Mulling Fall Fertilizer Decisions

Consider Agronomics, Economics When Deciding Whether to Apply Fertilizer in Fall or Spring

Russ Quinn
By  Russ Quinn , DTN Staff Reporter
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The best time of year to apply fertilizer is something that farmers need to keep in mind. (DTN file photo by Pamela Smith)

OMAHA (DTN) -- Farmers face tough nutrient management decisions as the fall fertilizer application season rapidly approaches. Issues with tight crop margins and possible uncooperative weather causing nutrient losses will factor into whether fall application is a logical option or not, according to nutrient specialists.

Dan Kaiser, University of Minnesota (UMN) Extension nutrient specialist, said during a monthly UMN Nutrient Management podcast last week that while there are some good crops in Minnesota, large amounts of rain earlier in the growing season have led to some issues with nutrient availability.

"There is a lot of thinking with nutrient management," Kaiser said. "You want to get away from the guessing game from one year to the next and really see what is your best option."

FALL N CONSIDERATIONS

Kaiser said one area that farmers should think about is nitrogen sources, rates and when the fertilizer is applied. The fall application of nitrogen must be done carefully to assure the nutrient is there in the spring for the plants and not lost, he said.

Nitrogen source and the timing of the application will determine how much nitrogen will be available to plants come spring, he said.

Fabian Fernandez, a UMN Extension nutrient management specialist, said applying too much nitrogen in the fall can lead to nutrient leaching in the soil as well as lower profits. Heavy rainfall in the spring may be pushing some to apply nitrogen in the fall, but shifting more application to the fall increases the risk of losing the nutrient, he said.

"I would really encourage people to look at if they are applying a lot of nitrogen in the fall to potentially changing this practice, even if this an acceptable practice in their area," Fernandez said.

Fernandez said a spring pre-plant application of nitrogen would be a better choice than a fall application in terms of nutrient availability. Typically, fall-only applications of nitrogen usually have the lowest yields in research, he said.

Jeff Vetsch, a UMN soil scientist, said the economics of applying fertilizer needs to be considered.

Over-applying nitrogen just to chase the last few bushels is not a good practice for the bottom line if the economics doesn't justify it, he said. In addition, when more nitrogen is applied, then a higher percentage of the nutrient will be lost if uncooperative weather (too much moisture) is present.

Vetsch said some western Minnesota farmers apply urea in the fall for a source of nitrogen, as this part of the state tends to be drier. This practice is happening less often as the region sees more heavy rains, he said.

"If nitrogen has to be applied in the fall, the best form would be anhydrous ammonia," Vetsch said. "And delaying this application as late as possible would be good as well."

FALL P and K

Phosphorus (P) and potash (K) application in the fall is also a common practice, and soil tests should be used to determine if these applications are necessary, Kaiser said. Eliminating applications on fields with a soil test showing high levels of P and K could be an effective way to cut costs.

Kaiser said that, for P, a soil test with 20 to 25 parts per million (ppm) is an adequate level, and in most situations applying more P at this point would not produce a yield increase. An application at this point would not be very economically feasible during this time of tight crop margins, he said.

Vetsch said that, with much of the farm land rented in the state of Minnesota, many farmers choose to apply a more maintenance level of P in the soil. Keeping the P in the soil in the 14 to 19 ppm range is a good plan both agronomically and economically, he said.

Fernandez said research has shown that, in many cases, farmers will over-apply P while at the same time under-applying K. Farmers should examine the results of their soil tests closely for both P and K and consider what to spend their money on, he said.

"In some cases, it might be better to apply potash than more phosphorous," Fernandez said.

Kaiser and Vetsch agreed that K is frequently an overlooked nutrient, as many soils in the region are often high in K levels. Many soils in the Western Corn Belt are naturally high in K but some soils would still need K applications.

If soils are lacking in K, it would be a smart investment to apply more K than spend more money applying P if the P levels are already high, Vetsch said. Potash levels from 120 ppm to 140 ppm are generally high enough not to need an application, he said.

Russ Quinn can be reached at russ.quinn@dtn.com

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Russ Quinn