By Laura Lindsey, Alexander Lindsey, Ohio State University Extension
Saturated soils after soybean planting can cause uneven emergence and stand reductions of varying extent depending on the stage of the soybean plant and other environmental factors including temperature and duration of saturated conditions. Additionally, increased disease incidence may further reduce plant stand.
Saturated soil prior to germination: While soil moisture is necessary for germination, soybean seeds will not germinate when soils are saturated because oxygen is limiting.
Saturated soil during germination: Saturated soils during soybean germination may cause uneven emergence. In a laboratory study, soybean germination was reduced by ~15% after only one hour of flood conditions (Wuebker et al., 2001). After 48 hours of flood conditions, soybean germination was reduced 33% to 70% depending on when imbibition (seed taking up water) began relative to the flooding conditions. Practically, for Ohio, this means if soybean seeds were further along in the germination process when flooding occurred, the seeds will be more susceptible to flooding stress.
Saturated soil during vegetative stage: Warmer temperatures will cause soybean plants to die faster. At temperatures, 80 degrees and greater, submerged soybean plants will likely due in 24 to 48 hours. However, cool, cloudy days (…and we’ve had plenty this year) and clear nights increase the survival potential of a flooded soybean crop. Flooded plants may also exhibit poor nodulation, resulting in yellow, stunted plants.
Evaluate stand: To quickly estimate stand, count the number of plants in 69 feet 8 inches of the row for 7.5-inch row spacing, 34 feet 10 inches for 15-inch row spacing, or 17 feet 5 inches of the row for 30-inch row spacing. These counts represent 1/1000th of an acre (i.e., 120 plants in 7.5-inch row spacing = 120,000 plants/acre).
Keep in mind, the effect of plant population on yield is very small over the normal range of seeding rates. For soybeans planted in May, final populations of 100,000 to 120,000 plants per acre are generally adequate for maximum economic return. For example, in our seeding rate trials in Clark County, 100% yield (77 bushels per acre) was achieved with a final plant stand of 125,000 plants/acre. However, a 95% yield (73 bushels per acre) was achieved with only 77,000 plants per acre. (This trial was planted the second half of May in 15-inch row width.)