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Nebraska Cooperative Extension G98-1367-A

Irrigating Soybean

Proper irrigation management is critical to optimize both yields and irrigation water resources. Irrigated soybean production in Nebraska encompasses a region with diverse soils and climate and different irrigation management strategies. This NebGuide details irrigation water management recommendations that accommodate Nebraska's soil, climate and precipitation variability.

Brian L. Benham, Extension Water Management Engineer
Joel P. Schneekloth, Extension Educator
Roger W. Elmore, Extension Crops Specialists
Dean E. Eisenhauer, Biological Systems Engineering
James E. Specht, Agronomy

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Soybean acreage in Nebraska increased from 1.6 million acres in 1979 to 3.5 million acres in 1997. About 32 percent of those 3.5 million acres are irrigated. The increased interest in soybean is due to favorable economic returns, a relatively good yield potential, the value of soybean in rotation with other crops and changes in farm legislation.

Plant-Water Relationships

Soybean plant development is divided into two major phases – vegetative and reproductive. The vegetative stages begin with seedling emergence and end with flowering. The reproductive stages begin with the appearance of the first flower and end at maturity. Normally, irrigation, or high amounts of rainfall during vegetative growth, are not beneficial, except when soil water levels are extremely low. In fact, excessive water during the early growth stages may stimulate vegetative growth and increase the potential for lodging with either no increase in yield or a yield decrease.

With determinate varieties, vegetative growth ends at flowering. With indeterminate varieties, the later phases of vegetative growth overlap the early phases of reproductive growth. Although irrigation is usually required only during the mid- to late-reproductive stages, this overlap may mean some water will be applied during the later phases of vegetative growth. Soybean reproductive stages are described in Table I.

Table I. Reproductive stages of soybean plant development (Ritche et al. 1994).




Beginning Flower


One flower at any node on the main stem.

Full Flower


Open flower at one of the two uppermost nodes on the main stem with a fully developed leaf (nodes with fully developed leaves are those that are below a node with a leaflet unrolled to the extent that its edges are not touching).

Beginning Pod


Pod is 3/16 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf. It is not uncommon to find developing pods, withering flowers, open flowers and flower buds on the same plant at this time.

Pod Development


A pod 3/4 inch long at one of the four uppermost nodes with completely unrolled leaves.

Beginning Seed Fill


The presence of bean seeds (felt when pod is squeezed) in pods at one of the four uppermost nodes with completely unrolled leaves.

Seed Fill


A pod with full-size green beans (bean is full size when it fills pod cavity) at one of the four uppermost nodes with completely unrolled leaves.

Beginning Maturity


One normal pod on the main stem has reached its mature color, normally tan or brown.

Full Maturity


Ninety-five percent of the pods have reached their mature pod color.

Soybean growth and water-use patterns are shown in Figure 1. The total water use by a fully irrigated soybean crop (evaporation plus transpiration) is 21 to 24 inches per year. About 65 percent of this water is used during the reproductive stages. The peak water use rate, about 0.3 inches per day, occurs during pod development. The average rate during the reproductive stages is about 0.25 inches per day.

Figure 1. Soybean crop water use (evapotranspiration) and growth stages.

The most important times for soybean plants to have adequate available water are during pod development (R3-R4) and seed fill (R5-R6). Irrigation may also be required during the flowering stage on sandy soils or during very dry years on medium and fine-textured soils. However, if water is applied during flowering, it is important to follow with adequate water during seed fill. Otherwise, more but smaller seeds will develop, reducing yields.

Although soybean roots can reach depths of 5 to 6 feet, the largest concentration of roots and the majority of soil water extraction occur in the top 2 to 3 feet of the soil profile. Therefore, irrigation water should not normally penetrate below 3 feet. Soybean produce highest yields on soils with good internal and surface drainage.

