Authored by Emma Matcham and Shawn P. Conley
In our past article about early season soybean irrigation, we shared some thoughts on soybean water needs from planting through vegetative growth. To recap, if you’re planting into dry conditions, you might need to apply some water to aid germination. Between soybean emergence and beginning R3 growth stage, yield reductions start when soil water deficits are in excess of 50-75%.
Water deficits quantify how much water is available within the soil profile compared to the maximum amount of water the soil could potentially hold. For instance, soils at 30% deficit contain 70% of their maximum water quantity (more explanation of depletion). Deficits occur because water is evaporating off the soil surface or being released through plants’ transpiration at a rate faster than soil water is being replenished by rainfall and irrigation.
Understanding why transpiration varies can help us understand why soybean plants need frequent rainfall or irrigation during reproductive growth. The biggest drivers of transpiration are environmental factors like temperature, relative humidity, and wind speed. But, plant species, size, and growth stage can also affect transpiration rates.
As soybean plants grow and develop their transpiration increases, and soil water depletion happens faster. Soybean vegetative growth only require 0.7 inches of water per week, but flowering (stages R1 and R2) plants need twice that amount (1.4 inches of water per week). Once bean pods are elongating (R3), soybean water use increases to 1.4-1.75 inches of water per week. A soybean crop uses more water and is more likely to have lower yields due to water deficit during the pod-fill stage than they are earlier in the season.
Water usage continues to increase throughout the season, to around 1.75-2.45 inches per week from R4 through R6. Soybean producers that apply 5-7 days’ worth of water at a time lose less water to canopy evaporation than more frequent applications, but they need to keep future water need increases in mind when planning irrigation.
As you plan future irrigation timings, you may choose to start your sprinklers before soil water deficit surpasses 50%, particularly on hot days when water is actively being depleted. Sprinkler systems take time to move across the entire field, so starting irrigation before the field reaches 50% depletion gives you adequate time to make sure that irrigation is completed before the end of the field incurs yield loss due to low moisture availability.
Looking towards the remainder of the season, soybean plants need less water during R7 than during R6; and roughly 0.3-1.4 inches per week to get from R7 to full maturity. Soybean only spends around 10 days in R7, so between R7 and maturity soybean only needs around an inch of water in total.
Even though soybean needs only a small amount of water in R7, yield losses due to insufficient water during this growth stage can be as high as 0.75 bu/acre/day on sandy soils. By entering the R7 growth stage with the soil in excess of 60% of its water holding capacity, you can reduce yield loss due to late season water stress.
Weather is the main factor that determines if more irrigation will be needed after entering R7. Precipitation in the forecast would negate the need for irrigation, but hot and dry conditions can increase transpiration rates, depleting the soil water before soybean reaches maturity. Typically, in similar environments to what we have in WI, additional irrigation after reaching R7 is not necessary. Using an irrigation planning tool like WISP can account for precipitation and soil variables, or you can follow step by step guides like those published by UMN to estimate if you will need to apply more water after reaching R7.