install more than one sensor at a site, see the method below .

If you don't have a soil probe, but you want to use this method, an old one inch diameter wood auger will make the hole. If you need to go deeper than the length of the wood auger you can weld a rod with a tee handle to it. I've seen them like this from 3 to 5 feet long.

Place the soil you excavated from the hole on something, especially in an established field. Putting it on the cardboard or cloth keeps it from damaging the crop, and makes it easer to get up. You'll need this soil later to seat the sensor and backfill the hole.

Figure 3 Make a hole with a one inch diameter soil probe to the average root depth of your crop.

Figure 4 Fill the hole about two inches with slurry to seal the GMS t the sides of the hole.

Mix up a slurry with some of the excavated soil and water. Pour some of the slurry into the hole to fill about two inches from the bottom. Position the GMS in the hole with the wires up. Push the GMS into the hole with a dowel stick, or piece of ¾ inch PVC pipe. Make sure the sensor is against the bottom of the hole. Tamp it a little to seat it in the slurry.

Pour the rest of the slurry on top of the GMS. Backfill the hole with soil. Pack the soil around the wire as you backfill.

Do not install more than one GMS in the same hole. If you want to place sensors at different depths use additional holes a couple of feet from each other.

You'll have to protect the wire from damage. In figure 5, we use the piece of PVC pipe that pushed the GMS into the hole. Use something that is tall enough to see over the crop. Many growers flag the spot with a stake and survey ribbon too.

You can add 100 feet of lead to these sensors. If you really want the leads out of the way route them to a windbreak or berm in a colter slice. You could then, do all of your field work without worrying about damaging the leads.

If you use a data logger at the site, to record the soil moisture, install a valve box flush with the surface. A four inch diameter valve box works good for our data loggers. We paint the cover with international orange to make it easy to find in the field.

You could use a smaller valve box, without a data logger, to protect the leads. Valve boxes are available at any place that sells landscape equipment

Figure 5 Backfill the hole and devise a way of protecting the lead wire. Here we use a piece of 3/4 inch PVC pipe.

Installing Watermark® Soil Moisture Sensors Without a Soil Probe and at multiple depths.

Figure 6 Placing more than one GMS at a site.

Figure 6 shows how to install GMS sensors without a soil probe, and multiple sensors, also. Dig a hole with a spade to about the depth that you want your sensor, or, if more than one, your deepest sensor. Then use a tool to make a three inch deep hole that is one inch in diameter.

I've used old wood augers with a make shift handle, or a sharpen pipe nipple, with good results. What you're trying to do here is have as much undisturbed soil contacting as GMS sides as possible.

If you're installing more than one sensor make sure you mark the leads so you'll know which are which. Tape and a permanent marking pen will work, but make sure the pen doesn't fade in sunlight. Another method of marking is to use bands of electrical tape around the leads. In this case, one band for GMS 1, two for GMS 2 and three for GMS 3.

In figure 6, we're placing one sensor at the upper root depth, GMS 1, one sensor at the lower root depth, GMS 2, and one sensor below the root depth, GMS 3. This will give us a good indication of the water in our root area, and GMS 3 will tell us if we're over irrigating.

Make a saturated paste of soil and water. The paste should be much thicker than the slurry used for soil probe holes. It should stick without running off the sensor, see figure 7 to get an idea of the consistency.

Pack the paste around the GMS. Immediately, insert the GMS in the hole before the paste starts to dry. Do the same to all the other sensors that you're going to place at this site.

Pack the top of the sensors with the left over paste. Check the marking on the leads again to make sure which is which. Route the leads out of the hole. Use the same methods of protecting the leads as show above in the soil probe install method.

Figure 7 Make a saturated paste of soil and water to seal the GMS in the hole.

Figure 8 Three sensors installed at a site. Notice that the position of the sensors offers the most contact with undisturbed soil.

Backfill the hole with the soil you removed from it. As you see in figure 8, the sensor's perforated screen surfaces are in the least disturbed soil.

Now you have three sensors at this site to monitor your irrigation. One in the upper root zone, another in the lower root zone, and another past the root zone.

If this is a seeded crop, you would water using GMS 1 early in the season. Later, when the root system develops, you'd use GMS 2 to monitor the irrigation. GMS 3 would be used all season to make sure you don't over water. Over watering not only cost more in irrigation expenses, but moves the fertilizer past the root zone of the crop making it unavailable to the crop.

A few hours after installing the sensors you should be able to read them. I would at least check them daily during hot dry periods. Remember, a plant can be moisture stressed for a long time before showing any signs. Even short periods of moisture stress can reduce yields.

You can get more information on irrigation at the Salcha-Delta Soil and Water Conservation District office. For specific questions you can contact me at my home page.

Last changed: 03/01/2005, 22:10:28 Copyright0 © 2005 by Chuck Mancuso for AEC Systems