Soybean Irrigation

Yield Response

Variety tests conducted at the Mississippi Agricultural and Forestry Experiment Stations and on private farms are excellent sources of data on varieties, soil types, maturity groups, and irrigated and nonirrigated yields of soybeans. Variety tests that are irrigated and nonirrigated show a significant advantage to irrigation. Look for locations with irrigation to compare yields. Stoneville is the only one with comparative irrigation and dry land tests side by side. Information includes how much irrigation was applied and when, amount and times of rainfall, disease ratings, lodging, and the maturity group. When selecting varieties, use information from test plots and demonstration fields that are similar to your soil types and irrigation systems.

The Soybean Management by Application of Research and Technology (SMART) Program in Mississippi or Verification Programs from other states are good sources for yield history and irrigation data for soybean producers. Annual SMART reports are available through the county and state Extension offices.

Daily and Seasonal Water Use

Generally, you can expect the soybean crop to use about 0.2 to 0.25 inch of water per acre per day during peak demand. Peak water use begins at flowering and lasts through the pod-filling stage. Seldom will water use exceed 0.25 inch of water per day because of the relatively high humidities and low wind speeds experienced in Mississippi during the growing season.

Figure 1 shows a general pattern of growth and water use for soybeans; do not consider it exact. Total seasonal use can be as high as 20 to 25 inches of water, depending on planting date and maturity length.

Estimated Daily Water Use

Effects of Drought Stress

Once a good stand is established and until beginning bloom, soybeans can tolerate short droughts with little or no effects on yield. The best program to follow is not to let soybeans stress at any time, even though some research shows no yield advantage to irrigation before bloom.

Plants must have height at beginning bloom to allow for better harvest efficiency. The biggest exception for not irrigating before bloom might be plantings made after mid-June or in double-crop situations. Early irrigation will help develop plant height and canopy.

Water demands are greatest during the soybean reproductive period. Drought stress from beginning bloom through the pod-fill stage can greatly reduce yields (Figure 2). This loss in yield may result in fewer seeds, smaller seeds, or a combination of the two. Good soil moisture conditions need to exist from rainfall or irrigation to ensure adequate moisture from beginning bloom until the beans are touching in the pods.

Crop Management for Maximum Irrigation Response

Research at Stoneville shows a yield advantage for 20-inch rows over 40-inch rows for full-season irrigated soybeans in a year that is extremely dry. In moderately dry and wet years, row spacing did not make a significant difference in irrigated soybeans. Narrow rows combined with irrigation may create an environment more favorable for the development of certain diseases because of poor air circulation and longer periods of wet conditions.

When To Irrigate

Start your soybean irrigation before any visible signs of drought stress. Research shows that, in most years, soybeans will not show a yield increase from irrigation until just before bloom. If it is extremely dry (especially with late plantings or double-cropping) during the vegetative growth stage, you may need irrigation to give the plants some height (harvestability) and a quicker canopy.

Irrigate soybeans as frequently as necessary until pods have completely filled. The number and frequency of irrigations will vary with the season, variety, soil, and irrigation system capability.

Failure to begin early enough can cause a yield loss that cannot be regained from later irrigations. Research at Stoneville shows that during an extremely dry season, delaying the beginning of irrigation until the pod-filling stage results in much lower yields, compared to beginning irrigation at prebloom or bloom stage. Waiting until pod fill to irrigate causes significant losses when compared to beginning irrigation at bloom.

Irrigation may not be needed by some varieties in some years. Fall subsoiling of Tunica clay soils with early planted Group IV soybeans has been as good as irrigation in most years. However, well-timed irrigations may add enough yield response to more than pay for the irrigation.

How To Determine Soil Moisture

Using the "feel test" on a soil sample from about a 12-inch depth is one of the easiest and least expensive methods to determine soil moisture. To use this method, sample two to three times per week to determine your soil moisture status. If silt clay or clay soil forms a ball that sticks together when squeezed in your fist or is slick or ribbons out with relative ease when pressed between the thumb and index finger, there is probably adequate moisture. The harder it is to make a ball and ribbon the soil, the drier the soil.

