By Dave Nanda, Director of Genetics and Technology for Seed Consultants, Inc.
There are several factors that affect soybean populations. Soybean plants have a great flexibility to adjust to plant populations. Unlike corn plants, soybeans have a tremendous ability to adjust to variations in population density. Soybean plants adjust by producing more branches per plant, and by increasing or decreasing the number of pods on both the main stem and branches.
Depending on the variety, each plant uses about 6 to 10 inches of space in all directions. There is little change in size of the beans and in number of beans per pod. If the population is too thick, plants will grow taller, pods are placed higher and there will be fewer pods on individual plants with fewer branches. Taller varieties will yield less if there are too many plants.
For double cropping, seeding rate should be increased following wheat because establishing stand may be more difficult. Recent studies at Purdue University suggest that 120,000 plant stand with good spacing may be sufficient to get optimum yield. A recent multi-year study conducted by Pedersen at Iowa State University indicated that populations above 150,000 plants do not increase and might even decrease yields. In general, plant 140,000 to 160,000 seeds per acre in most situations for economic yields. These recommendations for soybean populations, though, are based on tests conducted by various universities over several years. However, I should clarify that university tests are generally conducted in ideal conditions, on uniform, fertile soils with high organic matter. In real life that is rarely the case. There are a lot of farms in our marketing area that are low in organic matter and have variable soils.
Optimum seeding rates will vary for each farm and depend on the following factors:
• Soil Type: Is it highly fertile ground or low in organic matter?
• Drainage: Is it well-drained, tiled farm or poorly drained ground?
• Soil condition at planting time: Is it good or poor planting condition?
• Planting date: Earlier planting dates require higher seeding rates.
• Seed treatment applied or are you planting untreated seed?
• Tillage : Conventional tillage requires less seed compared to no-till situations.