With low commodity prices showing no signs of stopping anytime soon, economics weigh heavy in almost every farm management decision these days.

Each input cost takes careful agronomic, environmental and economic consideration. Growers across the Midwest now facing the decision of whether to apply fall fertilizer must think about money on top of weather, timing and what next year’s crop will be.

The advantages of fall fertilizer applications are clear. Soil moisture is generally lower in the fall, so the risk of soil compaction is lower than with wet spring soils. If fall tillage is planned, phosphorus and potassium can be incorporated into the rooting zones of next year’s crops. It also ensures that nutrients get applied, something that could slow spring planting depending on whether conditions. Rick Gilbertson, crop consultant with Pro Ag Crop Consultants Inc., says that farm operations spend more time analyzing economics of input decisions than in years past, and that fall fertilizer can be an attractive option as fall prices are generally lower than spring prices. Also, dealer equipment and applicators are more available.

Just the facts
University of Minnesota Extension soil fertility specialist Dan Kaiser encourages growers to use soil tests as the basis for their fertilizer decisions.

“My advice is to target your fertilizer to the areas of greatest economic benefit, and reduce in areas where there is little economic benefit,” says Kaiser. “Fertilizer is one of many input costs in crop production, and knowing which will make most economic sense increases the chance for a profitable cropping season.”

Within rotations, applying the right nutrient at the correct rate is more crucial than the timing of the application. Kaiser says there are four nutrients that provide the greatest return on investment in Minnesota. Nitrogen is the most consistent need for non-legume crops, while the need for phosphorus, potassium and sulfur varies.

In a recent study funded by the Agricultural Fertilizer Research and Education Council, U-M researchers calculated return on investment for P, K and sulfur applications in a two-year corn-soybean rotation (see table). Phosphorus and sulfur were the clear winners when it came to increasing profit at sites where soils had a low probability of supplying either nutrient. Sulfur has typically worked more effectively on soils in south-central and southeastern Minnesota with organic matter contents of around 3.0% or less.

Kaiser notes that potassium responses have been more variable with some low-testing soils, producing large and profitable yield responses, and in other situations where yield increases are modest, barely covering the cost of the fertilizer.

Jeff Vetsch, researcher and soil scientist at U-M’s Southern Research and Outreach Center, suggests growers apply fertilizer potassium annually on coarse textured soils, and strongly consider it on medium-and fine-textured soils if soil test K is less than 100 ppm.

“Our research has shown that an annual application of 60 to 90 pounds K₂O on low-testing sandy soils has consistently provided maximum yield for corn and soybeans, which is less than what we would currently suggest for corn or soybean production,” he says. “On medium- to fine-textured soils, applying fertilizer K to medium-testing soils — 81 to 120 ppm ammonium acetate test — can give a sizable ROI.”

Vetsch encourages growers to discuss their soil test results with an independent crop consultant or another trusted ag adviser before applying potash fertilizer this fall.

Trends in fall fertilizer
“The top concern I’ve seen farmers have when making fall fertilizer decisions is uncertainty about the commodity mix for each field in the coming year,” Gilbertson says. “Without a clear plan of desired crop in the field, it's difficult for them to apply appropriate amounts of fertilizer ahead of time.”

Gilbertson says that generally, those farmers who have historically gotten into the practice of fall-applying fertilizer continue to do so, expensing that item in the previous year relative to when the crop is grown. He says most of his clients are becoming conservative in their fall fertilizer applications, content to keep soil test fertility levels in a "medium" category rather than pushing to climb into a "high-testing" category.

Research included in this article was funded by the Minnesota Agricultural Fertilizer Research and Education Council.

Source: University of Minnesota