Good Land Drainage Increases Nitrogen Efficiency

Prepared by John Heard, MB Ag Soil Fertility Specialist

Just before a 3” rainfall, a farmer was working feverishly to improve his nitrogen use efficiency.  But instead of using a fertilizer applicator he was using a tractor and scraper. 

Indeed, it is wet soils that tend to reduce nitrogen efficiency in many of Manitoba’s soils.  And many of our “Best Management Practices” for fertilizer application consider the appropriate timing, placement, form and rate based on the drainage characteristics of the soil.

Most nitrogen losses are associated with the combination of excessively wet soil and nitrogen in the nitrate-N form.  Nitrate-form N may exist naturally in the soil or may originate from fertilizer and manure applications or organic matter decomposition.  Once in this nitrate form the N is vulnerable to losses.   

On well drained coarse textured (sandy) soils, excessive moisture will move this nitrate-N downward.  Under excess rainfall, this N can leach below the crop’s rooting zone and sometimes into the groundwater.

On poorly drained soils or those with very slow permeability, like clays, excessive water will saturate the soil.  Under these conditions, bacteria denitrify the nitrate-N to the gaseous forms N2 or N2O (nitrous oxide).  Both are lost to the crop and the grower, but N2O is the most troublesome since it is a powerful greenhouse gas.  The magnitude of this loss increases with soil temperature, but even under cool spring conditions this denitrification can cause flooded fields to lose 2-4 lb nitrogen/ac/day.

If farmers use practices to reduce excess water soaking into or flooding their land, then nitrogen losses will be reduced and efficiencies are gained.

Improved drainage will not only reduce N losses but permit crops to make use of the nitrogen that is present in making more yield.  A 60 bu/ac wheat crop will take up and remove twice as much nitrogen as a water-stressed 30 bu/ac crop – better for the grower and the environment.

Manitoba farmers now require drainage licenses to complete the needed surface drainage on their land.  For the sake of improved nitrogen efficiency, farmers should pursue these licenses and get the excess water moving.