Original article posted on C.O.R.N. Newsletter.
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 33-70% 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.
Evaluate Stand: If soybeans were planted shortly before heavy rainfall, it is important to evaluate soybean stands in the next couple of weeks. To quickly estimate stand, count the number of plants in 69’8” of row for 7.5-inch row spacing, 34’10” for 15-inch row spacing, or 17’5” of 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. At this time of the year, final populations of 100,000 to 120,000 plants/acre are generally adequate for maximum economic return. For example, in our seeding rate trials in Clark County, 100% yield (77 bu/acre) was achieved with a final plant stand of 125,000 plants/acre. However, 95% yield (73 bu/acre) was achieved with only 77,000 plants/acre. (This trial was planted the second half of May in 15-inch row width.)