Evapotranspiration (ET) Crop water use, or evapotranspiration (ET), is the movement of water through evaporation (E) from the soil and plant surfaces and transpiration (T) through the plant. Transpiration is the movement of water from the soil into plant roots, through plant stems and leaves, and back out into the atmosphere. Transpiration is an important concept because yield is linearly related to the amount of water a soybean plant transpires. Understanding soybean water use and the factors that affect it can help guide more efficient irrigation applications.
Factors Influencing ET Soybean water use will fluctuate throughout the season depending on weather conditions and crop growth stage (Figure 1). Factors that will affect ET and irrigation decisions include:
Crop growth stage. Crop water requirements vary depending on growth stage (Table 1). Young plants transpire less than larger plants due to a smaller leaf surface area. Soybean requires the most water from flowering through seed fill.
Relative maturity. Longer-season soybean products will require more water over the growing season than short-season products. While longerseason soybean products use more water, they may also have a higher yield potential if growing conditions are optimal.
Weather conditions. The ability of the atmosphere to evaporate water is the driving force for ET. Daily ET is influenced by solar radiation, air temperature, relative humidity, and wind. High air temperatures, low humidity, clear skies, and high winds cause a large evaporative demand.
Soil water holding capacity and soil water content. A soil’s water holding capacity indicates the maximum amount of water that will be available for plant use when the soil water profile is full (field capacity). Fine textured soils can hold more water than coarse textured soils. As the soil dries, it becomes more difficult for plants to extract water. At field capacity, plants use water at the maximum rate. Plants use less water as the water content of the soil drops.
Residue management / tillage system. Minimizing soil disturbance from tillage and increasing surface crop residue can reduce soil water evaporation. Reduced tillage practices, particularly no-till and strip tillage, can help the soil capture and retain more moisture. Tillage increases the exposed surface area of the soil for evaporation and increased runoff, destroys crop residue that can catch rain and snow, and may reduce water infiltration because of compaction that can be created during the tillage process.
Surface residue helps increase infiltration rates by limiting the amount of surface sealing created by rainfall and irrigation droplets. Surface residue also helps reduce runoff by creating obstructions that limit water movement and allow more time for water to seep into the soil profile.
Crop population. When water resources are limited, populations can be lowered to reduce the transpiration component of ET and better match precipitation, stored soil moisture, and irrigation water supplies to crop requirements. However, soybean has the ability to adjust growth and development to compensate for different plant densities with a tendency to produce more branches and pods in low populations.
Seasonal water requirements for soybean range from 15 to over 25 inches depending on planting date, maturity group, location, and weather.1,2 The most critical time to avoid water stress is during the mid- to latereproductive stages. When soybean does not receive enough water to meet ET demands during this critical water use period, significant reductions in yield can occur.
Emergence and vegetative stages. Soybean uses very little water during the seedling stage; water demand increases significantly during rapid vegetative growth. Unless the soil is extremely dry, supplemental irrigation is generally not needed during germination or vegetative growth stages. Too much water early in the season can prolong the vegetative growth stage, which can result in delays in flowering, increased plant height, and lodging.2 Limiting early-season irrigation encourages plants to develop stronger, healthier root systems that grow deeper. Farmers should rely on stored soil moisture and natural precipitation as much as possible during the early growth stages.
Reproductive stages to maturity. Soybeans are most sensitive to water stress during the mid- to late-reproductive stages: pod development (R3 to R4) and seed fill (R5 to R6). Water stress during pod development and early seed fill can have the greatest impact on yield potential and result in a reduced number of seeds per pod and reduced seed size. Irrigation may be required during flowering on soils with an insufficient water holding capacity (sandy soils) or when conditions are exceptionally dry.2 When water is applied during flowering, it is especially important to supply adequate water during seed fill. This is because irrigation during flowering usually increases the number of seeds produced, but subsequent water stress during seed fill will reduce the seed size which can result in greater yield penalties than would have occurred if the crop had not been watered at all during flowering.2
Soybean requires adequate water through the reproductive stages in order for seeds to achieve their maximum weight (Table 2). Discontinuing irrigation before physiological maturity can result in yield penalties if the soil water content is not sufficient.
It is important to know when crop water demands will become greater than precipitation, which often occurs during the critical reproductive periods. Under hot, dry conditions, peak water demand can reach up to 0.5 inches/day in semi-arid regions like western Nebraska.2 Plan ahead by knowing the capacity of the irrigation system to ensure that adequate water is available to the crop through maturity. Refer to agAnytime.com for publications on the topic of irrigation scheduling for soybean
1 Soybean irrigation and water use. University of Missouri Extension. http://crops.missouri.edu/irrigation/.
2 Kranz, W.L. and Specht, J.E. 2012. Irrigating soybean. NebGuide G1367. University of Nebraska-Lincoln Extension. Other Sources: Helsel, D.G. and Helsel, Z.R. 1993. Irrigating soybeans. Publication G4420. University of Missouri Extension. Web sources verified 4/1/15. 150315145957