Soil testing critical to seeding success in season after flooding

MELVILLE, Sask. — Soil tests this spring are critical after a couple of years of flooding, says soil scientist Jeff Schoenau.

Farmers face planting into soil that is not in optimal condition after lying under water or covered in weeds, the University of Saskatchewan professor explained at a recent Indian Head Agricultural Research Foundation crop management seminar.

Water fills the pores in the soil when fields flood.

“It greatly slows the movement of oxygen,” Schoenau said.

In fact, the oxygen moves 1,000 times more slowly.

Plant roots run out of oxygen, as do micro-organisms that live in the soil.

The microbes begin to use nitrates instead, which they convert to gas and the soil then loses to the atmosphere.

Nitrogen loss through this process, called denitrification, is the most common problem most producers will face, he said.

“There can be rapid and extensive losses of nitrogen,” he said.

It can even happen on anaerobic micro-sites in fields where the water has drained, he added.

Nitrogen losses are variable.

Losses of up to 50 percent of applied nitrogen were reported last year, Schoenau said, but soil tests in the spring showed more available than what Schoenau had expected.

Heavy clay soil is more susceptible to denitrification, while sandier soil is more susceptible to leaching, in which the nitrogen could potentially move below the root zone.

“After a wet year, it’s a good idea to go deeper in soil sampling,” Schoenau said.

Sulfur can also be affected by excess water. He said soil that smells rank means microbes are converting sulfates to sulfide.

Sulfur loss through gas isn’t usually a problem, but sulfur is mobile in the soil and can affect fertility in that way. If the sulfur has migrated upward, a soil sample could show high available sulfate close to the surface.

Phosphorus and potassium are not affected by flooding and cannot be lost in a gaseous form.

Schoenau said soil tests are also more important this year because fertility will be highly variable. Portions of fields may have been seeded and portions left flooded or weed-covered.

Producers may face a number of soil scenarios this spring:

Q: What should a farmer expect if it was too wet to seed last year and the soil stayed saturated throughout the growing season?

A: “Any residual available N is probably gone,” Schoenau said, even though there was no crop to use it.

Denitrification would have occurred, and mineralization would have been limited. Mineralization is the conversion of organic nitrogen to the plant available forms of ammonium and nitrate.

However, mineralization can take place as the soil dries.

Q: What should a farmer expect if a field was too wet to seed but it dried out and could be kept free of weeds?

A: Schoenau said the loss of available nitrogen will be lower than if the field had been flooded the entire season. Some fields may have very high levels of available nitrogen.

Microbial activity will be releasing nutrients, and an accumulation of available nutrients in those soils would likely occur.

When he compared wheat stubble with an unseeded, tilled, weed-free field, Schoenau found higher levels of nitrogen, phosphorus, sulfur and micronutrients in the unseeded field.

For a 30 bushel-per-acre crop, the farmer would need to apply 80 pounds of nitrogen on the stubble and 20 lb. on the fallow.

“Save yourself the fertilizer dollars,” he said.

Q: What should a farmer expect if weeds grew in fields that were too wet to seed?

A: Schoenau said it could be a mixed blessing because the plants at least take up nitrogen and prevent it from being lost.

“In the short term, that’s not so good,” he said, because the nitrogen may not be released in time for the upcoming crop to use it.

Mature weeds have a high ratio of carbon to nitrogen and the release of nitrogen would be slow.

However, if the weeds were controlled when the plants were young, a lot of the nutrients they took up would siphon out of the weed biomass into plant available forms, he added.

Q:What should a farmer expect if a field was seeded and fertilized but then flooded?

A: Schoenau said nitrogen losses likely weren’t that great. Ammonium nitrogen does not leach or denitrify and farmers might have more available nitrogen than they think.

He said farmers who use no-till systems might need to do a light tillage to smooth ruts. Research work he did three years ago found that a light till didn’t result in any measurable changes in soil properties.

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