It’s harvest time across Canada, and potato producers across the country are starting to judge how well their agronomy programs worked with Mother Nature to produce a high-quality potato crop. A myriad of factors are responsible for producing a profitable potato crop, and they rarely stay constant from year to year. While optimizing crop nutrition is essential for all producers, how individual farms manage fertility can differ widely across the country.
One essential nutrient that has been garnering significant attention in recent years is nitrogen. Nitrous oxide emissions are one of the leading sources of greenhouse gas emissions from agriculture, as one molecule of nitrous oxide is equivalent to 300 molecules of carbon dioxide for atmospheric warming potential. Nitrous oxide emissions are most often released when there is an excess of soil nitrate in damp soil. Here in Eastern Canada, the highest risk of nitrous oxide emissions is from “orphan” nitrogen left over post-harvest. This is nitrogen in excess of crop needs that is at risk of being lost to the environment. Not only can this be an environmental issue, but it also represents the cost of unused nitrogen).
In my experience, nitrogen is the most complex nutrient to manage, and that it can be more complex depending on where in Canada you grow potatoes.
If you utilize fertigation and can spoon-feed nitrogen each week to your crop, the risk of nitrogen losses is lower than farms where most nitrogen is provided up-front. Regions with warm and wet autumns are also at higher risk of nitrate losses than cooler and drier regions.
Nonetheless, it is important for all producers to look at nitrogen fertility and ensure that their program is optimized to have adequate nitrogen available to the plant when it needs it without large amounts of residual nitrate at the end of season.
Beyond environmental concerns, an important reason to ensure that we haven’t applied too much nitrogen is that plants with excess nitrogen availability can experience delayed maturation and tuberization. This can result in reduced tuber count and size, particularly in regions with shorter frost-free windows. Particularly for indeterminate growth varieties such as Russet Burbank, finding the right balance between enough nitrogen to meet yield expectations and not too much to delay maturity can be tricky.
At the heart of any successful nitrogen program is accounting for all sources of nitrogen, including:
- Chemical fertilizer, dry or liquid: What forms of nitrogen are being applied? How quickly do they covert from urea to ammonium and from ammonium to nitrate? When are you applying fertilizer (pre-plant, at planting, in-season)? Are you using inhibitors or polymer coatings? These delay nitrogen availability to when the crop needs it and reduce risk of early season losses.
- Manures and compost: Are you testing for nitrogen content in your amendments? How accurate is your rate per acre? Are you incorporating/injecting liquid manure soon after application? Accounting for both the amount of nitrogen available and the timing of release is important for an accurate nitrogen plan.
- Nitrogen from legumes: Are you growing forage legumes like alfalfa or clover ahead of potatoes? If you are, you should be accounting for nitrogen credits in your fertility program. These credits will differ depending on the percentage of legume in your forage stand, the number of years in forage, timing of termination, and climate conditions. Here in P.E.I., I am leading some on-farm trials under the AAFC’s Living Labs program to dial in nitrogen credits following legumes in our growing system. Consult with your agronomist or nutrient management specialists in your region to help quantity N credits where you farm.
- Nitrogen from organic matter: What is the organic matter percentage in your fields? Higher organic matter fields (greater than 3.5 per cent) will mineralize more nitrogen each year than low OM fields, and this should be factored into your nitrogen program.
A simple way to gauge whether you are being efficient with your nitrogen program is to take soil samples for nitrates after harvest. This will indicate how much residual nitrogen is left over at the end of the season. For those with primarily sandy-loam soils, a post-harvest cover crop can help scavenge some of that residual nitrate and carry it over for next year, reducing the risk of nitrate losses.
Recently, the acreage of nitrogen-efficient varieties has been increasing in many provinces. Lower N requirements while producing the same or better yields than standard varieties is a good news story for all involved, but it requires work by the industry to understand the N requirements for newer varieties under different management systems. There is a project under the FVGC National Cluster (led by Dr. Mario Tenuta) looking at this very question, and other trials are underway across Canada to optimize nitrogen management.
Best wishes to all producers as you lift another season’s promise from your fields. Please prioritize safety with your family and employees at this busy time of year.
