Ted W Tyson, Agricultural Engineer-Irrigation

Larry M. Curtis, Agricultural Engineer-Soil & Water

Scheduling irrigation is determining when to irrigate and how much water to apply. Both are important decisions for an irrigator. Not enough moisture in the soil can cause crop stress while too much water can wash fertilizer from the root zone. Crop yields suffer from either extreme. Excess water also wastes pumping energy.

Irrigation scheduling seeks to prevent yield-reducing plant stress caused by lack of soil moisture while, at the same time, preventing waste of labor and pumping energy due to over-irrigation.

Keeping Moisture In The Soil

A common guideline in irrigation scheduling is to maintain soil moisture above 50 percent. This is a general guideline and will vary with different crops. The majority of Alabama crops are most sensitive to moisture stress when they are in the reproductive (flowering and early seed fill) stages of growth. Crops are less sensitive later in the growing season as they reach maturity.

A good irrigation scheduling scheme will begin irrigation early enough to avoid stressing the last part of the field before completion.

This publication describes the soil moisture tension (SMT) method of scheduling irrigation. If used properly, it is an excellent tool for deciding when and how much to irrigate.

Soil-Water-Plant Relationships

You must understand soil-water-plant relationships before you can properly manage soil water and before you can get the maximum benefits from irrigation.

Evapotranspiration

The soil may be thought of as a reservoir, storing water from rainfall and irrigation. Water leaves the soil by evaporation from the soil surface and by transpiration from plant leaves and stems. Both of these losses together are called evapotranspiration (ET). ET depends on the type of crop, stage of growth, temperature, sunshine, wind speed, relative humidity and amount of soil moisture. For maximurn growth and production, soil moisture must be maintained in a range that permits water to be taken in by plant roots as fast as ET losses occur.

Available Moisture

A soil's available moisture-holding capacity is the moisture that a plant may extract from the soil. Field capacity is the amount of water the soil will hold against drainage-one or two days after a good wetting. At the permanent wilting point, plants wilt and remain wilted until water is added to the soil. Plants may die at this point. Available moisture is the difference in moisture content between wet soil at field capacity and dry soil at the permanent wilting point.

Soil texture is the major factor affecting available moisture-holding capacity. Texture refers to the amount of sand, silt, and clay particles in soil. Clay soils may hold as much as three times the water of sandy soils.

Soil Moisture Tension

The force that holds moisture in the soil increases rapidly as plants near the wilting point. This force per unit area is called soil moisture tension (SMT). SMT is generally measured in units of pressure (or suction) called bars. One bar equals one atmosphere of pressure (14.7 pounds per square inch) or 1,030 centimeters of water column height.

Soil Moisture Release Curves

Figure 1 shows variations in available soil moisture content with soil moisture tension for five commonly irrigated Alabama soils. These moisture release curves show that, at equivalent soil moisture tensions, soils with larger amounts of silt and clay hold more available water than sandier soils.

Plant Root Zone

Each crop has a typical root zone depth. The root zone determines to what depth the plant can extract water from the soil. Plant roots extract most of their water from the upper root zone. Therefore, less than the fully developed root zone may be used for managing irrigation water.

Always make sure soil throughout the root zone is moist at or near the beginning of crop emergence and growth. If necessary, make the root zone wet by irrigating. If there is a dry layer of soil in the root zone, roots cannot grow through it and a reduced rooting depth will result.

The relationship between available soil moisture and soil moisture tension (SMT) is the basis for two commonly used devices for monitoring available soil moisture. These two devices, tensiometers and electric resistance sensors, provide the most practical methods for determining soil moisture on the farm. The SMT method of scheduling irrigation uses either device.

For accurately determining the soil moisture in field crop root zones, these moisture monitoring devices should be placed in the field no later than 30 days after planting.

Placing the instruments early in the growing season allows the roots to grow and develop around the sensing devices. Soil moisture determinations in the undisturbed root zone gives more accurate readings of field moisture.

Tensiometers

A tensiometer is a sealed, water-filled tube with a porous ceramic tip on one end and a vacuum gauge on the other (Figure 2). Most tensiometer vacuum gauges are calibrated in hundredths of a bar (centibars, or cb) and graduated from 0 to 100

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As soil dries, water is drawn out of the instrument, reducing the water volume in the tube and creating a partial vacuum that is registered on the gauge. The drier the soil, the greater the force per unit area holding the remaining water in the soil, and the higher the reading.

When soil receives moisture through rainfall or irrigation, the vacuum inside the tube draws water back into the instrument from the soil, thus lowering gauge readings. Reduced gauge readings from rainfall or irrigation will usually show up in 24 hours.

Tensiometers work best in sandy soils at tensions of 0 to 70 cb.

