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Fertilizer applications might have to wait until spring

Agriculture.com Staff 12/03/2009 @ 2:15pm

In the Corn Belt, most phosphorus (P), most potassium (K) and some nitrogen (N) is applied typically in the fall. This year, the late harvest and wet conditions have worked together to make nutrient applications a challenge. Most producers prefer to apply fertilizers in the fall because they are not as busy as in the spring planting season, labor and equipment can be better distributed, and soil conditions are more favorable for avoiding compaction. Obviously, the excessively wet conditions starting in October this year have negated this last advantage.

In essence, all the N applied in the fall is anhydrous ammonia. To apply this form of N, soil conditions have to be adequate to lower the risk of ammonia losses and/or crop injury the following season. When the pressurized ammonia (NH3) comes out of the applicator knife, it quickly moves through the soil and reacts with the soil water present to produce ammonium (NH4+).

When soils are wet, two possible scenarios are likely. The first is that the soil will likely not close behind the applicator knife. If that occurs, the ammonia gas escapes to the atmosphere, resulting in N loss. The second scenario is that ammonia likely will not disperse very far in the soil volume and will remain concentrated in a band at the point of injection. Since ammonia is toxic to plants, the new roots of emerging seedlings can be severely damaged as they grow into that concentrated band in the spring. Another possibility is that as soils dry in the spring, they might crack along the path of the applicator knife; when that happens, ammonia escapes, not only resulting in N loss, but also damaging the canopy of the young corn plants.

Given current weather conditions and the factors I've described, I strongly discourage making nitrogen applications this late in the fall. While it might create some hassle to have to apply all your N in the spring, weighing the potential for N loss, crop injury, and soil compaction alone makes waiting worthwhile.

This fall a lot of farmers were planning to put P and K on their fields, because for the last two years many had reduced or eliminated those applications due to high fertilizer prices. This fall P and K prices are finally lower, but conditions are not conducive to application, mostly because of the potential for compaction or getting stuck in the field.

While it might be tempting to apply P and K on frozen ground, consider the risks. Frozen soils typically get covered with snow. Once the snow starts to melt, it dissolves the fertilizer, and if the water moves out of the field, it carries with it the applied nutrients. Runoff when water is moving on the surface of frozen or water-saturated soils can be significant even on fairly flat ground (as we have seen in many fields these last two years). This runoff can result in important economic losses to your operation as well as environmental degradation concerns.

If your soil test values indicate you are below the critical level (30, 40, and 45 lb P/acre for the high, medium, and low P-supplying regions of Illinois, respectively; and 260 and 300 lb K/acre for the low and high CEC soil regions of Illinois, respectively), it is important to apply the needed nutrient, but I recommend not applying it on frozen ground. As with N, the disadvantages under the conditions this fall can outweigh the advantages. Applying P and K is possible in the spring, and there are no agronomical drawbacks.

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