The past couple of years have illustrated how a condensed spring fertilizer application season can put significant pressure on the entire fertilizer supply chain — from producer to retailer to farmer.
One of the ways to reduce the risk of a short spring season is to apply your fertilizer requirements in the fall. The questions are: what are the major benefits to applying fertilizer in the fall? And, are there agronomic differences between fall and spring application for nitrogen, phosphorus and potassium? The primary reasons for making fertilizer applications in the fall are time, labor availability and open fields. READ MORE
Once crops are harvested, farmers generally have time to collect soil samples and retailers have time to communicate plans for the upcoming season. Fields tend to be relatively dry (although not every year) and available for fertilization, and tillage can be done to prepare the field for next spring’s planting.
Fall fertilizer application can also help the supply chain more evenly distribute deliveries throughout the year. When the fall application window is missed, a significant amount of pressure is put on the entire fertilizer supply chain. Producers have to be able to get product in place and that can be a challenge during peak winter/ spring periods. Retailers must have adequate storage to meet customers’ needs, and must have equipment and open weather to cover acres missed in the fall. This can result in serious logistical challenges if the weather does not cooperate.
From an agronomic standpoint in North America, there is little difference between fall and spring fertilization for potassium and phosphorus. Fall and spring fertilization of potassium or phosphorus is about ensuring an adequate supply of exchangeable/ available nutrients for the subsequent crop during the growing season, so the actual application timing is not super critical. Most soils in North America are not considered high-fixing soils, so application timing for potassium and phosphorus does not greatly impact availability. Plus, most soils have a low potential for loss of these two nutrients. There are a few exceptions for certain soils, climates and crops.
Coarse-textured, sandier soils should avoid fall potassium application—especially in higher rainfall environments because of the potential to leach. Crops that demand higher potassium during certain physiological periods (cotton is an example) can benefit from applications closer to uptake, especially in sandier soils.
Fall fertilization of pasture/turf is advantageous because it allows these crops to develop strong root systems for over-wintering.
When it comes to potassium and phosphorus fertilization there are a couple of factors to remember. If at all possible, do not rely solely on a single soil test to determine your next crop’s fertilizer need. A more reliable indicator is to track soil test levels over time and incorporate estimates of crop removal as a guide to rate decisions. For this, you can use the Nutrient Removal Calculator on the PCS eKonomics website (www.potashcorp-ekonomics.com). Additionally, residue from the most recently harvested crop (especially corn) contains a considerable amount of potassium. If that material has not broken down sufficiently, soil test levels may come back lower than expected. Keep this in mind when collecting soil samples this fall and evaluating soil test levels.
Phosphorus can also benefit from fall application from an environmental perspective. Unlike spring, fall is typically characterized by less intense rainfall events meaning gentler rains can incorporate surface applications. Once the dissolved fertilizer interacts with soil it is much less susceptible to runoff from future rainfall and more is left in the soil system for subsequent crops.
Fall nitrogen applications are subject to dramatically different rules. Nitrogen applications should be delayed until soil temperatures reach 50 degrees F. If applying anhydrous ammonia, the subsurface temperature (or zone where the nitrogen will be applied) should be at or below 50 degrees. The reason scientists recommend delaying application until soil temperatures cool is because of the ability of ammonium to be converted to nitrate. Once nitrogen is present in the nitrate form it can be subject to leaching, and ammonium applied at cooler soil temperatures is not converted as quickly to nitrate.
Avoid fall application of nitrogen sources that contain nitrate. This is especially true in higher rainfall environments that are much more subject to leaching. If using urea as your nitrogen source, ensure that the product is incorporated to minimize volatilization losses of ammonia. Also, consider using a nitrification inhibitor with fall nitrogen applications to further slow the conversion of ammonium to nitrate.
The predominant phosphorus fertilizer materials marketed in the U.S. are ammoniated phosphates. Products like monoammonium phosphate (MAP), diammonium phosphate (DAP) and ammonium polyphosphate supply both phosphorus and nitrogen to soil solution and ultimately a crop. Since these products do have nitrogen, you should be mindful of soil temperatures when making fall applications. Fall is a great time to get fertilizer applied in preparation for next year’s crop. Pay attention to your soil test levels, be mindful of crop removal based upon this year’s yield and think about soil conditions at the time of application. Doing things right in the fall can alleviate some of the constraints faced during the busy spring season and help get your crop started on a solid footing.
Originally published in Today's Farmer Magazine by Dr. Robert Mullen
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