Seeding operations have generally been delayed because of wet soil conditions in much of the Red River Valley. As seeding is backed into June there are some fertility and nutrition concerns with nitrogen and phosphorus that should be recognized.
Cool season crops generally respond less to applied nitrogen fertilizer as seeding is delayed. Two reasons account for this decrease.
Yield potential of cereals, canola and flax has declined. According to Manitoba Crop Insurance past records, the yield potential in the Red River Valley area will be about 85, 80, 75,70 and 70 percent of average for canola, flax, wheat, barley and oats, respectively.
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Growers could consider this reduced yield potential in recalculating nitrogen fertilizer requirements. Most nitrogen recommendations are based on potential yield so tables or formulas are available for recalculating nitrogen needs as yield potential changes.
Wheat requires 2.75 pounds of nitrogen per bushel of yield potential. If yield potential at the optimum seeding date was 50 bu. per acre, then 135 lb. of nitrogen (soil plus fertilizer) would be required. If yield potential is estimated to be only 75 percent of this, nitrogen rates would drop to 105 lb. of nitrogen per acre (soil plus fertilizer). The difference is 30 lb. of nitrogen per acre of fertilizer.
Nitrogen is mineralized and released from soil organic matter and crop residues. Usually this release is too late to meet the bulk of the needs of early seeded cereal and oilseed crops. However, as seeding is delayed, this nitrogen release will more closely match the crop’s uptake pattern. The rate of mineralization microbial activity is driven by soil temperature, so the warmer soils become, the more rapid the release of nitrogen.
Two factors should be considered in phosphorus fertilization. First, phosphorus supplementation is most important for early season growth in cool soils. Phosphorus diffuses short distances in the soil to root hairs and the cooler the soil, the slower this diffusion and the growth of roots. Phosphorus fertilizer placed close to the seed is important in meeting this initial phosphorus need.
As soils warm, the rate of root growth and diffusion increase, so soil phosphorus more adequately meets the crop need.
When long-season crops such as corn are grown in short-season areas, the race to maturity is critical. Studies in Wisconsin indicate that the advancement in maturity and yield with starter phosphorus fertilizer is still warranted for grain corn, even when seeding is delayed until soils are warm.
For crops where advancement of maturity is not as large a concern, then fertilizer phosphorus rates can be reduced, especially where soil test levels are medium or greater.
