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  Sensors Aid Irrigation Management
David Zoldoske and Greg Jorgensen

CATI Publication #900607
© Copyright June 1990, all rights reserved

There are two basic approaches to obtaining the information needed to schedule irrigations. One approach is to measure factors influencing crop water use, such as weather and crop stage of growth, and then calculate the moisture status of the soil in the root zone. A more direct approach is to sense directly the soil moisture status, or plant stress condition, triggering an irrigation when some critical level is reached. Even those using a weather based approach to irrigation scheduling often do so in conjunction with soil moisture or plant stress sensors. This article reviews the basic types of sensors and techniques used for direct measurement of soil moisture or plant stress measurement.

One of the oldest and simplest methods of measuring the soil water content is to squeeze a handful of soil between the thumb and index finger. This so called "feel" method may be considered crude by some irrigation technologists, but can give the experienced irrigator a quick, in-the-field check, and is widely used. A shovel, push tube or soil auger may be used to obtain a sample.

The gravimetric method gives a more precise measurement of the soil water content. Samples are selected from various soil depths, as with the feel method. The soil samples are immediately placed in airtight containers and taken to the laboratory for analysis. There the soil samples are weighed; then dried in an oven at 105°C for 24 hours; and finally reweighed. The soil water content is calculated from the difference between the wet and dry weights. One of the disadvantages of this method is that it is destructive in the sense that it requires sample removal from the field. This makes it impossible to make another measurement at a future date at exactly the same point. A further problem is the 24 to 36 hour delay between sampling and having the results available.

Soil Moisture and Appearance Relationship Chart
This Chart indicates approximate relationships between field capacity and wilting point.
For more accurate information the soil must be checked by drying samples.
(loamy sand)
(sandy loam)
(clay loam)
(field capacity) (field capacity) (field capacity) (field capacity)
.0 Leaves wet outline on hand when squeezed. Appears very dark, leaves wet outline on hand, makes a short ribbon. Appears very dark, leaves wet outline on hand, will ribbon out about one inch. Appears very dark, leaves slight moisture on hand when squeezed, will ribbon out about two inches.
.2 Appears moist, makes a weak ball. Quite dark color, makes a hard ball. Dark color, forma a plastic ball, slicks when rubbed.
.4 Appears slightly moist, sticks together slightly. Dark color, will slick and ribbons easily.
.6 Fairly dark color, makes a good ball. Quite dark, forms a hard ball. Quite dark, will make a thick ribbon, may slick when rubbed.
.8 Dry, loose, flows through fingers. (wilting point) Slightly dark color, makes a weak ball. Fairly dark, forms a good ball.
1.0 Lightly colored by moisture, will not ball. Fairly dark, makes a good ball.
1.2 Very slight color due to moisture. (wilting point) Slightly dark, forms a weak ball. Will ball, small clods will flatten out rather than crumble.
1.4 Lightly colored, small clods crumble fairly easily. Slightly dark, clods crumble.
1.6 Slight color due to moisture, small clods are hard. (wilting point)
1.8 Some darkness due to unavailable moisture, hard & cracked clods (wilting point)
2.0 --- --- --- ---

Field Method of Approximating Soil Moisture for Irrigation, from Am. Soc. Agri. Engr. Vol. 3, No. 1, 1960, by John L. Merriam, California Polytechnic College.

The tensiometer is a device for measuring matric potential (capillary tension) in soil. Matric potential is analogous to the effort required to draw fluid through a straw. It is the force that must be exerted by the roots to remove water from the soil. Tensiometers measure the matric potential directly. The tensiometer functions between 0 and -0.8 bars ( 0 to 80 centibars) tension, which is a small, but significant part of the entire range of available water. Tensiometers are likely to break tension or suction beyond -1.0 bar, hence, tensiometers are best suited to systems and irrigation management that maintain a high moisture level in the soil. They are typically placed at several depths in the root zone with the difference in readings monitored to initiate the irrigation event.

