Will fall dry fertilizer provide a return?

October 9, 2025 | Kriss Nelson

Faced with rising fertilizer costs and falling commodity prices, many farmers are questioning whether fall dry fertilizer applications of phosphorus and potassium will provide a return.

“It opens the door for a broader conversation. I tend to take a 30,000-foot view. I like to look at the big picture and fertilizer is part of that discussion,” says Mark Jackson, a farmer from Rose Hill and past president of the Iowa Soybean Association (ISA).

Jackson explains that some of his farmland’s nutrients come from livestock. But with his other fields located away from livestock, he has been forced to consider a tough question: Should he apply fertilizer when prices are higher than last year, especially as the outlook for corn and soybean prices declines?
“I think you go back to the basics,” he says. “Look at your soil test and the type of soils you have. If you’ve got strong CEC (cation exchange capacity) and medium or higher test scores, you can sometimes skip a year during high-cost periods.”

Soil testing makes the difference

Antonio Mallarino, professor emeritus of nutrient management research and extension at Iowa State University, agrees that soil testing is key.

“Most Iowa farmers are maintaining soil phosphorus and potassium levels higher than needed to optimize yield,” he says. “It’s been a decades-long fight to convince them, and dealers, that they could save a lot of money by letting levels drop to what should be maintained, using reliable soil sampling and variable-rate technology to account for wide variation in soil tests and nutrient removal.”

Mallarino recommends testing soils every two years. For corn–soybean rotations, if phosphorus levels test above about 35 parts per million (ppm) using common soil test methods, no additional fertilizer would likely be needed. The same is true if potassium levels are above 200 ppm on moist tests or 250 ppm on dry tests, for at least five to six years.

“With reliable and frequent soil testing, there is essentially no risk,” he says. “Skipping applications in high-testing soils can increase profits while sustaining productivity.”

Mallarino suggests farmers and agronomy professionals refer to ISU Extension publication PM 1688, which was updated in 2023.

“The publication provides optimum soil-test levels to be maintained and how to estimate removal based on yield of harvested plant parts for several crops,” he says.

Balancing priorities

Jackson points out that phosphorus is now more expensive than potash. The most important thing is to find out where fertilizer works best.

“Phosphorus is more important to corn than soybeans,” he says. “A lot of farmers used to fertilize for two years by applying before corn and letting soybeans use what’s left. But that mentality has been shifting. More farmers are feeding each crop every year. You see old-school practices blending with modern thinking.”

Jackson foresees no changes to his regular fertilizer program this year, noting the strong yields his farm has seen over the past three seasons. Soybeans have averaged 70 bushels an acre or higher, with some fields reaching 80 to 85 this fall. Corn yields have also climbed, averaging 250 to 275 bushels an acre.

“That definitely mines the soil,” he says. “Those yields take nutrients out. You have to maintain your soil. It’s like the old adage: the banker needs money, so you sell the sows, but then you have to think, what about pigs next year? That’s the mentality.”

A no-till farmer, Jackson uses cover crops to capture nutrients following soybeans and corn. “They convert tied-up phosphorus into organic forms available to plants,” he says. “Instead of running into the water, phosphorus gets recycled in the soil. Cover crops can reduce phosphorus loads in water by nearly 47%. That’s a big win for clean water and could also support future yields.”

Weathering change

Farming since 1974, Jackson has seen markets rise and fall. His focus remains steady.

“You see them come and go. The key is maintaining the source of your product—the soil. That’s always my priority,” he says.

Jackson acknowledges current uncertainty around tariffs but believes disruption should be viewed in the context of long-term reset.

“I look at tariffs like the interstate system. When they built it through Iowa, it caused an uproar and hurt some people, but long term it’s been a tremendous benefit. Same with building a house—you have to dig and disrupt before you can enjoy the result.”

He’s frustrated by those who blame tariffs alone for low prices.

“Prices were already sliding,” he says. “Fertilizer costs peaked in 2022, but nobody complained because we were selling $5 to $7 corn and strong soybeans. Now we’re still spending about $900 an acre on corn and $600-plus on soybeans, while prices have dropped by 50%. That’s the real pressure. And yes, tariffs don’t help, but they aren’t the sole cause.”

Phosphorus efficiency trial

ISA’s Research Center for Farming Innovation (RCFI) continues to help farmers identify areas  where they can boost the productivity of their fields and increase the efficiencies of their farms. 

With the recent release of enhanced efficiency fertilizers, RCFI  is exploring the potential benefits of these products.

“This fall, ISA’s RCFI is running a trial with Phospholutions’s RhizoSorb product,” says Alex Schaffer, ISA’s research agronomy lead. “The company claims that with the product’s enhanced plant efficiency, phosphorus fertilizer rates can be reduced by 50 percent without a yield loss.”

The trial will compare the application of Monoammonium Phosphate (MAP) or Diammonium Phosphate (DAP), whichever the farmer prefers, comparing the yield with full and half rates of RhizoSorb. These products will be applied in strip till with a third optional treatment of MAP or DAP, applied in either conventional or no-till systems.

“This will give us an idea if this product has a greater return on investment than traditional phosphorus sources,” says Schaffer.

Farmers interested in participating in RCFI trials can learn more at iasoybeans.com/what-we-do/productivity/work-with-us or by contacting Schaffer at aschaffer@iasoybeans.com or 815-274-2998.

Written by Kriss Nelson.

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