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Soybean Irrigation Scheduling
May 22, 2022
The scheduling of soybean irrigation is crucial for helping to maximize soybean yield while managing costs and reducing agronomic issues caused by over-watering. A fully irrigated soybean crop can use between 20 to 26 inches of water throughout the growing season. Sixty-five percent of this water is utilized during the soybean plant’s reproductive stages.1 Three important factors that need to be monitored for irrigation scheduling include:
- The amount of plant available moisture in the soil profile
- The water movement from the soil into the air known as evaporation
- The water movement through the crop into the air known as transpiration.
Factors affecting plant available moisture in your soil.
The most important factor affecting soil moisture holding capacity is the texture or make up of the soil. A medium soil has the highest holding capacity, coarse sandy soils have the lowest holding capacity, and a fine clay soil is in the middle. Table 1 from the University of Nebraska shows a common range of available water holding capacity in varying soil textures.
Table 1. Available water capacity based on soil texture*
Textural Classes Available water capacity in inches per foot of depth Coarse sand 0.25 – 0.75 Fine sand 0.75 – 1.00 Loam sand 1.10 – 1.20 Fine sandy loam 1.50 – 2.00 Silt loam 2.00 – 2.50 Silty clay loam 1.80 – 1.70 Silty clay 1.50 – 1.70 Clay 1.20 – 1.50*Table 3.1. Burr, C., Yonts, D. and Benham, B. Irrigation Chapter 3 – Soil Water. Irrigation Home Study Course. Plant & Soil Sciences eLibrary. Permission to use table provided by Dr. Burr, University of Nebraska.
Other factors affecting plant available moisture in the soil profile include:
- Soil Structure (Compaction reduces pore space which lowers water storage capacity)
- Natural Restrictive layer (Layer of rock or other restriction that inhibits water infiltration)
- Organic Matter (OM) Content (Water holding capacity can be increased by increasing OM).
The percentage of plant available water in a soil can be determined by using the hand and feel method. With this method, a soil core is physically taken from the root zone and rolled in the hand to estimate the percent of moisture. This technique can vary from one person to the next and takes time and skill to become proficient. Another option is the placement of a soil moisture probe in a representative spot or spots in the field to monitor the soil moisture level. Either way, it is good to continually monitor the soil moisture levels throughout the entire growing season.
Soil Water Use and Loss
Water loss from evaporation of soil moisture to the air and plant transpiration of moisture into the air is collectively known as ET (Evapotranspiration). This process can be monitored/measured through local evapotranspiration gauges placed by yourself or potentially those managed by other sources such as NRD-NRCS or University Extension. An ET gauge can really be thought of as a reverse rain gauge. A known volume of water is placed in the ET gauge and the loss of water through evaporation is monitored and recorded over a day or period of days to determine the rate of ET occurring per day from this gauge. A factor is then applied to this ET gauge reading to correlate the water use to the specific crop and growth stage. Soybean water use varies quite drastically over the growing season, starting off low in early vegetative stages and raising quite high during reproductive stages, especially the later vegetative stages (Figure 1).
Factors Affecting Evapotranspiration (ET)
- Air Temperatures (High temperature increase Evaporation and Transpiration)
- Air Humidity (Low humidity increases Evaporation and Transpiration)
- Wind (High wind increase Evaporation and Transpiration)
- Solar Radiation (High amounts of sunlight increases Evaporation and Transpiration)
- Soil residue (More soil cover lowers evaporation)
- Crop canopy (Increased foliar growth lowers evaporation by shading the soil but increases transpiration)
- Crop Growth Stage (Water use is increases as plant enters Reproductive stages)
Soybean Water Needs by Growth Stage
As mentioned previously, soybean water needs vary dramatically from early vegetative growth (0.05 to 0.10 inches water use/day) to late reproductive stages (0.25 to 0.33 inches water use/day).1 Excess irrigation in vegetative stages can cause excess vegetative growth that can cause lodging. This excess vegetation and/or lodging can lead to poor air flow into and through the soybean canopy leading to potential fungal diseases which can reduce yield potential.
