Fall soil tests — tips

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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 isn’t always practical.

In the fall, wait until soil temperatures drop below 7°C before sampling. Microbial processes in the soil slow down at cooler temperatures, so samples taken from cool soils provide a closer estimation of nutrient levels at seeding next spring. Winter sampling also works, but you want to make sure you can still get the probe down 24”.

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.

Ideally, you want to submit three samples per zone: 0”-6”, 6”-12” and 12”-24”.

If you don’t plan to test every field, 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 find, in many cases, that the fields are too wet to spray. Weed growth adds 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.

For more on soil tests, click here for the Canola Council of Canada factsheet “Understanding soil test recommendations.”

Published on Thursday, October 28, 2010

Canola Watch