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Optimising fertiliser source and rate to avoid root death

A new study assembles canola root’s dose-response curves for nitrogen sources.

The American Society of Agronomy reports on a new study that aims to determine what is called a dose-response curve. Researchers are trying to find the ideal fertiliser rate and source that will help the plants – in this case canola – without damaging the roots.

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Banding

The rate and source of fertiliser can make the difference in crops, especially in a method called banding. It’s a method where the fertiliser is placed in a band in the soil below the seed. While banding has many advantages, it can also cause damage to the plant roots if used incorrectly.

“While banding below and to the side is the recommended practice, banding directly below the seed continues to be a common practice used in dryland systems,” says Isaac Madsen. “It can reduce the number of passes across the field required in a growing season. Banding also allows you to put on all or most of your fertiliser at one time. This generally causes less disturbance in a minimum or no-till system.” Madsen is a postdoctoral research associate at Washington State University.

Canola roots interacting with fertiliser

Researchers like Madsen are trying to find the ideal rate and source that will help the plants without damaging the roots. Madsen and his team studied canola in particular. New imaging and analysis techniques allowed them to see canola roots interacting with fertiliser in a way never previously done.

“In this study, we imaged canola roots growing into a fertiliser band and measured the damage to the canola tap root,” Madsen explains. “Using scanner-based rhizoboxes (glass boxes filled with soil), we were able to collect a series of root images over time. This allowed us to look at the effect of fertiliser rate and source on the seedling root systems.”

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The researchers used 3 sources of nitrogen fertiliser in their study: urea, ammonium sulfate, and urea ammonium nitrate. Each one reacts differently in the soil, and the researchers thought they would cause different amounts of damage to the roots. - Photos: American Society of Agronomy
The researchers used 3 sources of nitrogen fertiliser in their study: urea, ammonium sulfate, and urea ammonium nitrate. Each one reacts differently in the soil, and the researchers thought they would cause different amounts of damage to the roots. - Photos: American Society of Agronomy

The reason the root damage is of such concern in canola is because it has a long tap root. These larger roots are especially affected by fertiliser. If the tap root is damaged, the plant can’t properly take up nutrients and water.

Dose-response curve

The team’s end goal was to determine what is called a dose-response curve. This will help farmers better apply their fertiliser and know if it will harm their crops’ roots or not. They used the data they collected from the root images to develop these curves for the different fertiliser sources.

“A dose-response curve helps determine the amount or dose of a substance that will result in a specific response,” Madsen says. “In this instance we modeled tap root survival, depth, and distance from fertiliser band.”

They used 3 sources of nitrogen fertiliser in their study: urea, ammonium sulfate, and urea ammonium nitrate. Each one reacts differently in the soil, and the researchers thought they would cause different amounts of damage to the roots.

Urea ammonium nitrate rather than urea

Madsen says their results show that banding urea ammonium nitrate rather than urea, along with keeping the rates low, is the best option for canola. He adds that this research is important for canola growers in the Pacific Northwest area. It helps establish guidelines for them to apply nitrogen fertilisers.

“I think banding is, indeed, frequently the best practice,” says Madsen. “But we need to be careful with our rates and sources. Banding of fertilisers will not cause problems as long as the rates used are low enough and the sources used are safe enough. My research goal is to develop rate and source guidelines that growers can use to minimise damage and maximise fertility.”

Also read: In-soil fertilisation leads to less phosphorous loss

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