Electrical Resistance Meters And Sensors

Electrical resistance meters determine soil moisture by measuring the electrical resistance of a soil moisture sensor. This sensor is made of a block of gypsum or similar material and is permanently embedded in the soil. The electrical resistance of the block varies with its moisture content, which is dependent upon the moisture content of the soil. As the soil dries, the block loses moisture and its electrical resistance increases. Most electrical resistance sensors are not very accurate at low soil moisture tensions and should not be used on coarsetextured soils. There are, however, some newer electrical resistance sensors that are accurate at lower soil moisture tensions. These new sensors last longer than conventional gypsum blocks, which usually last only one season.

Resistance blocks are generally calibrated for soil moisture tension so that readings will be applicable for all soil textures. If a calibration is stated in terms of percent available soil water, it must be made for specific soils. Blocks made by different companies vary considerably in their response to changes in soil moisture tension; consequently, manufacturers should furnish calibration curves for their own instruments and blocks.

A calibration curve for one of the new type electrical resistance- sensors is shown in Figure 3. Once readings are understood in terms of soil moisture tension, interpretation is the same as with tensiorneters. This particular sensor requires that you correct for field soil temperature. This is easily done by the operator when reading the meter.

You can use one meter to read several sensors, one at a time, by simply reconnecting the meter leads to each.

Interpreting Readings

As soil moisture suction increases and decreases each day in a pattern similar to air temperature, you should take and use only the very early morning tensiometer/sensor readings to schedule irrigation. Using afternoon readings could result in too much irrigation.

If possible, refer to the soil moisture release curve for the particular soil where each tensiometer/sensor tip is located. Read from the curve percent available soil moisture corresponding to the measured soil moisture tension reading in centibars.

For example, from Figure 1, an SMT reading of 30 centibars from an instrument located in a sandy loam soil would contain 65 percent available moisture. An SMT of 50 centibars would indicate 45 percent available moisture in the same sandy loam soil.

The following general guidelines can be used for interpreting soil moisture tension readings for medium to moderately coarse textured soils:

• Reading of 0 to 10 centibars (cb). The soil is at or near saturation. This often occurs for one or two days following rainfall or irrigation. Plant roots may suffer from lack of oxygen if readings persist in this range.
• Reading of 10 to 20 cb. The soil is near field capacity. Sandy soils will be near field capacity in the lower range while clay soils will be near field capacity in the upper range.
• Reading of 20 to 30 cb. This is the normal range for starting irrigation. Irrigation at this point ensures readily available soil moisture at all times and provides a safety factor in case of practical problems such as delayed irrigation or the inability to obtain uniform distribution of water to all parts of the field.
• Reading above 40 cb. Field crops in most moderately coarse and coarse-textured soils in Alabama are moisture stressed at such a reading. Although all available water is not used up, readily available water is below what is required for maximurn growth. At readings above 40 cb, tensiometers may break tension (vacuum is destroyed) before irrigation is completed. This is especially true in coarser textured soils. You should suspect this problem if a gauge reading remains constant with increasing soil dryness or if the gauge reading falls without rainfall or irrigation. Service the tensiometer if this occurs. Take immediate steps to irrigate to reduce the gauge reading to an acceptable level.

Location Of Moisture Monitoring Stations

A moisture monitoring station consists of one or two moisture monitoring devices placed in the row drill about halfway between plants. These monitoring stations are located in each major area of the field that has a different soil type and top soil depth. Leave the monitors in place throughout the growing season.

Usually, at least one station is needed in each area of the field that can be irrigated in one day. Avoid low spots when you select representative areas of the field for station locations.

Be sure to mark stations so equipment operators can avoid them during field work. 

Installing And Servicing Moisture Monitors

Follow manufacturer's recommendations when preparing a moisture monitor for installation. This includes filling the tensiometer with solution, removing air from the gauge, and removing air from the pores of the ceramic tip and from all internal plastic parts. A service unit, which includes a hand vacuum pump for removing air and testing the gauge, is available from some manufacturers.

Where the newer type electrical resistance sensors are used on annual crops, one-half inch PVC pipe may be used to allow easy removal of the sensor. Install the sensor wires through the pipe that sits firmly on the sensor. Cover the above-ground, open end of the pipe with duct tape to prevent moisture from entering. Such moisture would give false soil moisture indications from the field. At season's end, remove the tape and pour water down the pipe to loosen the soil. The pipe, wire, and sensor unit are then removed from the loosened soil.

