OMAHA (DTN) -- Fertilizer prices are on the rise. They were already fairly high to begin with, and the military conflict between the United States and Iran has made these nutrients even more expensive.

Farmers often ask if they can delay their phosphorus (P) and potash (K) applications until the next year, when prices might be lower. The quick answer is yes -- if their soil test levels are near recommended levels, according to University of Missouri Extension state nutrient management specialist John Lory in a news release.

Lory likens a soil test to the gas gauge of a car. A soil test at or above recommended levels is much like a gas gauge reading full.

"If your gas tank is full, you know you can travel 500 miles before the tank runs dry," Lory said. "If you are making a trip that is 200 miles, you can, with confidence, make that trip once without refilling your tank."

The second trip is likely OK as well. But you know you need to refuel before completing the third trip, he said.

On most Missouri fields, optimum soil test levels mean you should be fine skipping one year without refilling the tank, he explained.

Many farmers already apply P and K on alternate years, Lory said.

But what if you did not apply last year? Can you take another year off? According to the University of Missouri Extension's recommendations, one more year should not be a problem, he said.

Lory suggested several options to reduce fertilizer costs this year. They include:

-- Do not apply fertilizer to fields that are at or above optimum soil test levels. Extensive research shows that soils at optimum soil test levels do not see increases in yield from the fertilizer applied that year. A benefit of following soil tests is you have the flexibility to skip a year of application if needed. Why not take advantage of this benefit in a year when P and K prices are high?

-- Not comfortable with going to zero? A second option is to cut the fertilizer rate by 50% or less of removal rate. Most yield response to fertilizer is driven by the first 30% to 50% of the fertilizer applied. If you are nervous about going to zero, going halfway will likely provide 100% of the yield benefit this year, even on low-testing soils.

-- The most conservative option is to limit the fertilizer rate to removal rate. In a year of high fertilizer prices, there is no reason to apply more than the crop removal rate.

"If your field happens to be responsive to added P and K this year -- a big if! -- crop removal rate is guaranteed to maximize yield," he said.

In some cases, however, soil test recommendations don't fully identify fertilizer needs.

Lory said he has seen potash deficiency symptoms associated with highly compacted areas of the field, because restricted root growth prevents the plant from getting to the potash in the soil. Cool soils also can temporarily limit the availability of phosphorus.

"Often these conditions lead to transient deficiency symptoms that the plant grows out of as soils warm and become less saturated," he said.

In addition, sandy soils and soils with low organic matter may not have the same capacity to withstand multiple years with no fertilizer application. Using the gas tank analogy, these soils have a smaller gas tank.

Lory said fertilizers help sustain crop production on agricultural soils. Soil testing helps farmers understand the nutrient status of soils.

"When fertilizer prices are high, we can also use soil testing to understand the risks of not applying fertilizer," he said.

To read the entire release, visit https://extension.missouri.edu/….

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

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