Early in the growing season, water loss from the soil occurs primarily through evaporation from the soil surface.
As the crop growth and more leaf area shades the soil, evaporation will decline as transpiration increases.
Crop residue on the soil surface can significantly reduce the amount of water lost through evaporation by reflecting solar radiation and protecting the soil from wind.
In the process of transpiration, plants take up water from the soil and transport it to the leaves. Small openings in the leaves (stomata) allow water vapor to pass from the plant into the atmosphere, cooling the plant.
The rate of transpiration increases with higher air temperature, solar radiation, and wind speed.
High humidity levels reduce transpiration by decreasing the difference in water potential between the leaf airspace and the ambient air.
Soybean Water Use Over the Growing Season
Daily ET varies greatly throughout the growing season due to day-to-day variability in weather conditions.
On average, daily ET increases through the vegetative growth stages, peaks during early pod fill, and declines as the crop approaches maturity. (Table 1).
Over 60% of total water use occurs during the R1 to R6 reproductive growth stages.
Seasonal soybean water use can range from 20 to 26 inches (50-66cm) during the growing season (Kranz and Specht, 2012) compared to a typical range of 21 to 28 (53-71cm) inches for corn.
Soybean Rooting Depth and Water Uptake
Well-developed root systems are essential for soybean water uptake and growth.
Soybean root systems that are unimpeded by soil factors can reach a maximum depth of over 60 inches (1.52m), similar to that of corn (Ordóñez et al., 2018).
The majority of soil water uptake by soybeans occurs within the top 2 to 3 feet (60-91 cm) of the soil profile (Kranz and Specht, 2012).
Table 1. Average daily soybean water use (ETc), water use per growth stage, and cumulative water use over the course of the growth season.
Impact of Water Availability
Soybeans can typically withstand moderate drought stress during vegetative growth with little effect on yield.
Excessive rainfall during vegetative stages can cause the plants to put on more vegetative growth that will not necessarily lead to higher yields. Larger plants can be more susceptible to lodging during thunderstorms later in the season.
Adequate water is most critical to soybeans during pod development and seed fill (R3-R6).
Ample water during flowering followed by drought stress during seed fill will result in smaller seeds.
Kranz, W.L., and J.E. Specht. 2012. Irrigating Soybean. NebGuide G1367. University of Nebraska-Lincoln Extension. https://extensionpublications.unl.edu/assets/pdf/g1367.pdf Ordóñez et al. 2018. Maize and soybean root front velocity and maximum depth in Iowa, USA. Field Crops Res. 215:122-131.