You can also use instruments to monitor soil moisture. One of the more commonly used tools in the Midsouth is called a tensiometer, which is a sealed, water-filled tube with a porous, ceramic tip and a vacuum gauge on the upper end. When properly installed and maintained in the soil, a tensiometer measures the soil moisture tension, which is an indicator of the availability of water to the plant. The greater the tension, the more tightly the water is held by the soil and the lower the availability for plant use.

Service the tensiometers properly before placing them in the field, and maintain them properly afterwards. Failure to do so will give inaccurate readings. Follow the manufacturer's instructions carefully.

Place tensiometers at representative sites within the field on the predominant soil type. For furrow irrigation, use four locations to determine soil moisture. Placements at the upper and lower ends of the field and in the first and last irrigation sets are most useful.

For flood irrigation, place the instrument in one station in the top levee interval and one station in the bottom levee interval. For center pivots, use four stations: two stations in front of the pivot -- one inside the first or second tower and one on the outside of the circle between the last two towers. The other two stations are in a similar location behind the pivot.

The location of the instruments allows measurement of the wet to dry gradient in the field from where the irrigation began to where it finished. This gives you an idea as to how fast the field is drying out and when to start irrigation again. Place the instrumentation in the drill to keep it from being damaged or broken by other equipment operating in the field. Mark each site with a flag tall enough to identify the location.

Depth of placement should be less than 30 inches or just above the hardpan if it occurs at a more shallow depth. Some producers use 12-inch and 24-inch placements at each site to better evaluate soil moisture distribution. Some growers use 9-inch and 18- inch depths, and some prefer 12 inches. The more readings, the better the understanding of what is occurring in the root zone. Also, average the readings from the same side of the field at all depths; this gives a better indication of the average soil moisture.

For center pivots, you may need a 6-inch placement, since it may be difficult to get effective watering deeper than 6 inches. Sometimes the pivots will not even wet a tensiometer at 6 inches.

Research shows irrigation needs to be initiated at 50 to 60 centibars at 12 inches. (This is the scale on the tensiometers and on the newer style of moisture blocks.) A reading of zero on the tensiometer or block indicates a soil is at field capacity. The higher the reading, the drier the soil. As the soil moisture levels increase from rainfall or irrigation, the reading should drop back to lower values.

Allow the instrumentation to stabilize for about a day after irrigation before a new reading is taken. If the average readings have not decreased significantly, then continue irrigation. If the readings drop back to zero or slightly above, delay irrigation until the first stations to receive water from irrigation reach the 50- to 60-centibar level again. If allowed to go completely dry, the tensiometer sometimes breaks tension with the soil and gives a zero reading; if this occurs, the tensiometer may need to be reset after the next irrigation.

Some good crop-water-use estimators are also available. These are called evapotranspiration models, and they use weather data to estimate daily crop-water use. Daily water use is balanced against available soil moisture and will indicate when 50 percent of the available moisture is depleted or when the allowable deficit of available moisture has been used. Many of the models predict when the next irrigation needs to be started if there is no rainfall or irrigation.

How Often To Irrigate

When starting irrigation for the first time, or after a rainfall, start soon enough to get across the field before stress occurs. Once you start irrigation, flood and furrow systems typically need water again in 8 to 10 days. Pivots should run continuously because most supply only an inch in 4 days, which nets about 0.8 inch in 4 days. If the pivot can put out more than 1 inch (gross) in 4 days, then adjust to about 0.25 inch per day (gross) and repeat accordingly.

Continue irrigation as necessary until pod fill is completed. The cutoff dates vary with the variety and growing season. Consider pods filled when the green seeds have developed to the point of touching inside the pods. Inspect several pods about four nodes down from the terminal of the plant to determine this stage.

