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Soil Tests, Not Yields, Tell Fertility Needs

Crop producers may have shied away from making phosphorus or potassium applications last year due to high fertilizer prices, but Robert Mullen says you shouldn't let bumper corn and soybean yields fool you into thinking an application this fall may not be needed.

The Ohio State University soil fertility specialist says that producers should look to soil tests to determine the level of potassium and phosphorus in the soil, then decide whether or not an application is warranted.

“Many growers didn’t supplement their crop with phosphorus and potassium and they had a tremendous growing season. So many farmers are asking whether they can get away with going another season without any application,” Mullen says. “You’re not always going to see a yield loss. But you run a greater risk of a nutrient deficiency and subsequent yield loss if you don’t make any applications and your soil tests are hovering at or below critical levels.

"This year just means that with the growing environment, a farmer was able to get away with no phosphorus or potassium applications.”

Mullen recommends producers conduct soil tests every 2 to 3 years to determine potassium and phosphorus levels, then make an economic decision based on where fertilizer prices are headed — if they can predict it.

“Fertilizer prices have dropped dramatically since last year, from $1,200 a ton to under $400 a ton for phosphorus and from $1,000 a ton to about half that now for potassium," Mullen says. "For those farmers who skipped applications last fall, it was a very wise decision.

"If soil tests are still high, then you probably still don’t have to make an application and it’s still a savings. For those whose soil tests are close to critical levels, don’t assume you can get away with not making applications.”

Mullen says that potassium and phosphorus are tricky to manage because they are not very mobile in the soil. Environmental conditions can lock up potassium, specifically, making it unavailable to the plant.

“Adequate phosphorus for a corn-soybean rotation is about 25 to 30 parts per million. Keep your phosphorus at that level and the risk of deficiencies is so negligible, you don’t have to make applications,” Mullen says. “Potassium is a completely different animal. You can have high potassium levels, but you can run into deficiencies because the plant can’t take it up.”

Mullen says that drought stress is the biggest factor that can lead to a potassium deficiency. If potassium deficiency is severe enough, a grower can lose 60% of the top-end yields.

Specialists recommend that growers supplement potassium or phosphorus when soil tests show low levels. When both are low simultaneously and farmers can only afford to supplement for one, research has shown that it’s better to invest in potassium than phosphorus.

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