Soybean Irrigation Water Management Research

The most convenient way to time soybean irrigation is crop growth stage. Stage-of-growth irrigation scheduling works well for crops like indeterminate soybean that respond well to water supplied during the later growth stages. However, stage-of-growth scheduling also depends on the capability of the irrigation system to supply sufficient water to the crop. Precipitation during the growing season, stored soil moisture prior to the growing season and irrigation system capacity combine to furnish water to the crop. Research has shown indeterminate soybean respond well to delayed irrigation. However, as rainfall and stored soil water decrease from east to west, delayed irrigation can reduce yields when compared to full-season irrigation.

Soybean irrigation research has focused on comparing full-season irrigation to growth-stage irrigation (irrigations timed to coincide with specific crop growth stages) to evaluate the effects of delayed irrigation on grain yield and water use efficiency. Four irrigation treatments were evaluated across Nebraska at Tryon, North Platte, Clay Center and Mead:

  1. Full-season (Full). If necessary, irrigation began prior to flowering to supply water according to the water use of the crop. Irrigations were scheduled to maintain the available soil water above the 50 percent depletion level in the active root zone.
  2. Full Flower (Flower). Irrigation began when a flower opened at a node immediately below the uppermost node on the main stem with a completely unrolled leaf (R2). During both flowering and pod elongation (R3-R4), a maximum of 3 inches of water was applied in two weeks. During seed fill (R5-R6), a maximum of 4.5 inches of water was applied in three weeks. Total maximum effective irrigation application was 10.5-inches. Irrigation amounts were adjusted weekly for rainfall.
  3. Pod Elongation (Pod). Irrigation began when a pod was 3/16 to 3/4 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf (R3-R4). During pod elongation, a maximum of 3 inches of water was applied over two weeks. During seed fill (R5-R6), a maximum of 4.5 inches of water was applied over three weeks. Total maximum effective irrigation application was 7.5-inches. Irrigation amounts were adjusted weekly for rainfall.
  4. Dryland. Water was applied only if needed for stand establishment.

Figure 2. Relative yeilds for soybeans irrigated at flowering (R2), pod development (R3-R4), and Non-irrigated dryland with respect to full season irrigation.

Figure 2 illustrates the average relative yields relative to the Full-season irrigation treatment yields by location and irrigation water treatment. Dryland relative yields were greater at Mead and Clay Center than the two west-central locations. More precipitation before and during the growing season at the eastern locations increased the dryland yields. Relative yields from the pod elongation treatments decreased from the eastern to the west-central locations. Soil water storage and rainfall were not enough to produce maximum yields from the pod-elongation treatment at the west-central locations; the pod-elongation treatment showed a positive yield response due to late-season water application at all locations.

Irrigation Water Management Recommendations

Coarse-textured soils

Water management for these soils is more difficult than for medium-textured soils. Soils in this classification include fine sands, loamy sands and fine sandy loams. Generally, these soils have a low (less than 1.5 in/ft) available water capacity. In addition, some sandy soils in Nebraska have root-restricting layers at shallow depths. The combination of low available water capacity and shallow rooting results in a small soil water reservoir. The available water-holding capacity in a 3-foot active root zone will be 3.0 to 4.5 inches. This low available water-holding capacity, coupled with the fact that sprinkler systems will likely be the irrigation used, means light, (0.75 to 1.5 inches), frequent water applications are necessary to recharge the limited soil water reservoir. Further, there is less room for error in timing irrigations.

The general recommendation for water management on coarse-textured soil is to allow no more than 50 percent depletion of the available soil water in the top 2 feet during flowering (R1-R2) and no more than 50 percent depletion in the top 3 feet during pod elongation (R3-R4) and seed fill (R5-R6). Soil water levels can be determined by combining the appearance and feel method with soil water-balance calculations using reliable evapotranspiration estimates. See NebGuides, G85-753, Irrigation Scheduling Using Crop Water Use Data; G84-690, Estimating Soil Moisture by Appearance and Feel; G82-602, Predicting the Last Irrigation for Corn, Grain Sorghum and Soybean for more information on these methods.