For sizes common to agricultural applications, tensiometers range in price from $35.00-$45.00. For routine maintenance, a vacuum pump is used to evacuate air from the tensiometer and, when equipped with a vacuum gauge, enables the user to check the accuracy of the vacuum gauge on the tensiometer. Service kits, including a vacuum pump and a chemical to prevent algae growth in the tensiometer, are available for under $25.00. In applications where tensiometers are subjected to freeze conditions, they must be protected by covering with soil, sawdust, or other insulating material.

Another common method of estimating matric potential is with gypsum or porous blocks. These devices contain two electrodes connected to a wire lead and embedded in a porous block of gypsum or fiberglass. These are buried at the desired depths and location, so that moisture can move in or out of the block, until the matric potential of the block and the soil are the same. The electrical conductivity of the block is measured using a meter which employs an alternating current bridge. The manufacturers of the blocks generally provide a calibration curve relating conductivity to the matric potential for any particular soil type.

The use of porous blocks has the advantages of low cost and the possibility of measuring the same location in the field throughout the season. The blocks function over the entire range of soil water availability. Disadvantages of this method include the facts that each block has slightly different calibration characteristics and that these characteristics gradually change over time. Blocks are prices from $5.00-$6.00 (depending on the length of the wire leads). A hand held meter is required to read the blocks, and these are available for $200.00-$250.00. The blocks will generally last up to two years, however under saturated conditions, their life can be substantially shorter.

A similar moisture block made of ceramic material rather than gypsum employs the principle of heat dissipation rather than conductivity to measure moisture. The blocks contain small heating and temperature sensing elements, and the wetter the soil, the quicker the heat is dissipated. These sensors are very accurate and long lasting, and are not affected by salinity. They are, however, expensive and require more calibration. The sensor sells for $115.00, and the meter for approximately $1,500.00. Life expectancy is reported to be 3-5 years.

The neutron probe has been popular in recent years to estimate the volumetric water content of the soil. This method utilizes fast traveling neutrons emitted from a radioactive device lowered down an access tube made of plastic, aluminum, or electrical metal tubing (emt). The neutrons collide with hydrogen atoms associated with water (H20) and are slowed. The numbers of slow neutrons counted bouncing back gives a good indication of the amount of water present. This method has the advantage of measuring a large soil volume at several depths at the same location. The instrument can be configured

to store data, which is then easily transferred to a computer for analysis and used to forecast the next irrigation. There are several disadvantages, including the high cost of the instrument, the radiation hazard, and the weight of the instrument (it can become heavy after lugging it through the field all day). Operators must be trained and licensed, and are required to wear a sensing badge to monitor exposure to the radioactive source. Also, depending on the radioactive source, there are restrictions on transporting the probe. Neutron probes sell for $3000. to $4500., depending on features such as micro processors for direct readout and data transfer capabilities.

A more recently developed technique to determine irrigation timing is based on plant canopy temperature rather than soil moisture. This is done through the use of an infrared thermometer which can quickly evaluate a crop for stress.

The principle involves the plant's ability to regulate its stomatal opening. The plant requires carbon dioxide (CO2) with the majority entering through the pores (stomata) in the leaf through which water also evaporates. The plant can control water loss by regulating the stomatal opening, however this reduces growth and yield. With the stomata closed, less water is evaporated and the leaf temperature rises. This is the rationale for measuring the plant canopy temperature. The difference between the plant canopy temperature and the ambient air temperature, with adjustments for humidity and wind provides a measure of plant stress.

The infrared thermometry method has been studied by various researchers for nearly 30 years with only limited application to commercial agriculture. Recently, advances in the technique have been made and instruments with a combination of the appropriate sensors to collect the required data along with a microprocessor have become available. Commercial units available today calculate plant stress based on the crop water stress index.

The advantages of this method include the ability to make many quick field measurements, portability, and direct estimates of the level of plant stress. One of the disadvantages is the initial high cost, $4000. to $4500., for the agricultural model.

All of these sensors can provide accurate and reliable information provided that they are calibrated, used and maintained correctly. It is important to match the proper sensing technique with your irrigation method, soil, and crop requirements. Your local farm advisors office should be able to help with the proper selection, and the recommended number of sensors or reading sites per field.