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Cooperative Extension Service |
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Agricultural
Experiment Station |
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Aquaculture
Dale Bumpers College
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Soil and Water Management
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| Table 1. General Soybean Growth and Water Use | |
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Crop Development |
Water Use |
| Germination and seeding | 0.05 - 0.10 |
| Rapid vegetative growth | 0.10 - 0.20 |
| Flowering to pd fill (full canopy) | 0.20 - 0.30 |
| Maturity to harvest | 0.05 - 0.20 |
Moisture stress anytime after planting can reduce growth and yield. The goal of early irrigation (prior to bloom) is to promote adequate vegetative growth and node development. Prebloom irrigation is almost always needed on late-planted and double-crop soybeans. The crop should be irrigated as needed to avoid moisture stress and to provide good soil moisture at seed fill (R5-R6 growth stage), ensuring that the seeds achieve their maximum size. Most growers realize the need to irrigate when the crop is blooming and setting pods. Experience indicates, however, that many growers tend to be late with the first irrigation and then quit before the crop can reach its full potential. The lack of early and late season irrigation is often responsible for a soybean crop not reaching its irrigated yield potential.
The timing of irrigation is commonly referred to as irrigation scheduling. Correct timing is critical to maximizing yield. Having the ability to irrigate is important, but it is also essential that a grower have a commitment to apply irrigation in a timely manner. Too often growers irrigate by the appearance of the crop. Visual stress, especially during bloom and pod set, results in yield loss. Also, once irrigation is started, the time required to finish a field will result in part of the crop suffering even greater stress. If the soil moisture can be determined, then irrigation timing decisions can be improved.
Determining the soil moisture by visual observation or by kicking the soil surface is difficult and can be misleading. The "feel" method can be used to more accurately determine the soil moisture condition. This method involves using a shovel or soil probe to pull a soil sample from the root area. In general, if the soil forms a hand-rolled ball, the soil moisture is adequate. A key to this method is to take samples across the field at different depths to better determine the soil moisture for the field. The challenge is to determine when to begin irrigation so the entire field can be irrigated before any part becomes too dry, but satisfactory results can be achieved with experience.
A more precise method employs tensiometers, a sealed, water-filled tube with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. The tensiometer is installed in the seedbed at a depth where the majority of the roots are located. A 12-inch depth is commonly used for surface irrigation, except where a hardpan exists, and there it is placed just above this layer. Shallower settings at about 8 inches deep are recommended for center pivots. Two or three tensiometers per field are recommended to avoid a problem if one of the tensiometers quits working. Starting irrigation at a vacuum gauge reading of 50-60 centibars on silt loam and clay soils, and 40-50 centibars on sandier soils, is recommended. Tensiometers are fairly reliable and effective when checked and maintained properly. However, the time and effort required usually results in most producers not being able to use them very effectively.
Soil moisture accounting is used to calculate the soil-water balance in the root zone throughout the growing season. This method is sometimes called checkbook irrigation scheduling because a record is kept on the water that enters and leaves the soil like an account balance is maintained in a checkbook. Two forms of the checkbook procedure are available through county Extension offices in Arkansas – the Checkbook User’s Guide and the Irrigation Scheduling Computer Program.
The Checkbook User’s Guide is used to keep a written record of the soil moisture balance when a computer is not available. It is a three-page handout that shows how to use a water use chart and a water balance table to monitor the soil moisture. The water use chart shows an estimate of how much water the crop uses each day based on the maximum temperature and the age of the crop. Daily water use and rainfall amounts are entered into a water balance table. Maximum temperature data can be taken from the weather, newspaper, etc., but the rainfall should be measured with a gauge at each field. Adding and subtracting these numbers in the table determines the soil moisture deficit. Table 2 shows the recommended allowable deficits that are included in the User’s Guide to help determine when to irrigate. The allowable deficits vary depending on the soil type, crop and irrigation method.
| Table 2. Recommended Allowable Deficits - Soybeans | ||
| Predominant Soil | Flood, Furrow or Border Irrigation (inches) | Pivot Irrigation (inches) |
| Clay | 2.00 | 1.50 |
| Silt Loam w ith pan | 1.75 | 1.25 |
| Silt Loam without pan | 2.50 | 2.00 |
| Sandy Loam | 2.25 | 1.75 |
| Sandy | 2.00 | 1.50 |
| with pan - with shallow (<10") restrictive
layer without pan - without shallow restrictive layer |
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The Checkbook User’s Guide, water use charts and water balance tables are available through your county Extension office at no cost. This method does require some record keeping, but it can be helpful in deciding when to irrigate.