In the early stages it is recommended to irrigate sparingly and mainly rely on natural rainfall and soil moisture to provide the crop with needed water. Only under extreme dry conditions should irrigation be applied during this time and some type of soil moisture monitoring should be done to check the available moisture in the root zone this entire time.
When soybean plants reach the reproductive stages, water use increases rapidly. However, even at flowering, irrigation should be limited to when the soil profile and natural rain events cannot provide the required moisture to the plants. The key reproductive stage to ramp up to full irrigation is during pod development (R3 growth stage). This higher irrigation rate should be maintained through the end of seed enlargement (R6 growth stage) to help reach maximum seed size and seed count. During this time, it is recommended to monitor soil moisture to avoid over irrigating and to make sure the crop’s water needs are being met. When the R6.5 stage has ended, about 3.0 inches of moisture is needed to finish the crop. At this point, the moisture in the soil profile and any potential for natural rain over the next couple weeks should be evaluated. If rainfall is deficient, another irrigation may be required to help avoid reduced seed size. Table 2 shows soybean water use by crop stage starting at R4 growth stage.
Table 2. Crop water demand for soybeans*
Crop Growth Stage1 Pod and Plant Development Days to Maturity Water Needed to Mature (inches)2,3 R4 End of pod elongation 50 – 60 -- R5 Beginning of seed enlargement 40 – 50 10.0 R6 – R6.5 End of seed enlargement to leaves beginning to yellow 30 – 40 4.71 R6.5 – R7 Leaves begin to yellow 20 – 30 2.9 R7 Beginning maturity 10 – 15 0.75 R8 Maturity 0 0.27*Henry, C., Francis, P.B., Espinoza, L., and Ismanov, M. Timing the final irrigation using Watermark™ Sensors. FSA59. University of Arkansas. Permission granted for use by Dr. Henry, University of Arkansas.
1Source: A visual guide to soybean growth stages (https://coolbean.info/library/documents/2017_Soybean_GrowthDev_Guide_FINAL.pdf).
2Results from sap flow experiments conducted in 2017 at Lon Mann Reasearch and Extension Center, Marianna, AR.
3Acre-inches per acre.
A useful tool for determining final irrigation requirements for soybean is the app provided by the University of Arkansas Division of Agriculture Soil Moisture Sensor Calculator. It’s a tool for interpreting Watermark™ Matric Potential Sensors for irrigation water management.3 The app is available on Google Play Store and Apple App Store.
In summary, it is important to know a soil’s water holding capacity, the crop’s growth stage, and the crop’s water use to determine proper irrigation. It is crucial to limit early season irrigation and only irrigate when rainfall is limited, and soil moisture levels are low. Yet, it is more important to provide plentiful irrigation in late reproductive stages to keep up or just ahead of crop water use to help maintain soybean yield potential.
References:
1Kranz, W.L. and Sprecht, J.E. 2012. Irrigating soybean. NebGuide G1367. University of Nebraska-Lincoln Extension. https://extensionpublications.unl.edu/assets/pdf/g1367.pdf.
2Tacker, P. and Vories, E. Irrigation. Arkansas Soybean Production Handbook, Chapter 8, MP197. University of Arkansas. https://www.uaex.uada.edu/publications/mp-197.aspx.
3Henry, C., Francis, P.B., Espinoza, L., and Ismanov, M. Timing the final irrigation using Watermark™ Sensors. FSA59. Agriculture and Natural Resources. University of Arkansas. https://www.uaex.uada.edu/publications/pdf/FSA59.pdf.
Burr, C., Yonts, D. and Benham, B. Irrigation Chapter 3 – Soil Water. Irrigation Home Study Course. Plant & Soil Sciences eLibrary.
Yonts, C.D., Melvin, S.R., and Eisenhauer, D.E. 2008. Predicting the last irrigation of the season. NebGuide G1871. University of Nebraska – Lincoln. https://extensionpublications.unl.edu/assets/pdf/g1871.pdf.
Austin Merz
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