Roughly 40% of the large audience that joined a virtual webinar focused on making decisions about fertilizer said they were listening because they wanted to know how to cut fertilizer costs. Record-high fertilizer prices have many more people thinking this way.

“There are two prerequisites which you must do first if you’re serious about adjusting your fertilizer budget and perhaps reducing rates,” emphasizes Eric Miller, a Pioneer field agronomist in eastern Indiana. Miller and Matt Clover, Pioneer’s agronomy sciences manager, conducted the webinar for farmers.

“It all starts with recent, accurate soil tests,” Miller says. “Unless you have up-to-date soil test information, you are just guessing. It’s absolutely not possible to make informed decisions about whether you can lower fertilizer rates unless you have these soil test results.”

Without them, Miller says two things can happen, and both are negative. First, you may not apply enough nutrients in areas of a field where phosphorus or potassium are in the low range. Test levels in the low range typically respond to fertilizer and produce more yield in the very first year after fertilization.

“You don’t want to risk hurting yields when both corn and soybean prices are high,” he says.

The other possibility is applying more nutrients than are needed. Applying high-priced nutrients on soils that already have enough nutrients doesn’t make sense.

Get pH right

Even once you have soil test results, you’re not ready to begin making decisions that might result in applying less fertilizer.

“The most important thing on the soil report is soil pH,” Miller says. “If soil pH isn’t in the right range in all or part of a field, you need to correct it. It’s likely too low, meaning the soil is acidic. Fertilizer isn’t used properly and fully available to plants if the soil pH is too low.”

Information compiled by the Indiana Aglime Council indicates that in soils with a pH of 5.5, only 48% of phosphorus applied as fertilizer is used. Even at a pH of 6.0, it’s only 52%.

“We can quibble about whether those numbers are spot on or not, but the concept is solid,” Miller says. “If you have low pH soils, you’re wasting fertilizer until you correct the pH problem.”

Efficiency suffers in low pH soils because nutrients are less available to plant roots, he adds.

“Phosphorus is most affected by low pH and acidic soils,” Miller continues. It’s used most efficiently when soils are in the 6.5 to 7.0 range.

Potassium can be affected, although not as much. Nitrogen is likely affected least, but there can be an effect. Miller notes that nitrification is inhibited at pH levels below 6.0. Those bacteria prefer pH levels around 7.0.

“We’re not saying that you won’t get use of nitrogen unless you keep your pH around 7.0,” he says. “However, the bottom line for all these macronutrients is that we need to maintain soils at pH levels of 6.0 to 7.0 if we want fertilizer nutrients to be available and used by plants.”