After a year of abnormal wet across most of the Prairies, nitrogen and sulphur levels in the soil may be lower than growers expect. On the flipside, areas that were abnormally dry — the Peace Region of northern Alberta and B.C. — may have higher than expected nitrogen and sulphur reserves. In both cases, fall soil tests are advised.
The best time to take fall samples is after soil temperatures drop below 7°C. That’s because microbial processes in the soil slow down as temperatures cool, so sampling late in the fall will provide a close representation of nutrient levels at seeding next spring. The cooler the better when sampling, but you want to make sure you can still get the probe down 24″.
Fall soil tests give growers time to process samples and get results and recommendations. That way growers can develop a fertilizer program for this fall or next spring, and have more time to order fertilizer, to take advantage of typically lower fall fertilizer prices, and to spread the workload out over two seasons.
“Spring soil tests provide a more accurate assessment of nutrient levels at the time of seeding, but holding up spring seeding while you wait for sample results to come back just isn’t practical,” says Doug Moisey, Canola Council of Canada agronomy specialist for central Alberta. “Fall gives you time to get a proper analysis and make a fertility plan.”
Unseeded acres, uncertain fertility
Acres unseeded due to excess moisture may not have the available nutrients growers expect from a fallow field.
Mineralization of soil organic matter should boost available nitrogen — as it would with any fallow field. But excess moisture will increase nitrogen losses from denitrification, which are losses to the air, and leaching, which are losses to water movement through the soil profile. Sulphur is also subject to leaching losses.
Losses will be higher in low spots that were wet for longer. “Follow the water,” says Cindy Grant, research scientist and crop fertility specialist with Agriculture and Agri-Food Canada in Brandon. Grant suggests that zone sampling — sending in separate samples for low spots versus the rest of the field, for example — may highlight important differences in available nutrients. If growers find that certain areas of the field are “normal” and others are deficient, “then site specific fertility management could be an option next spring,” she says.
Moisey recommends three samples per zone: 0″-6″, 6″-12″ and 12″-24″.
“Ideally a grower will sample every quarter but we know that’s not always possible,” he says. “The next best is to concentrate on a couple of fields, taking samples by zone and by depth and using that information to estimate what’s happening across the whole farm.”
Big weeds as green manure
Growers with fields too wet to seed also found, in many cases, that the fields were too wet to spray. Weed growth was much worse than usual, which added to short-term nutrient removal from the soil.
A high number of big weeds will likely increase the weed seed bank and growers will have to be rigorous with their weed control next year. But it’s not all bad news. Those weeds may have done growers a fertility favour in preventing permanent nutrient losses. The weeds took up nitrogen and sulphur, protecting these nutrients from denitrification and leaching — losses that growers can never get back. Whether sprayed or turned under, this weed biomass — like green manure — will release nutrients back to the soil as it decomposes.