Variable rate fertiliser can slash N leaching into the Mississippi

Nitrate pollution is a big issue in the Midwest with nitrogen fertiliser applications on fields with tile drains in the Upper Mississippi region, contributing to one-third of the total N flowing into the Gulf of Mexico.

This nutrient rich water results in a “dead zone” near the mouth of the Mississippi, where oxygen levels are low. An US task force has set a target maximum area for the zone, which will require a 45% reduction in nitrogen loading.

One way is through better use of fertiliser. David Mulla, researcher at the University of Minnesota, compared fields that received a fixed rate of 151kg/ha (135lb/acre) pre-drilling nitrogen with areas that received variable rates.

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Variable rate fertiliser

These areas got 50kg/ha (44lb/acre) before planting and the rest was applied at growth stage V6, the amount based on crop-sensing data. “We saw that the amount of nitrogen leaching was 13% lower when using variable rate side-dressed nitrogen than the uniform applied areas.”

This was due to improved efficiency of nitrogen use, with overall nitrogen fertiliser use cut by 30% while yields were unaffected.

Dr Mulla was speaking at the recent European Conference on Precision Agriculture, held in Edinburgh. Read what Neale Postlethwaite (Society of Precision Agriculture Australia) said at the conference as well.