How deep is too deep when chasing moisture for soybeans?

A University of Manitoba researcher thinks there should be more attention paid to soybean seeding depth

Soybean growers may have been tempted to dig deep for seeding this year, but University of Manitoba researcher Kristen MacMillan says the data may not back up that practice.

The Manitoba Pulse and Soybean Growers puts ideal seed depth between three-quarters of an inch and an inch and a half below the soil surface. Dry conditions this spring, however, led some farmers to question that range.

Cassandra Tkachuk, MPSG eastern production specialist, said she heard several farmers considering deeper seed in order to chase soil moisture.

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Much of agricultural Manitoba was in the grip of a months-long dry spell during ideal soybean plant dates in mid-May.

“Some of the soybeans planted deeper than 1-1/2 inches, we saw some yellowing and swollen hypocotyls, some of your classic symptoms of deep seeding, but from what we’re seeing now, and I would have to look at those crops specifically and see how they fared compared to other crops, but the soybeans look quite good,” Tkachuk told an MPSG SMART Day here last week.

The group did note emergence issues early in the season, although Tkachuk said it was difficult to separate seed planted too deep with crops that were struggling with lack of moisture.

The discussion highlights MacMillan’s ongoing research into seed depth, a factor she says is often overlooked in soybeans compared to planting date, fertility or other strategies. She set out to put the recommended seeding depth range to the test last year. Now collecting her second year of data, she is testing seeding depths from a quarter of an inch to 2-1/4 inches deep at Carman and Arborg.

Kristen MacMillan.
photo: Alexis Stockford

“So far, we typically see reduced establishment with very shallow seeding, so that’s about a quarter-inch and that’s as expected because we’re leaving the seed in a vulnerable place in the topsoil where wetting and drying is very common. They’re very prone to desiccation and typically the top layer of that soil is dry,” she said.

“We are also seeing reduced establishment at the deep seeding depths, although so far it hasn’t been as frequent, and I think that also has to do with the soil type and whether or not there’s compaction and soil crusting.”

Arborg’s heavier clay soils showed little difference in either emergence or yield across seeding depths last year, although mechanical issues meant that plots could only extend to an inch and three-quarters deep, just outside the recommended range.

“At that site it was planted into fallow, and so we had a loose, mellow soil,” she said. “Again, thinking about soil conditions, there’s less of an impediment for emergence, even at the deeper depth of 1.75 (inches).”

In the lighter, sandier, soils near Carman, however, the shallow-planted plots struggled to emerge, and yield dropped 25 per cent in the deepest plots compared to those in the recommended range.

The preliminary data might suggest that the risks may not be worth the benefit from chasing deeper moisture, MacMillan said, although the study is far from complete.

Running out of moisture

Those who seed deeper may also not be doing themselves any favours if that depth gives seeds enough moisture to germinate, but not enough to maintain growth if no rain falls.

MacMillan said a soybean seed needs to absorb half its weight in moisture to germinate, not counting the continued moisture to keep growing.

Her results are still preliminary, but she has urged producers to pay more attention to matching seeding depth with the conditions that would affect soil moisture.

“Really, we want to stay within that range if possible and we really need to think about the factors that are going to affect the seed depth, whether that’s soil type and its ability to retain moisture, where the moisture is at the time of seeding, what the forecast looks like and do you foresee that, if you put it into dry soil now, (whether) there’s rain in the forecast and you’re likely to have moisture reach that seed,” she said.

Most producers will be familiar with their soil type’s impact on water, but MacMillan also urged them to consider things like tillage and residue (which might affect whether seed is buried or left vulnerable on the residue surface), soil organic matter structure (which will affect infiltration), and to take a considering look at their equipment.

“If you have a type of equipment that creates a big furrow and then you’re rolling it closed, you’re adding a lot more depth,” Tkachuk said. That same roller might lead to crusting and low emergence, should the field get a rain right after being rolled.

MacMillan also urged producers to double-check their seed depth even after emergence, something she says few farmers do, but that could be invaluable in tracking emergence or yield issues from deep seeding. Measuring the white section of a seedling from the top of the root hairs to where the plant starts to green will give a good indication of seed depth, she said, while physical symptoms like swollen hypocotyls may also be a clue that seeds were buried too deep.

“It’s trying to build up pressure to try and get through (the surface),” she said.

“Often I’ll see lost cotyledons that will get sloughed off,” she added. “Typically, if you lose one cotyledon, it’s OK, but there has been documented yield loss if you lose both.”

MacMillan plans to continue the project into 2019, both to see if the yield reduction at deeper seeding depths is consistent and to better hone in on the factors leading to that yield reduction.

About the author

Reporter

Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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