Nitrogen fertilizer: how much more efficient can we get?

Monday, February 20, 2012

Eventually, we will hit the biological limits of N use efficiency. But, for now, there is still room for improvement

by KEITH REID

Nitrogen (N) fertilizer has improved grain yields with huge economic benefits for the agricultural sector. However, it is a resource that needs careful management. Too little and crop yields suffer. Too much and it increases the risk of losses of ammonia and nitrous oxide to the air and nitrate to groundwater, not to mention increasing overall costs. 

In the past, N fertilizer was cheap enough that it made sense to apply a little extra as insurance. Today's economic and environmental realities, however, demand that we do better. This does not necessarily mean applying less nitrogen, but it does mean getting more yield out of each unit of nitrogen that is applied.

The challenge is finding the tools to match N fertilizer application to crop requirements more closely. This is not necessarily easy, given the variabilities in soil and weather, but there are tools available and more will soon be available.

How we can improve N efficiency today? The first step is to make sure your N application rates are based on what your fields are capable of producing, rather than "yield wishes" (or unrealistic yield goals). There is a link between higher yields and higher optimum N rates, but simply adding more fertilizer will not overcome other yield-limiting factors in the field.

Second, be sure you are accounting for all of the sources of N that are on the farm.

Plowing down an established forage legume will provide close to the total N requirements for a crop of corn, so adding more fertilizer to these fields will not increase yield but rather can lead to losses. Manure or biosolids can also provide significant amounts of N for the crop, depending on the nature of the material, and when and how it is applied.

When you are applying fertilizer, make sure you minimize the risk of losses. A good example is surface-applied urea, where half or more of the applied N can be lost to the air as ammonia if there isn't enough rain after application to soak the material into the soil. Subsurface banding or timely incorporation can avoid this, but if these are not possible, then it will pay to use a different form of N fertilizer.

Finally, match your application timing as closely as possible to when the crop needs the N. Since N does not bind tightly to the soil, the longer it sits before being taken up by the crop, the greater the risk that it will be lost. If you can side-dress corn, for example, rather than applying all the N at planting time, you can reduce the amount you need to apply by up to 20 per cent for the same yield.

Where do we need to improve in the future? Progress on improving N use efficiency will occur on three fronts – better tools to predict the amount of existing N in the soil or how much additional N is needed; better fertilizers to match N release to crop uptake; and genetic improvements to crops to get more out of applied N.

n Real-time sensors will be able to assess the N status of the crop on-the-go, either from plant-based or soil-based measurements (or most likely from a combination of both).

This, combined with weather data and sophisticated computer programs, will be able to target N applications to areas within the field where they will be most beneficial in any given growing season.

n Controlled-release fertilizers are in their infancy, and we can expect further progress in developing coatings that respond to soil conditions that match crop N uptake, or that are actually signalled by plant roots to release N to meet crop needs.

n There is no such thing as an "N response gene," but there are two promising areas for improving N use efficiency. The first is to enhance the uptake of N from the soil by plant roots through increasing the size of the root system or by increasing the uptake of N per unit of root length. The second is to change the physiology of the plant itself, so that each unit of N that is taken up by the plant is converted into more harvestable yield. 

All of these future tools are based on work that is already happening in the lab and in the field, although we can expect progress to be faster in some crops and approaches than others. Eventually, we will hit the biological limits of N use efficiency, but for now there is still room for improvement. BF

Keith Reid is Manager (Eastern Canada) – Soil Nutrient and GHG Management, Agriculture and Agri-Food Canada, Guelph.