By Andy Lenssen, Department of Agronomy
Farmers and non-farmers alike are asking the questions, "What is going to happen to soybeans this year? What level of yield can we expect?" Given the lack of rain in Iowa and much of the U.S. soybean belt, current prices and high demand, these are legitimate questions.
Most soybean is at growth stage R2, flowering at each node along the entire length of the stem. Some soybean fields are at growth stage R3, "beginning pod development." Soybean is most susceptible to drought stress at germination and during reproduction. The most critical period to avoid drought stress during reproduction is the period from growth stage R3 through R6, beginning pod development to "full seed." This is the time when nitrogen (N) fixation must proceed at a high rate to attain good seed yield. Soybean has a very large requirement for N during seed fill because soybean seed have high N concentration.
Soybean plants obtain about half their N from soil and half from N fixation. Soil N, in nitrate form, is taken up by the plant with water during daylight hours. This occurs as long as plants are able to transpire and nitrate is available from soil microbial breakdown of organic matter in the top soil. Unfortunately, N fixation is very susceptible to drought stress, and even low to moderate levels of drought stress will cause fixation to stop. Unlike perennial relative alfalfa, soybean plants will rarely reinitiate N fixation once it has stopped, particularly at stages R3 and beyond. Once fixation stops due to drought stress, yield will largely be determined by how much N has already been accrued and stored in the plant. Little N can be taken up from soil during drought because microbial activities necessary for nitrate evolution and water for transport are greatly decreased. However, much of the N previously stored in leaves, stems and roots will be remobilized and used for seed development.
Yield comes down to light, water and nutrients. Light energy is more than adequate during periods of drought; water is what soybeans lack, along with N due to compromised fixation and uptake from soil. Fortunately, soybeans are very plastic and capable of responding to improved water availability after undergoing significant drought stress, particularly when they have not yet attained R3-R3.5 stage.
How much longer can soybeans "hold on" this summer and still produce a reasonable yield? At this time it depends largely on available soil water content, until adequate rainfall commences again. Soybeans likely are using anywhere from 0.15 inch to 0.25 inch of water per day now, unless they were planted rather late. A common assumption is most soils in Iowa hold about 2 inches of water per foot of soil depth. Soybeans can root up to 6 foot depth in well-drained, well-watered soils, but compaction or poor drainage can reduce rooting depth to 4 feet or less. Yield potential at this time depends on how much available soil water is left in the rooting zone. Ground penetrating radar and other experimental techniques are not currently available to accurately determine available soil water for soybean growth and development. Soil sampling can be done in soybean fields to estimate available water content, but this rarely is done in Iowa during the growing season or at other times of the year. However, growers and agribusiness personnel with access to soil sampling equipment such as hydraulic probes can take soil samples and determine soil water content. Sampling should be done to rooting depth. To determine actual water content, soil core samples should be weighed, oven-dried, and reweighed. Gravimetric water concentration can be calculated by subtracting dry weight from wet weight, and dividing that answer by dry weight. Multiplying gravimetric water concentration by soil bulk density provides volumetric water content, and from that number it is possible to estimate available soil water. Farmers or others who wish to estimate available soil water are welcome to contact the author for additional information on sampling procedures or subsequent calculations.
If a dry trend continues into next year, managing water capture and soil moisture will be increasingly important.
Andy Lenssen is a soybean systems agronomist. He can be reached at 515-294-1360 or e-mail firstname.lastname@example.org.