Install monitors in the active root zone. Only one monitor is needed if plants have active rooting zones that are less than 15 inches deep. Set the electric resistance sensor or tensiometer tip about half of the depth of the rooting zone, but not less than six inches deep. If two moisture monitors are used per station, set one with the sensor about a third of the depth of the rooting zone. The other monitor should be set with the sensor about three-quarters of the depth of the major rooting zone (Figure 4). When soil moisture tension and soil type (see Figure 1), are known at these depths, an accurate estimate can be made of water conditions throughout the root zone.

Figure 4: Installation Of Tensionmeters In A Field For A 24-Inch Moisture Control Zone.

Typically, with a two-instrument station, readings from the shallow instrument determine when to start irrigation. Readings from the deeper instrument show whether the amount of water applied was enough (readings lowered) or not enough (readings remain high or increase) to reach the lower root zone.

Some manufacturers recommend inserting the sensor or ceramic tip into a prepared hole so the walls of the tip are in close contact with undisturbed soil and roots. You can prepare such a hole by driving a steel rod or pipe-the same diameter as the moisture monitor tube-to the desired depth.. Carefully remove the rod and push the moisture monitor to the bottom of the hole. Press the soil around the monitor tube at the surface; then, slightly mound some soil around it so surface water will not collect at the monitor and seep down along the tube.

The following installation method is often used with equally good results:

• Bore the hole with a soil auger (11/4 inches in diameter) to the placement depth.
• Make a slurry using screened soil in the bottom of the hole.
• Place the sensor (tip) into the hole.
• Backfill with screened soil.
• Tamp the soil firmly around the tube or pipe with a half-inch dowel.

A tensiometer may have to be refilled with water occasionally. The best time to add water is after an irrigation when the vacuum is low. After refilling, the vacuum pump may be used to remove air bubbles.

Scheduling With The Soil Moisture Tension Method (SMT)

Scheduling irrigation using soil moisture tension is based on frequently determining soil moisture tension (at least every other day during the peak water use period) and then irrigating to maintain a minimum soil moisture tension level based on the particular crop grown and soil present.

For field corn, this peak use period is from early tassel (beginning 65 to 70 days after planting) to full dent (around 120 days). For high yields, corn can use up to 0.33 inch of water per day during this period. For peanuts, the peak water use period is from early pod setting/peak flowering (around 60 days after planting) through peak pod filling (around 95 to 100 days after planting). During this period, peanuts can use up to 0.26 inch of water per day. Supplemental water from irrigation is used to maintain soil moisture tension at an acceptably low level based on the crop grown and soil type present.

To get best use from the SMT method, SMT is recorded when it is determined and is then used to project SMT levels two to four days ahead. Charts provided by the instrument companies provide a good method of doing this (Figure 5). This projection time is determined by the capacity of your irrigation system. Always begin irrigation early enough to maintain soil moisture tension at the minimum level in the last part of the field to be irrigated.

The biggest advantage to this method is that immediately after reading either the tensiometer or electric resistance sensor, you know when you need to begin irrigating. The biggest disadvantage to the method is the time invested in reading from two to eight SMT devices per field each time the field soil moisture is determined. This could add up for daily SMT determinations made during the peak moisture use period. One other disadvantage with tensiometers is the servicing time that may be required for each instrument. This servicing time will vary from weekly to once per season depending on soil type and how good a job you are doing irrigating your crop.

The Bottom Line

Irrigation requires large investments of equipment, power, maintenance, and labor. For an initial $2 to $4 per acre, soil moisture tension instruments and this SMT method of scheduling irrigation will, with reasonable care and proper use, greatly increase the financial return from a properly designed and installed irrigation system. Returns from making the right decision on when and how much water to apply can easily be 50 times the investment in time, effort, and instruments to do the job.

For More Information

The following references were used in developing this publication and provide additional detailed information on irrigation and irrigation scheduling.

Conklin, W. Franklin, and others. 1983. Irrigated corn production in Georgia. University of Georgia, Georgia Cooperative Extension Service Bulletin 891.

Curtis, Larry M. 1982. Irrigated corn. production. Auburn University Alabama Cooperative Extension Service Circular ANR-165.

Hartzog, Dallas L. and others. 1981. Peanut production in Alabama. Auburn University Alabama Cooperative Extension Service Circular ANR-207.

Hartzog, Dallas L. 1985. Physiology of peanut crop growth. 1985 Alabama Peanut Pest Management Scouting Manual. Aubum University Alabama Cooperative Extension Service.

Rochester, Eugene W, and others. 1984. Irrigation schedules for peanut production. Auburn University. Alabama Agricultural Experiment Station Bulletin 556.

Samples, L. E. 1981. A guide for peanut irrigation. University of Georgia, Georgia Cooperative Extension Service Circular 685.

Wright, a L., and others. 1981. Irrigated corn production. Institute of Food and Agricultural Sciences, University of Florida, Florida Cooperative Extension Service Circular 486. 

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.