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How much nitrogen will carry over to next year?

By Jeremy Boychyn (M.Sc)(P.Ag), Agronomy Research Extension Specialist | Alberta Wheat and Barley Commissions

Producers make fine-tuned nitrogen (N) decisions based on soil tests, expected yield, and environmental expectations. However, mother nature has a way of evading our plans. Over the 2021 season, rainfall in Alberta was significantly lower than normal, and heat stress reduced yields. This means that most crops were small and utilized less of the applied and soil available N. Can producers account for this unused N in their 2022 fertility plans?

The truth is that this question can only be partially answered.

With plenty of days left before winter freeze-up and rain beginning to make an appearance, there is still plenty of mineralization that can occur. This means soil N levels can still fluctuate. To gain an understanding of how much N may carry over to the next season, it is vital that producers take soil samples. However, because fluctuations in soil available N can still occur, it is recommended that soil sampling occurs in late October (when soil temperatures have cooled but before freeze-up) or in the spring. However, spring is typically a more challenging time to collect soil samples due to the time constraints involved.

Carryover N depends heavily on the amount of N used by the 2021 crop by also by the amount of mineralization that occurs in the soil. Mineralization is the conversion of organically bound soil N into inorganic forms of ammonium and nitrate. However, mineralization is tough to predict. Conversion occurs through natural processes involving soil microbes. These soil microbes heavily rely on temperature and moisture to function. During a dry year, mineralization will decrease due to reduced microbial activity while plant N use is going to go down due to reduced growth. This means that although less nitrogen is being taken up by the crop, there is also less N being released from the soil by microbes for plant use.

Although soil sampling is the most common method to determine soil available nitrate-N, we can make some assumptions about potential carryover N based on rainfall compared to average. Areas receiving less than 60% average rainfall have an increased chance of N carryover, as the crop used much less of the N supplied from the fertilizer. However, those dry conditions can negatively decrease N mineralization, increasing complexity determining how much total N will be available to the 2022 crop. Areas which received over 80% of the average rainfall are less likely to see N carryover impacts, as the crop likely used most of the N supplied from fertilizer.

In short, those areas that saw below average rainfall (or well below average rainfall) should pay closer attention to their potential N carryover. This means waiting until just before freeze-up to take soil samples to assess potential N carryover. Those areas with closer to average rainfall will likely not see a huge variation from the norm. However, producers on either side of this fence will benefit from a properly timed soil test to determine available nitrate for the 2022 growing season.