Potatoes in Canada

News Fertility and Nutrients
AAFC: Soybean over red clover for potato rotations


March 12, 2021
By Agriculture and Agri-Food Canada

Topics
Red clover growing at Harrington Research Farm prior to potatoes. The white pipes hold tubing to collect soil and water samples used for analyzing nitrates. Photo courtesy of AAFC.

Prince Edward Island farmers commonly plant forage legumes, like red clover, in year two of their conventional three-year crop rotation prior to planting potatoes. Agriculture and Agri-Food Canada (AAFC) researcher, Yefang Jiang, recently completed a six-year study to find out how this rotation affects nitrogen levels in soil and water.

Red clover is planted and then plowed into soil in the fall prior to potatoes being planted, becoming a source of nitrogen and acting as a natural fertilizer that helps potato plant growth.

“However, without adequately accounting for the nitrogen from the clover, farmers can over-apply synthetic fertilizer to potatoes if they follow the recommended P.E.I. provincial guidelines (17 kg N/ha),” Jiang explains.

This excess nitrogen can enter groundwater, streams and estuaries in the form of nitrates which can affect water quality. Excess fertilizer can also suppress potato yield.

Jiang discovered that planting soybean in the rotation instead of red clover improved nitrogen usage efficiency by as much as 1.6 times and increased the potato yield by 13.4 per cent.

“Soybean can create a better economic return for farmers in the short term, but it can reduce soil organic matter over the long term,” Jiang says.

To improve soil organic matter, Jiang recommends that farmers considering adding soybean to their rotation should plant a high residue crop, such as sorghum-sudan grass, as a companion crop.

“To preserve groundwater quality, farmers should take into account the added nitrogen put into the soil from the plowed-down red clover before fertilizing for potatoes, or replace red clover with non-leguminous plants in their rotation.”
Yefang Jiang, research scientist, Agriculture and Agri-Food Canada 

Dr. Yefang Jiang is an AAFC Charlottetown-based sesearch scientist specializing in agriculture impacts to groundwater. Photo courtesy of AAFC.