How drought affects soybeans
Widespread drought across the entire state rarely occurs in Iowa or other parts of the Midwest. However, the 2012 growing season is one of those rare years. Normally, there is more concern with draining excess water. This year has put more focus on conservation of water for crop production.
Precipitation has been below normal for most parts of Iowa since July 2011, and this summer the recent extraordinarily high daytime and nighttime temperatures have put many corn and soybean plantings at risk for below-normal yields.
Soybean plants are most susceptible to yield loss from drought stress at two key developmental stages: germination and the reproduction-seed development stage. For germination to occur in soybeans, seed must imbibe 50% by weight in water. Seed coat swelling occurs during imbibition, causing the coat to rupture. If soil water is insufficient for radicle (root) and hypocotyl (shoot) development and emergence, the plant will die.
When seeds are planted in dry soil, or not placed into soil, germination will not be initiated, and as long as seed coat integrity is maintained, seed can survive until adequate moisture is received. Seedlings that recently germinated can undergo drought stress as early as two days following imbibition.
Water deficiency at this time results in poor hypocotyl growth, but root growth and elongation may be unaffected. This evolutionary response in soybean allows the seedling to seek more soil water while having an overall low water use, promoting survival and subsequent development.
Soybeans in Iowa may undergo relatively short-term drought stress at planting, but adequate rainfall typically occurs shortly thereafter, and the soybean plant responds to reinitiate shoot growth. Shoot growth after short-term drought stress can occur at a faster rate than what was observed prior to the stress. This is called compensatory growth.
Moderate, short-term drought stress during vegetative growth stages does not typically impact soybean yield. However, when vegetative stage soybean plants are subjected to long-term severe drought, the stress may be great enough to cause substantial yield loss or even plant death under extreme conditions.
When crop is most vulnerable
Soybean yield is most impacted by drought stress during reproductive phases. Soil water deficits during reproduction increases flower abscission, abortion of embryos and reduced pod number, and surviving seed are smaller.
Nitrogen fixation is a key biochemical pathway for soybean yield, and nitrogen fixation can be severely limited or completely stopped during moderate drought stress. Soybean is an annual plant, and unlike related perennial legumes such as alfalfa, nitrogen fixation is rarely reinitiated once it has stopped. The halting of nitrogen fixation in the soybean plant essentially fixes yield to the nitrogen previously stored in the root, stems and leaves for subsequent seed development. The large impact of available stored water in soil on nitrogen fixation and subsequent yield in soybean cannot be understated.
Numerous research studies have tried to replace or augment nitrogen fixation by applying nitrogen fertilizer. However, fertilizer nitrogen use by the plant requires adequate water availability for root uptake. Water uptake simply is inadequate during periods of drought stress, and fertilization will be ineffective unless rainfall is very timely and substantial, and the soil water infiltration rate is adequate.
Most Iowa farmers do not have irrigation available to compensate for inadequate or untimely precipitation. However, there are well-known soil management practices that can greatly enhance water capture and storage for subsequent use by soybeans or corn.
Zero tillage or reduced tillage systems that leave more crop residue on the soil surface can reduce evaporation losses.
Evaporative losses of soil water to the atmosphere are increased by conventional tillage, and evaporative losses never contribute to crop growth or yield. Conventional tillage systems such as moldboard plowing or disk-chisel tools decrease soil aggregation. Reduced or zero tillage can result in soils with higher levels of aggregation, lower bulk density, less compaction, higher levels of organic matter, greater resistance to erosion, greater water infiltration rate and greater water storage capacity.
Increase nitrogen fixation
Conserving additional available soil water during soybean reproduction can increase the length of time nitrogen fixation occurs. Increased nitrogen fixation has a strong, positive effect on soybean yield, particularly during growing seasons with inadequate or untimely precipitation.
At this point in time, there are few options for Iowa soybean farmers to improve available soil water status for the current crop, unless irrigation systems are already in place. However, soil management decisions to improve available soil water content should be carefully considered by soybean farmers following this year’s harvest. High-intensity tillage systems are not a solution to improve soil water content or long-term soybean yield.
Lenssen is the Iowa State University Extension soybean systems agronomist at Ames. Contact him at firstname.lastname@example.org.
This article published in the August, 2012 edition of WALLACES FARMER.