Recommended Seed Depth
Plant soybeans at 0.5–1.5 inches deep for the best emergence and yield potential. To determine the appropriate depth to target within this range in a given year, consider soil moisture, soil type, residue management, land rolling, moisture holding capacity, compaction risk and the rain forecast. Ensure good seed contact with soil moisture, if possible, and avoid planting deeper than 1.5 inches.
In dry seedbeds, the decision often comes down to dusting in the seed and hoping for rain, or planting deeper than 1.5 inches into soil moisture. If you choose the hope-for-rain approach and seed is placed in dry soil, your seed will need a really good rain to properly germinate and take off. Alternatively, deep planting (>1.5 inches) poses a risk of seedling death. If seeds germinate and do not receive adequate moisture afterward, they can desiccate and die off. Deep planting also causes stress to seedlings early in their development, resulting in poor, non-uniform or delayed emergence.
How to Assess Seed Depth
- Check seed depth a few times while seeding – when you first start and a couple times later on in different parts of the field. This is to ensure your equipment is functioning properly and seed depth is still uniform, and to account for any seedbed variability across the field.
- Get on your hands and knees to dig for seed carefully. Use a good digging tool! This means something more precise than your shoe and something that has a ruler (various companies offer seed depth indicators). Gradually scrape off layers of soil within the furrow until you find seed. Measure the distance from the seed to the top of the true soil surface. This does not always equate to the top of the furrow. Check different furrows along the seeder, noting whether you are in a wheel track or not.
- Learn what your machine is doing. If your seeder has openers that dig a deep furrow and create bigger hills, and/or you have wide packing wheels that push more soil over the furrow, your seed may end up deeper than intended. Rolling afterward may add even more soil to the furrow. Try to anticipate how much additional soil will be pushed onto the furrow after rolling. Check seed depth after rolling if you’re not sure so you can accurately estimate seed depth based on your practices in future years.
- Do you need to be rolling at all? If you don’t have stones that pose a risk of equipment damage at harvest or large soil clods that risk earth tag in the sample, save yourself the fuel and avoid rolling altogether. If you need to roll, consider seedbed conditions – dry soil is more prone to erosion after rolling and wet soil is prone to sealing and crusting, which can inhibit emergence.
- Symptoms of deep seeding may include a swollen hypocotyl, elongated hypocotyl arch and yellow cotyledons above or below ground.
- Post-emergence seed depth assessment can be done by measuring the length of white space along the hypocotyl (see image below).
Research Results in Manitoba
Kristen P. MacMillan, of the MPSG-funded pulse and soybean agronomy lab at the University of Manitoba, has been examining the effects of various soybean seed depths in Manitoba. Soybean seeding depths between 0.25 and 2.25 inches were tested at Arborg (clay soil) and Carman (loam soil) in 2017 and 2018. Trials were seeded between May 14 to 24 into dry soil conditions. At the time of seeding, soil moisture was at about 1.25 inches in 2018 and 25 mm of accumulated rain took about 14 and 21 days in 2017 and 2018, respectively.
The recommendation to plant soybeans within 0.5-1.5 inches deep stems from this research. At Carman in 2018, soybean yield was reduced by 20% with shallow seeding (0.25 inch) compared to seeding at 1.25 to 1.5 inches. The other depths were statistically similar to all others. Even though yield loss from deep or shallow seeding is not extensive every time, it is substantial when it does occur (one in four environments to-date in this study). Read the full article on this research for more details.