Deep medium and fine-textured soils

These soils (silt loams, silty clay loams, silty clay) generally have an available water capacity of more than 1.5 inches per foot. The available soil water at field capacity is between 4.5 and 6.0 inches in the top 3 feet. Applying irrigation water when the available soil water is depleted to 50 percent in the top three feet of the root zone after the full flower stage (R2) will generally result in maximum yields. The same methods mentioned for the sandy soils can be used to estimate soil water in these soils.

An alternative scheduling approach on deep- and fine-textured soils is stage-of-growth scheduling. This method works if the soil water reservoir is at or near field capacity to 5 feet at planting time. In the eastern half of Nebraska, this usually occurs if the soils were irrigated during the previous season and there was sufficient off-season precipitation to refill the profile.

For soybeans, between 10 and 11 inches of water are required from full flower (R2) to beginning maturity (R7). Therefore, effective irrigation plus rainfall should equal about 3 inches during full flower (R2), 3 inches during pod development (R3-R4) and 4.5 inches during seed fill (R5-R6). With adequate rainfall, optimum yields will be obtained with two, 3-inch net or effective furrow irrigations (typically at full flower or pod development and beginning seed fill). With systems such as center pivots applying smaller amounts of water per irrigation, it will be necessary to make two to four revolutions to apply the desired 3 inches during a particular growth stage. In dry years, an additional 3 to 5 inches of effective irrigation may be required.

If irrigation is started or unusually significant rainfall occurs during the beginning flower stage (R1), it is especially important adequate soil water (50 percent available soil water or greater) be maintained during the remainder of the growing season. If you are limited in the amount of irrigation water you can apply during the season, you will get the maximum benefit of this water if it is applied during the pod development (R3-R4) and seed fill (R5-R6) growth stages. However, when the rainfall is below normal during the vegetative and flower stages, a yield reduction may occur.

With furrow irrigation systems, it is generally not advisable to wait until pod development (R4) before applying the first irrigation, as this will probably cause extremely dry furrow conditions, making it difficult to get water through the field. An earlier irrigation date, perhaps beginning during the full flower stage, is advised. Individual effective irrigation applications should not exceed 3 inches.

Because precipitation decreases from east to west across Nebraska, a full soil water reservoir may not exist at planting time in the western half of the state. In this region, delaying irrigation until pod development may result in yield reductions when compared with full-season irrigation.


When irrigating soybean in Nebraska:

  1. Growth-stage irrigation scheduling for soybean should be limited to deep medium- to fine-textured soils. If soil water is at field capacity at planting, irrigation can be delayed until full flower (R2) and perhaps as late as beginning pod (R3).
  2. If one or more of the following exists, irrigation should be scheduled according to soil water depletion and depletions should not exceed 50 percent:
    1. Soil texture is sandy loam or coarser
    2. The root depth is impeded (shallow, limits available soil water reservoir size)
    3. Irrigation system capacity is 1.5 inches per week or less

While it is convenient to base irrigation timing decisions on crop growth stage, and growth-stage scheduling may work well for a crop like indeterminate soybeans, the use of growth-stage scheduling should be limited when soil texture and climate are not favorable.


  • Elmore, R.W., D.E. Eisenhauer, J.E. Specht, and J.H. Williams. 1988. Soybean yield and yield component response to limited irrigation capacity sprinkler irrigation systems. Jou. Prod. Ag., Vol. 1(3). p196-201.
  • Klocke, N.L., D.E. Eisenhauer, J.E. Specht, R.W. Elmore, G.W. Hergert. 1989. Irrigation of soybean by growth stages in Nebraska. Trans. ASAE, Vol. 5(3). p361-366.
  • Ritchie, S.W., J.J. Hanway, H.E. Thompson, and G.O. Benson. 1994. How a soybean plant develops. Cooperative Extension - Iowa State University, Ames, Iowa. Special Report No. 53. p20.

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File: G1367 under: FIELD CROPS
A-13, Soybeans
Issued November 1998

Electronic version issued January 1999
Need assistance?

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

University of Nebraska Cooperative Extension educational programs abide with the non-discrimination policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.


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