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Fall Fertilizer (08/27/20)

With the memory of no fall fertilizer in 2019 and the logistical and practical problem of applying all fertilizers to spring crops in a short period of time in 2019 and 2020, even with a large acreage of what would become 2020 Prevent Plant, fall fertilizer is on many growers’ and fertilizer providers’ minds.

With the memory of no fall fertilizer in 2019 and the logistical and practical problem of applying all fertilizers to spring crops in a short period of time in 2019 and 2020, even with a large acreage of what would become 2020 Prevent Plant, fall fertilizer is on many growers’ and fertilizer providers’ minds.

For phosphate (P) and potassium (K) fertilizers, this is the time for strip-tillers to take advantage of any opportunity to make strips for spring and apply their P and K at the same time. Fertilizer P and K can also be applied prior to any fall tillage operation. Keep in mind the importance of a starter P band for small grains, canola, corn and potato at seeding next spring. For no-tillers, application of P and K is best applied below the soil surface. There is always a risk of off-sight movement with heavy rain or spring snowmelt. Movement into streams/rivers from surface application and subsequent movement with excess water results in public/government pressure for tougher regulations. Therefore, application of P fertilizers near sensitive waterways should probably delayed until spring, or applied below the soil surface.

Fall N application is possible in soils not at risk of leaching or spring flooding. That means fall N application is not a wise decision in soils of loamy sand/sand textures, nor is it wise in fields next to major waterways where spring flooding has been a problem in the past. Along the Red River, this restriction may be wise for several miles from the river, depending on past experiences. These experiences should be fresh in many growers’ minds.

The most important factors to consider in fall N application are N source and timing. The general rules are these:

Do not apply any N fertilizer before October 1.

  • After October 1, wait until the soil temperature measured between 6AM and 8AM drops to 50oF. Then the risk of N loss from anhydrous ammonia application is low enough to reasonably profit from it.
  • For banded urea, with an air-seeder or similar equipment, wait another week after the anhydrous ammonia date, then risk of N loss is low enough to begin application.
  • For broadcast urea, wait 2 weeks after the anhydrous ammonia date, then begin application.

In addition to timing, but not as a substitute for timing, a nitrification inhibitor can protect the fertilizer from excessive transformation to nitrate in a longer-than-normal fall season. The most consistent nitrification inhibitor is nitrapyrin for anhydrous ammonia. Other nitrification inhibitors with activity to slow the rate of nitrification are the nitrapyrin in Instinct and DCD products at the proper rate. None of the nitrification inhibitor products completely stop nitrification, but they can slow it to varying degrees and keep more of the ammonium N in the ammonium form longer.

Sulfur is not a fall fertilizer. Sulfate moves with water, so ammonium sulfate applied in the fall would leach most in soils most at risk of S deficiency. If elemental S is applied in the fall, some might oxidize by spring planting, but the sulfate produced would be at risk for leaching loss and be unavailable to the crop, leaving behind elemental S of little use to the crop. Therefore, the best strategy is spring applied S fertilizer as ammonium sulfate, or sometimes ammonium thiosulfate liquid if not applied with the seed.

Remember that rates of P, K and N in North Dakota should be based on soil testing. The S rate is not related to a soil test result. Ideally each field that will receive an N application should be sampled. Zone sampling is the best form of sampling strategy in most ND fields.

 

Dave Franzen

Extension Soil Specialist

701-799-2565

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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