Soybean Irrigation

Irrigated soybean systems are the most productive in the U.S, averaging over 48 percent more yield than dryland systems. Drought is the most damaging abiotic (non-living) stress to soybean crops. Overcoming drought is a key factor to sustaining maximum soybean yields, but only about 8 percent of U.S. soybean acres are irrigated. The ability to continue using irrigation will rely on maintaining the quantity and quality of ground and surface water resources.

Soybeans are a relatively drought-tolerant crop but respond well to irrigation. While irrigated soybean acres can be found in every soybean producing state, the largest percentage of irrigated acres occurs in the Central and Northern Great Plains, particularly Nebraska, Kansas and the Texas Panhandle.

Plant Characteristics
Soybeans are a relatively deep-rooted crop. In deep, well-drained soils with no restricting layers, roots can penetrate to 6 feet. As with all crops, most of the roots are concentrated in the upper half of the root zone. Managing a root zone of 3 feet is the general irrigation recommendation. Water use requirements, also known as evapotranspiration or ET, for soybeans range from 17 to 28 inches depending on climatic conditions.

Water use rates
Daily water use varies with the stage of growth and weather conditions. The typical peak water use rate is about 0.35 inch per day as typical for all summer-grown field crops, which normally occurs near the beginning of the pod fill stage Figure 1.

Figure 1. Soybean water use of daily evapotranspiration (ET) from a well-watered crop (Nebguide 1367 UNL).

Single-day peak water use rates can approach 0.5 inch per day. Water use is low at the germination and seedling stages, peaks at or near the full-bloom stage, and then declines with maturity. The most critical time for adequate soil water availability is during the end of the reproductive period when pod fill begins. Soybeans produce many flowers relative to the final number of pods, so losing a few flowers to light water stress earlier in the reproductive cycle is not as critical to final productivity as the same water stress during pod fill. Net irrigation requirements for soybeans in dry years range from around 14 inches in western production regions to less than 5 inches in the east. Requirements in an average rainfall year will be 2 to 4 inches less.

Irrigation Timing
Research studies throughout the High Plains confirm that the most beneficial timing for a limited amount of irrigation is during the latter part of the reproductive growth stages rather than earlier. This is generally true because early-season growth and development can be satisfied by typical rainfall and stored soil water. When full irrigation is possible, a managed allowable depletion level of 50% in the managed root zone is the recommended management guideline; the typical managed allowable depletion for most field crops. The peak water use rate is generally later in the season than corn, which means soybeans may be used as a field acreage split with corn as a way to reduce water stress potential at tasseling for corn crop.

Irrigation scheduling using the soil water depletion method is a best management practice. Irrigation scheduling in this form can be accomplished using either soil water measurement devices (sensors/probes) or climatic-based (also known as evapotranspiration-based) irrigation scheduling. Many states provide an irrigation scheduling program. One example is the K-State Research and Extension KanSched irrigation scheduling program. Other free irrigation decision support software is also available.

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Source: Information on soybean irrigation is provided by Douglas J. Jardine, Professor Emeritus, Kansas State University 7/2020