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Water Quality Data
Measuring Soil/Plant Moisture Tension
Measuring water volumetrically, that is, in terms of actual water content is important. It gives us an idea of how much water to replace at an irrigation. Remember though, the plant does not care how much water is actually in the soil. It only cares how hard that water is being held by the soil. Thus, to prevent stress you would like to be able to measure this water-holding force, the soil moisture tension.
The most well known methods are the tensiometer and gypsum blocks. Both are in wide-spread use. The are relatively inexpensive and easy to use. However, like a neutron probe access tube, once installed they can only measure soil moisture tension at that one spot. Care must be taken in choosing the measuring site and the depths of installation.
Also, for complete water management there should be a way to relate a relative soil moisture tension reading to the soil moisture depletion to allow for efficient irrigations.
This is a very important point. The measurement of soil moisture tension provides an indicator for WHEN to irrigate. We want to irrigate before excessive stress on the plant, to provide for optimum plant development. The measurement of soil moisture content provides the estimate of HOW MUCH to irrigate. This is how much water to apply when we do irrigate.
The tensiometer tries to mimic the root as closely as possible. There are three main parts to the instrument, illustrated in link below. They are a water-filled tube (of varying lengths depending on what depth of soil is to be measured), a vacuum gauge on the above ground end of the tube, and a porous, ceramic tip on the other.
http://www.sowacs.com/sensors/index.html
The tensiometer is inserted into the soil to the desired depth. As the soil moisture level decreases at that depth, the soil will try to draw water out of the tube through the porous tip. This creates a vacuum in the tube which is read on the gauge. It is a direct reading on the stress that the plant roots must overcome. If irrigation (or rain) water is added, the vacuum in the tube will draw water back into the tube, decreasing the vacuum and reducing the gauge reading.
Tensiometers will generally read a range of from 0 to 70-80 centibars. Above 70-80 centibars, the water flow between the soil and the ceramic tip breaks and suction (and thus, the vacuum in the tube) is lost. The reading will go to zero.
This is an important point, especially with deep tensiometers. A zero reading on a tensiometer can either mean a very wet soil, or a very dry soil.
"Gypsum block" is a generic term. Modern blocks may be made of fiberglass and other porous materials. They consist of a cylinder of porous material, about 1 inch in diameter and two inches long, with two wires embedded in it but not touching.
http://www.sowacs.com/sensors/index.html
The block is buried at the desired depth with the electric leads extending above ground. When measuring tension, an electric voltage is sent through the wires. The resulting current flow is read with a sensitive instrument. The porous block will contain more or less water as the soil dries or wets. The wetter the block, the more current will flow. Instruments used are commonly calibrated so that the current flow is read as centibars.
An advantage of gypsum blocks are that they can be used to read very high soil moisture tensions.
Another instrument that may be called a gypsum block (because they are very much alike in appearance) is the thermal dissipation sensor. Wires are again embedded in a porous ceramic block. When wet, the block will dissipate (throw off) heat rapidly. When dry it dissipates heat much slower. An electric circuit is used to quantify this change as the soil wets or dries.
With a neutron probe, the access tube allows you to measure at any depth or depths desired. But once buried, the gypsum block or tensiometer is in one place. Thus, tensiometers and any form of "gypsum block" are usually used in "banks". That is, two or more of the instruments will be installed at the same field site but at different depths.
Measuring Plant Moisture Tension
Measurements of moisture tension can be soil-based or plant-based. Soil-based measurements measure the tension in the soil, as with a tensiometer or gypsum block. This is a direct reading on the tension that the plant must overcome. There is an instrument that measures the moisture tension within the plant directly, the plant leaf pressure chamber (also called the "pressure bomb").
This device consists of a pressure chamber with a special removable cover, pressure gauge, and pressure source. When using the pressure chamber, a leaf/petiole sample is cut. The petiole is inserted through a self sealing hole in the pressure chamber cover. The cover is then screwed on the chamber with the leaf inside the chamber and the cut end of the petiole left out. Then pressure is slowly introduced to the chamber. At some point, sap will be seen to bubble from the petiole. The pressure gauge is read in centibars of pressure. The reading is a direct measurement of the plant moisture tension and thus, the stress in the plant.
http://www.sowacs.com/sensors/index.html
Leaf chambers have been very effective in scheduling irrigations (particularly the first seasonal) for cotton. The UC Extension has done much testing to develop specific recommendations. They can suggest desirable pressure readings at the first seasonal and following irrigations in different varieties.
Pressure chambers are flexible in that you can go anywhere in the field to cut petiole samples. However they are restricted to 2-3 hours of sampling time a day, usually around solar noon (when the sun is highest).
Again, tension measurements are essential to prevent stress from lack of soil moisture. However there must still be some way of knowing how much water to apply during an irrigation. Effective, efficient irrigation management combines tension measurements with volumetric soil moisture measurements.
The Thermal Infrared Thermometer
Another instrument in use is the thermal infrared thermometer (sometimes called the "infrared gun" because of its appearance). This instrument is a thermometer that measures temperature by reading infrared radiation. The instrument is aimed at whatever you wish to measure the temperature of.
In use the thermometer is aimed at the crop canopy so that the average temperature of the crop's leaf surfaces is measured. At the same time a measurement of the ambient air temperature is taken. The difference between the two temperatures indicates the amount of stress on the crop. The basic theory being that if ETc is normal, the leaf temperatures will be lower (as water evaporates through the surfaces).
The thermometer is very fast and easy to use. You can point it anywhere in the field. However, it should be used at about the same time every day, it should be used on a full canopy (so that bare soil is not read), and also in low wind.
One of the keys for effective use of any form of soil or plant moisture measurement is picking the correct field site and depth in the root zone to sample. Do you want to irrigate to the driest part of the field, or the wettest? Do you irrigate to the sand streak, or the rest of the field. This is again an example of the art and science of irrigation management. Science has provided the tools for measuring soil moisture. You have to decide where best to measure and interpret the results.
Last updated September 2000