Methods of Irrigation

Slopes greater than 0.1 percent should be used; preferably, 0.15 percent to 0.3 percent slopes are better for moving surface water off in a timely manner. Also, drains and ditches around fields should be clean, large enough, and sloped enough so they can carry water away from the field in a short period of time.

Flood, border, furrow, traveling gun sprinklers, center-pivot sprinklers, and linear-move sprinklers are all systems that can do a satisfactory job if properly designed and managed. They vary in efficiency and cost. Base your selection on these factors, your field's topography, the soil's physical factors, labor costs, and on your management capabilities.

Flood -- This irrigation method requires levees that take away land area for production, take time to survey and construct, and take time to get out after the irrigation. Generally, levees are surveyed on elevations of no more than 0.4 foot.

Flood irrigation is very risky for a germination irrigation or irrigation of small beans. Getting levees to hold water can be another problem with a flood system. Getting water on and off rapidly (within 48 hours) is critical for good soybean growth and development. Flood irrigation is typically 50 to 60 percent efficient.

Border -- This system is a cross between flood and furrow irrigation and best fits straight levee rice fields or fields with no side slope. It is a flush system that moves water down the slope in a shallow flush between two small levees or dikes (borders). The border spacing is based on the well's flow rate and the length of the field.

Most border systems are designed to move water through a bay in 12 to 24 hours maximum. The cracking clays make this a uniform system for soybeans and simple to operate once it is set up. The efficiency is 60 to 80 percent in most cases and can be used on small beans. It is similar to furrow except it is ideal for flat-planted beans and works best where there are no side slopes involved. Border irrigation is not recommended for germinating soybeans but could help with germination in spotty stands, if it is not too hot.

Furrow -- This is perhaps the most common system of irrigation in the Delta. It requires grading the land to a slope of 0.05 to 0.5 foot per 100 feet of row. The best row grades are 0.15 to 0.3 percent.

Gated or roll-out pipe is most often used. Considerable labor is required for handling pipe; consequently, more and more farmers are buying enough pipe to put in place for an entire season or using roll-out tubing.

Problems are often associated with uneven distribution of water down the row. It helps to keep irrigation runs to 1/4 of a mile or less and to water clay soils before they crack too badly. Large stream sizes per row will get water through the field quicker and more efficiently than will a small stream size that may take up to 3 days to get down a row. Furrow irrigation efficiency can range from 50 to 70 percent.

Sprinkler -- A sprinkler system (especially center pivot) can supply smaller amounts of water more frequently than can the flood or furrow methods. Most pivots in Mississippi are nozzled with capacities to apply 1 inch of water in 4 days. This is a net application of 0.2 inch per day, which just meets normal demand for soybeans. With this type of system capacity, the pivot is designed to run 24 hours per day, 7 days per week during peak water use.

Center pivots require little labor but high capital investments. Efficiency of water applied is about 80 to 90 percent, depending on weather conditions.

Traveling Gun -- The traveling gun sprinkler system has an advantage in irrigating small, irregular-shaped fields. Size this system at nothing less than 5 gpm per acre to be irrigated. It typically has a higher cost per acre than does a center pivot and requires more labor and energy. The application efficiency is about 70 to 80 percent for the traveling gun.

Management Tips Summary

Meet all sound agronomic production practices before considering irrigation. Select a system to fit soil, labor, capital, and management needs. Provide good surface drainage for all irrigation systems. Begin watering soon enough to avoid stress. Avoid overwatering clays and silt loams. Use flood, border, or furrow irrigation on clay soils before they are heavily cracked. Apply at least 1 inch of water per application with sprinklers during peak water use,and repeat based on a 0.25-inch-per-day gross use. Repeat furrow and flood irrigations regularly (every 8 to 10 days) during peak water use, unless adequate rainfall occurs. Continue irrigating until beans are touching in the pods.

ByJames G. Thomas, Leader, Extension Agricultural Engineering, and Alan Blaine, Ph.D., Extension Soybean Specialist

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Publication 2185
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director