Improving nitrogen-use efficiency should be a priority
/by Amy Sangster
More than half the world’s population is nourished by crops grown with nitrogen fertilizers, and yet nitrogen remains one of the most challenging nutrients to manage.
Day to day, nutrient loss on farms goes undetected; we cannot see or smell it. Research suggests this undetected loss is especially significant after harvest when there is no crop to utilize the residual nitrogen. This results in significant nitrogen losses from the field both to the air and to water, and can also result in a significant loss to the grower’s bank account.
Improving nitrogen-use efficiency, which is the fraction of fertilizer nitrogen that is actually taken up by the plant, results in increased crop yield and decreased environmental risk. So this should be a priority for any grower and agronomist.
However, it’s challenging to match crop demand with soil nitrogen availability. Plants require nutrients in relatively small amounts in the early spring as they begin growing, then a greater nutrient requirement in the mid-season as vegetative growth accelerates, and then a smaller nutrient requirement again as they mature later in the season.
Trying to meet all of the crop’s nutrient needs with just one nitrogen application in the spring leaves many avenues for nitrogen loss throughout the season. In an attempt to circumvent this, we split fertilizer applications. The additional application can be made later when the crop’s requirements are greater. However, the nitrogen cycle is a leaky one and nitrogen is often over-applied in order to have some assurance that the crop will find enough nitrogen in the soil to complete its growth.
The most commonly used commercial fertilizers are water-soluble fertilizers that release plant-available nutrients very quickly. For example, ammonium nitrate (AN or 34-0-0) provides readily available nitrogen in both the ammonium and nitrate form. Most agricultural crops prefer the negatively charged nitrate form of nitrogen. But some, such as lowbush blueberries, prefer the positively charged ammonium form.
Urea (46-0-0), on the other hand, must undergo some biologically mediated transformations before it’s converted into the plant-available nitrate. This happens by what you can imagine as a production line, and workers (microorganisms) on the line have tools (enzymes) that ultimately convert urea into plant-available nitrate.
The global fertilizer industry has been working to develop fertilizers called controlled-release fertilizers and slow-release fertilizers. However, both of these terms can be somewhat misleading, and these products might be better described as delayed-release fertilizers.
Controlled- or slow-release fertilizers can be classified into two basic groups. One group contains compounds of low solubility and coated water-soluble fertilizers. Other products, known as nitrogen stabilizers or bio-inhibitors, reduce nitrogen losses by slowing nitrogen transformations, or, in other words, control the tools along the “microbial production line.”
Controlled-release fertilizers are typically coated or encapsulated fertilizer granules resulting in a controlled rate, pattern, and duration of plant nutrient release. The most common coated controlled-release fertilizers are sulphur- and polymer-coated products.
Sulphur-coated urea is an older product that was first produced in 1972. The polymer-coated urea (i.e. ESN) has more recently become popular, and its release is regulated by polymer chemistry, coating thickness, soil moisture, and soil temperature. It should be emphasized that these controlled-release products should only be considered controlled as much as you can control soil moisture and temperature.
Bio-inhibitors are generally either urease or nitrification inhibitors. When urea is applied, the bio-inhibitor NBTP (N-(n-butyl) thiophosphoric triamide) inhibits the urease enzyme, which is the first “tool” to be used in the eventual conversion to nitrate. A commercially available example of the NBTP inhibitor is Agrotain. A common commercial nitrification inhibitor is DCD (dycandiamide), which goes by the trade name SuperU.
The NBTP inhibitors are marketed to reduce ammonia volatilization. However, with our wet springs, we typically get enough rainfall to prevent extreme ammonia volatilization from spring urea applications. Both inhibitors can then be used to slow the “production line,” which ultimately produces nitrate.
Research in Ontario showed that these two inhibitors were effective in delaying nitrification when spring-applied to corn. This trial showed peak conversion to nitrate occurred at about two weeks in the regular urea applied treatment and occurred at about three weeks in the Agrotain treatment, while the SuperU treatment’s peak nitrification was delayed past 30 days after application. Again, this delay is dependent on soil climatic conditions, so it’s difficult to predict the exact time of release after application.
Regardless of the fertilizer product used, proper nutrient management should include the “four Rs” of fertilizer use: apply the right nutrient, at the right rate, at the right time, and in the right place for the selected crop. These fertilizer technologies can’t replace common sense or the value of observing the unique conditions of your farm.
Finally, don’t forget that some of the most valuable slow-release nitrogen fertilizers are manure and compost, which give far-reaching benefits beyond just slowly releasing nitrogen through the growing season.
(Amy Sangster is a soil specialist with Perennia Food and Agriculture Inc. based in Bible Hill, N.S.)
