Fort Collins Conservation District

Soil Moisture Sensor Guide

WATERMARK Soil Moisture Sensor



Why Use Soil Moisture Sensors?

Soil moisture sensors will provide the producer with information about how much moisture is in their field below the surface — the soil profile. Equipped with this information, the producer can irrigate more accurately and only when needed. Even a small reduction in the length of an irrigation, over the course of a season, can result in significant savings in money, time, and water. Soil moisture information can also help the producer decide when additional irrigation is needed.

How Many Sensors Will You Need?

Since every field has variations, at least two sets of sensors will need to be placed in a field. A set consists of two or three sensors placed near each other and buried at different depths within the field. The following is a general guide to help you decide how many you might need:

The sensors must be buried at two or three different depths in order to provide data that represents soil moisture data within the changing crop root zone. Crop type and management style will determine installation depths for the sensors. Common depths include:

Some growers manage at the top of the soil profile and include an eight-inch sensor in their set. Guidelines on proper depths for specific crops and soil conditions can be obtained from your local Extension office.

Consider Your Soil Type

If your field is large it will probably vary in its soil type. A loam field may contain a gravel strip or an area of hard clay. Check the Web Soil Survey for your soil type in the field where you will be placing sensors. Soil depth and texture will effect the rooting depth of your crop. Guidelines on proper depths for specific crops and conditions can be obtained from IRROMETER as well as your local farm advisor.

Instructions for Different Forms of Irrigation

Center Pivot Irrigation — Place one to three sets of sensors in the pivot field. These can be located according to crop management. For instance, in an alfalfa field it might be useful to place them within a few feet of a wheel track so that the harvester can make a sweep around the flagged sensors. Sensors can also be moved if a better location is determined. In general, each set can be placed within the field between the first and last wheel tracks, in the different soil types, or on and below hills.

Furrow or Flood Irrigation — Locate sets of sensors about two-thirds the way down the run, just ahead of the tail or backup water. This is the area where water penetration is usually the poorest.

Drip or Micro Irrigation — Sensors must be placed in the crop row. With drip emitters, this is usually 12 to 18 inches from the emitter. With micro-sprinklers, usually 24 to 36 inches is best. Monitor often enough to get a good overall picture of the field, or irrigation “block”, and consider the soil variations which exist. Keep in mind that light soils dry very quickly and heavy soils more slowly.

Preparation of the Sensors Before Installation

The sensor has two wires that are read with a meter. The meter has two clips that clip onto the two wires respectively. It does not matter which clip fits with which wire.

Step One

The plain sensor is about four inches long. For easier installation and removal it is recommended to attach a length of PVC (1/2 inch class 315 PVC or 3/4 inch CPVC pipe) and a PVC “T” at the top as a handle.

On one end of the PVC you need to drill a hole about a 1/4 inch from the end with a 1/8 inch drill bit.


Step Two

The wire from the sensor is threaded through the PVC tube and “T” so that it can be accessed by the meter to take readings.


Step Three

The sensor has a small slot in the top near the wire. This slot is lined up with the hole in the PVC to allow for drainage.

Glue the sensor to the PVC tube using a PVC/ABS cement (IPS Weld-On #795 or equal). The PVC “T” joint will also need to be glued to this main tube containing the sensor.


Installing Sensors in Your Field

Step One

Place each set of sensors in an area that allows for ease of access and the cutting of hay or any tillage.

Once the location has been selected, a hole can be made to the desired depth with an Oakfield soil probe or a similar soil probe.

The Oakfield probe also comes with extensions and different tips for different soils.


Step Two

Place the prepared and pre-soaked sensor in the hole. It is important to have a tight fit between the sensor and the surrounding soil.

A mud slurry using soil removed from the hole can be caked around the sensor before it is put into the hole to improve contact.


Step Three

After putting the sensor in the hole, a thin steel “tamper” is used to tamp the soil snugly around the sensor and tube.


Step Four

The soil around the top of the hole is tamped and leveled to prevent any “ponding” during irrigation or a rain event.


Step Five

Be sure to adequately flag the location once all of the sensors are in the ground.


Troubleshooting

From IRROMETER’s Installation and Operating Instructions:

Every now and then you may encounter a situation where the sensor doesn’t seem to be working properly. Please follow the steps below to determine if the equipment is functioning correctly or to determine if the field condition needs modification.

  1. Check the field conditions.
    1. The sensor does not have a snug fit in the soil. This usually happens when an oversized access hole has been used and the backfilling of the area is not complete. Reinstall the sensor nearby, carefully backfilling the access hole.
    2. Sensor is not in an active portion of the root system, or the irrigation is not reaching the sensor area. This may happen if the sensor is sitting on top of a rock, or below a hardpan, which may impede water movement. Reinstalling the sensor should solve the problem.
    3. If the soil dries out to the point where you are seeing readings higher than 80 centibars, the contact between the sensor and the soil can be lost. The soil starts to shrink away from the sensor. If the irrigation only partially re-wets the soil (soil suction above 40 centibars), it will not fully re-wet the sensor and may result in continued high readings. Fully rewetting the soil and sensor usually restores the contact. This is most often seen on heavier soils during peak crop water demand periods when irrigation may not be sufficient. Plotting your readings on a chart provides the best indication of this type of behavior.
  2. Check the meter.
    1. Is the battery O.K.? It should be replaced at least once a year, more often with frequent use. Check to be sure the battery contacts are clean and tight on the battery terminals.
    2. Follow the test procedure on the meter.
    3. If there has been some wire damage to the meter’s leads, it could malfunction. To check this, clip the leads to each other and push the “READ” button. The number 0 should appear in the display. If it does, then the leads are O.K. Moving the wire leads while reading will help to show if there is an intermittent wire connection problem. Holding the “READ” button down will result in continuous reading, while you move the wires. Replacement cables are available as a spare part.
    4. The LCD display on the meter has three digits. If you see only partial digits, the LCD may be suspect and should be returned for examination and/or repair.
  3. Then check the sensor.
    1. With a sensor submerged in water, your meter reading should be from 0 to 5. If the sensor passes this test, go on to step B.
    2. Let the sensor air dry for 30 to 48 hours. Depending on ambient temperature, humidity and air movement, you should see the reading go right up from 0 to 150 or higher – even off scale (LCD will read 199 when it reaches 199 centibars or more).
    3. Put the sensor back in water with the meter leads attached. The reading should return to between 0 and 5 within two minutes. If the sensor passes these tests, it is O.K.