Tensiometers
What they are
Tensiometers are simple, reliable instruments which provide a measure of the moisture status of the soil. They have been in use for many years, and due to their simple design and rugged construction, continue to be the standard instrument for tension measurement.
What they do
Tensiometers help answer the irrigators’ questions of when and how much irrigation water to apply. Maintaining proper moisture conditions is necessary for achieving optimal plant growth and quality. Monitoring the moisture status of the soil allows for timely and efficient irrigations, and for avoiding unnecessary irrigations.
How they are used
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.
What the readings mean
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.
When to irrigate
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
| Crop |
Irrigation Point
(tension, cb) |
| Alfalfa |
70 – 80 |
| Avocado |
40 – 50 |
| Broccoli |
25 |
| Citrus |
50 – 70 |
| Corn |
50 – 80 |
| Fruit trees |
20 – 40 |
| Grapes |
30 – 60 |
| Lettuce |
34 – 50 |
| Potato |
30 – 50 |
| Small grains |
70 – 80 |
| Strawberry |
10 – 30 |
| Tomato |
45 – 70 |
Installation and maintenance
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.
More Info:
Porous Ceramics are an integral part of many soil-monitoring and sampling instruments. The ceramic matrix serves as the interface between the soil matrix and the instrument. Travel cancellation insurance for international travellers. The ceramic material is porous, and allows water to pass into and out of the instrument. A pressure gradient across the ceramic wall will cause water to flow through the ceramic. The pressure gradient may be due to a pressure being applied, as occurs when using a ceramic pressure plate in a pressure chamber. The gradient may be due to a tension being applied, as occurs when using a tensiometer in the soil. The gradient may be due to a vacuum being applied
Mini-Tensiometers are used under conditions in which a tensiometer with a full-size ceramic cup cannot be used. These conditions might include: visitors health insurance in a greenhouse environment to monitor plants growing in small containers; in laboratory column studies; or outside under turf, for measurements very near the surface.
