Dr. Merrin Macrae: They not only looked at phosphorus but also used ‘tracer’ compounds like bromide and chloride that can be tracked more easily

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What about no-till and phosphorus run-off?

Macropores – worm holes and old root channels – have been identified as pathways for dissolved phosphorus moving from the field to the tile system

By Peter Reschke

Once touted as the poster child for soil conservation and environmental sustainability, no-till has recently come under fire from some who are concerned about phosphorus movement into watercourses.

Without regular tillage to break them up, those macropores – worm holes and old root channels – have been identified as pathways for dissolved phosphorus moving from the field to the tile system. Some have even suggested that water quality may be the trade-off for no-till’s other soil benefits.

Well, it turns out the critics spoke a little too soon. New Ontario research has shown that there’s no reason why no-till and minimal loss of P have to be an either-or proposition. It just comes down to how you apply fertilizer.

The key factor is sub-surface banding. “We often see reduced loads (of P) coming out of fields where sub-surface placement is used,” says University of Waterloo researcher Dr. Merrin Macrae, who co-authored the new paper with MSc student Kirsten Grant as well as Fereidoun Rezanezhad and Vito Lam. The work is published in the most recent edition of the Vadose Zone Journal.

The question they attempted to answer in the paper was ‘why is it happening’? Their experiments looked at clay and silt loam soil under environmental conditions commonly seen during the non-growing season in Ontario, where most of the P runoff typically occurs: autumn rains, periodic snow melts and freeze-thaw cycles during winter, and the soaking spring showers that can create “duck pond flooding” in fields, Macrae notes.

To really get a feel for nutrient movement they not only looked at phosphorus but also used ‘tracer’ compounds like bromide and chloride that can be tracked more easily, she explained.

The soil columns they used were not small cores but, what Macrae calls, “soil monoliths”, 30x30x30 chunks excavated from the fields of the farmer co-operators. They then compared broadcast and sub-surface banded fertilizer.

“We wanted to know how all these things relate to preferential transport through the macropores,” she says.

They found greater flow of nutrients through the clay soil. “That was nothing new,” Macrae notes. But while there was generally less flow through the silt loam, things changed when the soil was frozen. At that point, preferential transport increased showing that these soils are also susceptible to losses.

In all cases, there was greater movement when fertilizer was broadcast on the surface. Once those nutrients were placed in a band, there was less chance of them ending up in the macropores, Macrae explains. The experiments proved that “banding cuts it off from that path.”

If, for example, a grower bands fertilizer in the fall after winter wheat, even if the soil freezes, “the odds of those nutrients getting down (into the preferential flow path) are getting a lot less,” she says.

Macrae sees this as the key part of this research: understanding why there’s a different outcome depending on fertilizer placement. “(With banding) you’re essentially cutting it off from those transport pathways, especially in clay but also in other soil types in winter.”

In terms of numbers, Macrae says when fertilizer was broadcast and banded at the same rate, there was a 60 per cent reduction “in what came out at the bottom” with banding. But because banding generally involves a much lower rate of P “the true benefits would be even greater,” she says.

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