At a typical tensiometer station, two tensiometers are installed. One, located in the upper root zone, monitors the active root area and is used to determine when an irrigation is needed. A second tensiometer, installed near the bottom of the root zone, is used to adjust the irrigation amount or system run-time in order to ensure that sufficient water is being applied, and to avoid over-irrigation and loss of water and chemical amendments due to drainage beyond the root zone.

The moisture status of the soil is monitored by reading the tensiometers periodically. When the tension readings in the upper tensiometer reach a certain level, an irrigation is needed. This tension level is determined by the irrigator, and depends upon such factors as crop type, soil condition, and root depth. The lower tensiometer is monitored to ensure that enough water has been applied to refill the root zone.

Tensiometers measure the tension with which water is held in the soil. A plant root must overcome this tension in order to extract water held onto the soil particles, and to take in water and nutrients. Low tension readings indicate moist conditions, with water readily available and easy for the plants to extract. As the soil dries, the remaining water is held more tightly to the soil. Tension readings increase, and plant roots have more difficulty extracting water and maintaining optimal growth. At some point, irrigation becomes necessary to maintain growth and quality, and to avoid plant damage.

Factors such as plant variety, soil texture, and growth stage determine the tension at which irrigation should commence. Some examples of recommended tension readings at which to begin irrigation are shown in the following table. For other crops as well as the ones shown, we recommend that you consult with your local USDA representative for recommendations because soil characteristics can greatly changed the recommended irrigation point.

Tensiometers are simple to install and use. A soil auger is used to make the proper-sized hole in the ground. The tensiometer is then inserted into the hole and left permanently. Periodically, the tensiometer may need to be refilled with water. Under irrigated conditions, however, the tensiometer will usually refill itself when irrigation water is applied to the field.

Low tension tensiometers work very much the same as the conventional tensiometers but in light sandy soils and in greenhouse media soils we have found these to give superior readings. The secret to the success of this instrument is in the liquid filled gauge which is more sensitive to the lower centibar readings of 6 to 12 centibars. This also makes it an ideal instrument to be used on crops where drip irrigation is used.

Low tension tensiometers are available in lengths of 6", 12", & 18"

Watermark sensors measure soil moisture indirectly by measuring the electrical resistance between two electrodes that are located at the top of the sensor. Once installed in the active root zone, the sensors are read by using a handheld portable meter.

The watermark sensor is basically an improved gypsum block. It has a sensing range of 10 centibars to 200 centibars but it is generally agreed that it doesn't have good sensitivity under 20 centibars. It's main advantages are:

• It has an internal salinity buffer that doesn't allow it to be affected by soil salts like the gypsum block.
• The sensors won't dissolve in the soil like gypsum blocks do.
• They are unaffected by freezing temperatures since there is no water involved.
• Not subject to vandalism, theft or machinery damage.
• They can be combined with automatic controlling devices to turn irrigation systems on and off.

Lincoln meters provide the greenhouse and nursery growers with excellent results because the soil area to be monitored is small and the instruments themselves don't take up much room in the pots. It also has the advantage against freezing because there is no water involved with this instrument.

Lincoln meters basically work on measuring the wilt point of plants. Units are available in a wide variety of sizes

Soil moisture measuring devices are only as good as you choose them to be. I have seen installations that we put in and then ignored and I have seen the skeptical grower come back and tell me it was the best thing he ever did. If you do nothing else when you put these units in, you should record the readings on a daily or every other day basis. This gets you familiar with the unit and helps you develop a pattern of when to water and how much based upon different times of the year.

There is nothing in this world that is good that doesn't involve some effort. All I can say is that your efforts will be rewarded in dollars to the bottom line.