Features Soybeans
Refining soybean seeding depth

Soybeans can be sown too deep or too shallow. That’s the findings of a research project that investigated the optimum seeding depth for soybeans in Manitoba.  

“Dry soil conditions have often led agronomists and farmers to chase moisture and seed soybeans at  two inches or deeper. Observations on the success of this practice have been variable, such as delayed or reduced emergence in some but not all cases,” says Kristen MacMillan, research agronomist in soybean and pulse crop agronomy and cropping systems, department of plant science at the University of Manitoba. “On the other hand, very wet soil conditions in spring have led some farmers to consider broadcasting and incorporating soybean seed. It was my goal to find out if very shallow or deep seeding of soybean impacts yield.”

At the time of MacMillan’s research (from 2017 through 2019), the recommended seeding depth was between 0.75 and 1.5 inches, based on recommendations from other regions. The objective of her research study was to identify the optimum seeding depth for soybeans in Manitoba, and to measure the impact of seeding depth on plant population, nodulation and root rot (Carman 2019 only), pod height (2018 and 2019 only), maturity and grain yield. Trials were seeded with a double disc plot seeder between May 14 and May 24 at 200,000 seeds per acre. The soybean variety DK 23-60RY was grown in Arborg, and DK 24-10RY was grown in Carman. All trials were seeded into tilled stubble, except the trial in Arborg in 2017, which was seeded into tilled fallow.

Growing season conditions in all environments were drier than normal, reflecting what farmers in the region have been experiencing as well. Cumulative spring precipitation in May and June equated to 2.2 to 5.7 inches (56 to 145 mm), which was 40 to 87 per cent of normal; and seed was often placed into dry soil. An accumulated one inch (25 mm) of rain occurred between 10 and 22 days after seeding in all trial environments. These conditions provided a good scenario to understand if deep seeding is warranted in dry springs.

MacMillan found that a seed depth range of 0.5 to 2.25 inches resulted in an established plant density ranging from 140,000 to 170,000 plants per acre, which is the current recommended plant stand range. A very shallow seeding depth of 0.25 inches resulted in significantly lower plant density of 81,000 plants per acre, which is only about 50 per cent of the recommended plant stand. 

“To answer a question that was received from farmers and agronomists, we rated nodulation and root rot at the Carman experiment in 2019,” MacMillan says. “We found no effect of seed depth on nodulation or root rot. Nodulation was good and root rot severity was very low overall.” However, ratings could not be measured at the shallowest 0.25-inch treatment due to very low plant establishment and inadequate sample size.

Soybean yield was significantly affected by seed depth, environment and their interaction, meaning that overall response to seed depth was similar in each environment but with varying magnitude. Overall, the relationship between seeding depth and soybean yield explained 68 per cent of the variation in soybean yield. 

The seed depth range that produced 91 to 100 per cent of maximum yield was between 0.75 and 1.75 inches, with maximum yield at 1.25 inches.

“This study provides evidence that even under dry soil conditions, there is no benefit to chasing moisture. We did not measure depth to moisture in each environment but seed was often placed in dry soil with rainfall to follow, sometimes up to three weeks later,” MacMillan says. 

Shallow and deep seeding had the most impact on soybean yield. Seeding at 0.25 inches reduced soybean yield by 19 per cent, and seeding at 2.25 inches reduced yield by 10 per cent, but with a range of zero to 36 per cent. Shallow seeding was more detrimental than deep seeding in this study. 

“As observed in the field, deep seeding only slightly reduced overall plant stand and is likely not the primary mechanism of yield loss. Other factors such as delayed emergence, reduced seedling vigour, cotyledon loss and chlorosis were observed and could contribute to yield loss with non-optimal seeding depth,” MacMillan says.

Pod height was affected by seeding depth but the differences were not agronomically significant, since height to the first pod-bearing node was high overall, ranging from 3.5 to 3.9 inches, and was statistically the same for all seed depths from 0.5 to 2.25 inches. Very shallow seeding at 0.25 inches resulted in significantly lower pod height at three inches. Growing conditions had a greater impact on pod height, ranging from 3.1 to 4.7 inches. “This is in agreement with other literature indicating that environmental conditions and genetics are known to influence pod height to a larger extent than management practices,” MacMillan says.

Seeding depth did not affect days to maturity.  

“Compared to other soybean management decisions that we have studied in the soybean and pulse agronomy program, including seeding date, fungicide and variety choice, ensuring seed depth is within the optimum range is likely the most influential to soybean yield,” MacMillan says.