An in-soil band of nitrogen at the time of seeding is the “gold standard for efficiency” on the Canadian Prairies, says AAFC soil fertility specialist Cindy Grant. That said, a top-dress of nitrogen or sulphur for canola can make financial sense in some situations. They are:
—Growing conditions improve after seeding. If conditions were too wet or too dry at the time of seeding, growers may have cut back fertilizer rates in response to lower yield projections. If conditions improve in June and a good stand emerges, growers may see a yield benefit from a nitrogen top up. In dry conditions, applying 66% of the recommended nitrogen rate at seeding then topping up with remaining 34% if conditions improve has shown to be an effective and economical practice.
—Saturated soils impede good seed placement. This expands on the previous point. When the only choices to get canola seeded are mudding in or broadcast, cutting back nitrogen rates at seeding may be a good risk management practice. Handling less fertilizer at seeding may help facilitate better seed placement. It may also put a smaller proportion of the fertilizer at risk for significant nutrient losses and allow assessment of stand establishment and crop potential before applying the balance of the fertilizer. If the crop becomes well established, an investment in more nitrogen fertilizer would be warranted.
—High nitrogen losses are likely. Wet soil conditions can accelerate nitrogen losses through leaching and denitrification. Fields may need a nitrogen top up to reach their yield potential, but make sure canola survived the wet conditions before investing in a fertilizer top up.
—The crop is showing signs of nitrogen deficiency. Nitrogen deficiency symptoms first show up in older leaves as pale green to yellow coloring, and sometimes purpling. Tissue analysis can confirm these observations, but tissue analysis is not an exact science. Sampling error can be a problem. Many labs offer tissue samples, so check with the lab for proper sampling procedures. Turn around time is another hurdle. Growers should sample as early as possible to get results in time to take action.
—A grower cannot efficiently place all the fertilizer needed through the seeding tool. Some growers will address this with a top dress application of nitrogen after crop emergence, which can reduce potential for losses if tillage practices do not allow for broadcast and incorporation prior to seeding.
—Growers could not put the desired rate on at seeding.
—Yield potential improved and growers want to add sulphur to their nitrogen top up. If field conditions have been excessively moist, sulphur may have moved lower in the soil profile. As canola plants grow, their roots will extend into these reserves. For that reason, growers who have been applying recommended rates of sulphur may not see as much economic return from a sulphur top up compared to a grower who has cut sulphur rates in recent years.
—Canola shows signs of sulphur deficiency. With sulphur deficiency, yellowing and leaf cupping tend to occur on new leaves first. Purpling of leaf edges can show up when deficiency is fairly severe. In fields short of sulphur, crops can usually find enough to get past the early rosette stage without visible symptoms. Deficiency symptoms often show up at flowering.
Click here for more information on products, rates and timing for best results from a fertilizer top-dress. This article has just been updated.
If you’re on the fence about a top-dress application, consider applying a rate high enough to make a difference — such as 30 pounds of nitrogen or 20 pound of sulphur — to part of a field. Mark the location and compare yields for treated and untreated areas. Increasing greening does not necessarily translate into increased yield, so a simple observation of crop colour is not enough to determine the value of the top dress. If you do see a yield difference, the best solution may be to increase fertilizer rates at seeding.