Fertilizer Timing

Farmers face tough nutrient-management decisions all the time, but deciding on a fall fertilizer application can be especially challenging.

Tight crop margins and potential for uncooperative weather that can cause nutrient loss have to factor into whether a fall fertilizer application is the best option this year.

Nutrient specialist Dan Kaiser, with University of Minnesota (UMN) Extension, notes while some crops in the state looked good as harvest neared, large amounts of rain earlier in the growing season created issues with nutrient availability.

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

FALL NITROGEN. A fall application of nitrogen must be done carefully, Kaiser explains, to assure the nutrient is there in the spring when plants need it. Nitrogen source and application timing will determine how much of the nutrient is available.

Fabián Fernández, also a UMN Extension nutrient-management specialist, says 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 push some producers to apply nitrogen in the fall, but shifting more application to the fall increases the risk of losing the nutrient, he notes.

“I would really encourage people … applying a lot of nitrogen in the fall to potentially change … even if this an acceptable practice in their area,” Fernández says.

He adds a spring preplanting application of nitrogen is a better choice in terms of nutrient availability. Typically, fall-only applications of nitrogen result in the lowest yields.

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

Overapplying nitrogen just to chase the last few bushels is not a good practice for the bottom line if the economics doesn’t justify it. In addition, he says, when more nitrogen is applied, a higher percentage of the nutrient is likely lost because of uncooperative weather (too much moisture).

“If nitrogen has to be applied in the fall, the best form would be anhydrous ammonia,” he explains. “And, delaying this application as late as possible would be good, as well.”

FALL P AND K. Phosphorus (P) and potash (K) applications in the fall are common, but use soil tests to determine if they’re really necessary, Kaiser says. Eliminating applications on fields where soil tests show high levels of P and K can be an effective way to cut costs.

Kaiser says for P, 20 to 25 parts per million (ppm) is an adequate level. Applying more P would not produce a yield increase and would not be economically feasible-especially during this time of tight margins.

Vetsch explains that on rental ground, many farmers are now choosing to keep P at what would be considered a maintenance level-a 14-- to 19-ppm range. That’s a good plan economically, and it works agronomically, as well, he notes.

Fernández says research shows, in many cases, farmers overapply P while underapplying K. He encourages careful examination of soil test results and consideration as to where those nutrient dollars will be best spent.

“In some cases, it might be better to apply potash than more phosphorous,” Fernández says.

Kaiser and Vetsch agree K is frequently an overlooked nutrient, as many soils in the region tend to be high in K levels. That shouldn’t lull producers into thinking there’s no reason to apply the nutrient.

“If soils are lacking in K, it would be a smart investment to apply more K rather than spend more money applying P, especially if the P levels are already high,” Vetsch says. Good K levels will range between 120 to 140 ppm.

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