Managing Winter Applications of P and K Fertilizers

Charles White, Penn State Extension

Understanding the behavior of P and K in the soil and crop will help you better manage winter fertilizer applications.

Wintertime is often seen as a good time to get ahead with applications of P and K fertilizers. After all, most Pennsylvania soils have plenty of capacity to store these nutrients until spring for the next crop. While wintertime application of blended P and K fertilizer may have its conveniences, there are some considerations that you should be aware of and plan to manage for.

First, P and K fertilizer applications should always be made based on results and recommendations from a soil test. Without a soil test to gauge the existing availability of P and K in the soil and determine how much fertilizer is needed to raise or maintain these levels based on expected crop yields, you could easily apply too much or not enough of the nutrient. If you haven’t gotten a soil test for your fields in the last 3 years, before committing to a fertilizer purchase, stop and get a soil test completed.

Once you have a soil test completed, you need to decide on the rate, timing, and method of P and K applications. The rate of P and K is determined by the existing soil test level and the expected level of crop removal based on the yield of the crops in the rotation. For K, crop removal is heavily influenced by whether the crop is harvested as a grain or as a forage, with forages removing 3 to 5 times as much K as grain crops. Removing residues, such as small grain straw or corn fodder, also dramatically increases K removal. So make sure your K fertilizer recommendations are based on an accurate portrayal of how you plan to manage the following crop (and how previous crops were actually managed). For instance, is the “corn” crop that is listed on your soil test report a corn silage crop or a corn grain crop, and is that actually how it was or will be managed? If last year’s K application was based on a corn grain crop and you actually harvested it for silage or baled up the fodder, you may need to make a higher K application this year to maintain soil levels. The other thing to keep in mind is whether you have added crop harvests to the rotation that weren’t reflected in the soil test recommendations. This most often occurs with cover crops that are harvested for a forage in the spring. Each ton (65% moisture) of silage harvested from small grain forages removes 26 lbs/ac of K2O, or about 160 lbs/ac of K2O from a typical 6T/ac yield. For reference, this is about three-and-a-half times more K2O removal than with a single corn grain crop.

The next thing to consider is timing of fertilizer application. Soils in Pennsylvania have enough cation exchange capacity to store K through the winter, so wintertime applications can be used for convenience. However, K is a nutrient that is susceptible to luxury consumption, or over-uptake by the crop. So think carefully if the rate you are applying is intended to feed just one crop harvest, or if it is intended for multiple crops or multiple cuttings of hay. If you apply the K needed for multiple crops or multiple cuttings all at once in the winter, it’s quite possible that the first crop of the season will take off more than its fair share of the nutrient, leaving the remaining crops short. This is particularly problematic with crops harvested for forage or where crop residues are baled and removed. For crops that are harvested as grain and the residues are returned to the field, any over-uptake of K by the first crop will be quickly recycled back to the soil in the residues.

Timing of P fertilizers is also important to consider, both for crop availability and environmental pollution. When P fertilizer is applied to soil, it starts to undergo reactions with the soil minerals that render it less available for crop uptake. So in the time that elapses between P applied in the winter and when the crop uses the next summer, the fertilizer P is becoming less and less available. This reduction in availability is particularly consequential if soil test P levels are below optimal to begin with, and less consequential if the fertilizer P is being added to maintain existing soil test levels. Another concern with P applied in the winter is that unless it is placed sub-surface, the fertilizer P granules can be solubilized by runoff water and carried off the field and possibly pollute nearby water sources. Winter is a time when soils are saturated or frozen, resulting in higher volumes of runoff water, exacerbating the risk of P losses.

Because of these issues with P fertilizer behavior, I generally discourage applying P fertilizers in the winter, especially on the soil surface. While applying K on the soil surface in the winter is okay, it would be better to wait to apply P until the spring and to find an application method that gets the P positioned into the soil profile where it can be effectively utilized by the crop roots, such as a 2×2 starter or in-furrow popup. This would mean forgoing the convenience of a blended fertilizer and managing the P and K fertilizers separately. When managing the P in starter fertilizer, be careful about salt injury that can occur from excessive fertilizer placed near the seeds. For 2×2 applications, no more than 70 lbs/ac of N and K2O combined should be applied and for in-furrow applications, no more than 10 lbs/ac of N and K2O combined should be applied. Also, be careful about ammonia injury that can occur from diammonium phosphate fertilizer that is used at too high a rate or too close to the seed.

Knowing the different behaviors of P and K in the soil, uptake by crops, and potential risks to the environment will help you make the best management decisions that balance, convenience, economics, yields, and environmental protection.

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