If a computer is available, then the Irrigation Scheduling Computer Program can be used for the record keeping. This program operates much like the Checkbook method just described except that the computer does the calculations. It also uses local daily maximum temperatures and rainfall measured at the field to determine a soil moisture deficit for the field. The program is being successfully used by growers in Arkansas, Mississippi, Louisiana, Tennessee and Missouri. It is also being used in numerous irrigation studies and demonstrations conducted in Arkansas. A three-year (1994-96) demonstration of the program at the Southeast Branch Experiment Station (SEBES) in Rohwer indicated that it successfully scheduled irrigations in a field situation very comparable to growers’ fields (Table 3).
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Table 3. Irrigation Scheduler Demonstration SEBES - Rohwer, Arkansas |
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| Treatment | Avg. 1994 Yield (bu/acre) | Avg. 1995 Yield (bu/ac) | Avg. 1996 Yield (bu/ac) | Avg. 3-Yr. Yield (bu/ac) |
| Irrigate at: | ||||
| 2 inch deficit | 62 | 63 | 61 | 62 |
| 3 inch deficit | 58 | 57 | 57 | 57 |
| 4 inch deficit | 51 | 52 | 50 | 51 |
| Nonirrigated | 25 | 16 | 22 | 21 |
| Variety NK S59-60, emergence mid-May, furrow irrigated, 19 inch x 1,000 foot rows, silty clay, 0.1 - 0.15% slope. | ||||
In field studies using both tensiometers and the scheduler program it was found that they are usually within one or two days of each other on indicating when to irrigate. However, the program is much easier to use and maintain than tensiometers. The program also has the option to predict when irrigation will be needed in the next 14 days if no rainfall occurs. This offers a real benefit to managing irrigation labor and sharing irrigation water with other crops. The program is available through the county Extension office.
Growers
are also faced with making the decision on when to stop irrigating soybeans. The
goal is to have adequate soil moisture to ensure that the seed will obtain its
maximum weight. Field experience indicates that inadequate moisture for full
seed development can result in as much as 10 bu/ac yield loss. A practical rule
of thumb for terminating irrigation is to determine if 50 percent or more of the
pods have seeds that are touching within the pod. (The upper two pods in Figure
1.)
If there is good soil moisture at this point, then irrigation can be ended. If the soil is becoming dry, an additional irrigation is needed to assure maximum seed weight. A final irrigation at this stage should be as quick a flush as possible if flood (levee) or border irrigating, or every other middle with furrow irrigation. About 1 inch should be applied with a pivot at this time, and in five to seven days the soil moisture and crop development should be checked again to determine if an additional irrigation is needed.
Surface and sprinkler irrigation methods are used on soybeans. Each method has different characteristics that could make it the best for a particular situation. No one method can be labeled as the best – each has its place. The SRVP has included the four primary soybean irrigation methods. Table 4 shows that the four methods averaged essentially the same yield over a 16-year period. See irrigation methods on menu.
| Table 4. SRVP Yields by Irrigation Methods (1983 - 98) | ||
| Irrigation Method | Avg. Yield (bu/ac) | Number of Fields |
| Center Pivot | 47 | 41 |
| Flood | 46 | 68 |
| Furrow | 49 | 86 |
| Border | 51 | 8 |
When considering the different irrigation methods, it is important to remember that any method that is well planned and is properly installed, operated and maintained can give the results desired. Every method requires time to irrigate the whole field, so it is very important that irrigation be started early enough that no part of the field suffers moisture stress.
Consistent and profitable soybean production is difficult without irrigation. Fortunately, once irrigation is in place, the energy cost for pumping water is relatively cheap at $2 to $5 per acre for each irrigation. This cost is easily justified by the yield increase that can result from the irrigation. The maximum profit usually results when the maximum yield is obtained, so the irrigation goal is to obtain the maximum yield by preventing crop moisture stress. Irrigation is not a cure-all. Maximum yield and profit will be achieved only when irrigation is coupled with other production practices that establish profitable yield potentials.
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University of Arkansas • Division of Agriculture |
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