In unsaturated soils, in which the void space is occupied by air as well as water, the pressure potential value is always negative. Pressure potential is usually expressed in bar (bar), centibar (cbar), millibar (mbar), cm water pressure, kilo pascal (kPa), or other units. The positive value of pressure potential is called tension. Thus, if one measures a pressure potential value of -15 kPa in a given soil, then the tension of that soil is 15 kPa.
There is a clear relationship between soil water tension and water content, which can be illustrated with a soil-water retention curve, or soil-water characteristic curve. This relationship is different for every soil. f soil dries out, tension increases (and pressure potential decreases, i.e. becomes more negative), leading to a reduction in plant growth as water becomes less available to plants. If soil is very moist, its tension is low, which might mean that there is too much water for optimal growth of most plants. Thus tensiometers are widely used to monitor the availability of water in soils for plants.
Tensiometers are also used to monitor the pressure potential at various depths in unsaturated soils. These measurements are used to calculate the gradient in pressure with depth in order to estimate if water is moving up or down in a soil profile or is not moving in either direction. Once the gradient and the unsaturated hydraulic conductivity of the soil are known, the rate of water movement in a field soil profile or in a soil column can be calculated.
Standard SMS tensiometers are made of 0.85 inch OD (21.5 cm) grey plastic pipe with a 0.875 inch (2.22 cm) ceramic porous cups at their lower end. The tensiometers have 5-cm sections of clear plastic tubing at their upper ends. In the field the grey plastic pipe is filled with water till 2 cm from the top, closed with a silicone filled rubber septum stopper, and placed in the soil. Once in contact with soil, water moves from the tensiometer through the porous cup and into the unsaturated soil. This reduces the pressure inside the tensiometer (water is incompressible).
The process continues until the negative pressure inside the tensiometer pipe equals the negative pressure in the surrounding soil. The pressure inside the tensiometer pipe, which is in equilibrium with the pressure in the soil, can now be measured with a tensimeter or with a pressure transducer.
The operating range of tensiometers is from 0 cm H2O (0 kPa) pressure potential when the soil is fully saturated to about -800 cm H2O (-80 kPa) when the soil is dry (i.e. too dry for most common agricultural and horticultural crops). Tensiometers can be ordered from SMS in lengths varying from 6 inches (15 cm) or less, to 6 feet (183 cm). Tensiometers longer than 6 feet are available on special request.
Tensiometers with pressure transducers are similar in appearance to standard tensiometers. They still have the clear plastic tube with septum stopper on top but they also have a tee near the top, just below the clear plastic tubing. Pressure transducers with a 0 to15 psi range (0 to1000 mbar or 0 to100kPa) are placed inside this tee. The transducers are connected to a data logger (such as the Campbell CR-510 (maximum 2 pressure transducers), CR-850 (maximum 3 transducers), CR-1000 (maximum 4 transducers), CR-10X (maximum 6 transducers)) to record the changes in tension over time. This way a continuous record of the soil water tension is obtained.
With these tensiometers it is possible to record the pressure with the transducer, and at the same time determine the pressure with a handheld tensimeter. This makes it easy to check the pressure transducers in the field and verify their calibration.
Pencil tensiometers are used in laboratory columns or in small containers (pots) for growing plants. Their cups are 2.54 cm long with an outside diameter of 1 cm. The tubing is transparent, and has an outside diameter of 1.37 cm (0.54 inches). A small rubber septum stopper with a silicone core closes the upper end of the tubing. These tensiometers can be made with an elbow, to better fit in a large diameter vertical soil column. Pencil tensiometers are ”read” with a tensimeter or can be equipped with pressure transducers, connected to a data logger.
Elbow tensiometers are made for larger soil columns, or for soil profiles that are accessible through a trench. They are similar to pencil tensiometers (made with the same 1 cm OD porous ceramic cups), except these tensiometers have an elbow and a short vertical section. The horizontal part of this tensiometer may be made as short as 9 cm (including the porous cup), and as long as 40 cm or more. The vertical part, with the septum stopper closure, is 5.5 cm long. When installed, the cup may protrude from about 1 cm to as long as the length of the horizontal section in the soil of the column or the soil profile.
When these tensiometers are installed in large soil columns, they are held in place with a compression fitting, which can be screwed into the wall of the column or flow cell. The hole in the wall needs to have threads made with a 1/2 inch NPT pipe thread. Once the compression fitting is installed in the wall, the tensiometer is inserted through the compression fitting into the soil. After pushing it into the soil, the fitting is tightened and the tensiometer stays in place. A tensimeter is needed to read the tension inside the tensiometer (and in the soil), but elbow tensiometers can also be equipped with pressure transducers.
Column or Flow Cell tensiometers are the smallest tensiometers available from SMS. The cups are 0.67 cm OD (0.38 inch) and 2.54 cm long. They are glued into one end of a 6 cm (may be ordered longer) section of clear acrylic tube. The tube with cup is 7.5 cm long (or longer on request). The other end of the tube has a three-way valve with attached pressure transducer (1 bar or 100 kPa). This tensiometer is attached to soil columns through a 3/8 inch NPT compression fitting, which can be screwed into the wall of SMS flow cells or columns.
The tensiometers can be filled with water by placing them with their cups submerged in water for one day. Once the tensiometer is half full of water, complete filling the tensiometer from the top. This can be done by attaching a syringe to the open port of the three-way valve, and sucking the water through the cup into the tensiometer.
After the soil column has been filled and wetted, the tensiometer is pushed through the compression fitting into the soil. By tightening the fitting, a watertight connection is obtained. The pressure transducer is then connected to a data logger in order to obtain measurements
