Farm Progress

The first step in increasing your soil phosphorus levels should be soil tests.

Tyler Harris, Editor

January 17, 2017

3 Min Read
REMOVAL HAS CHANGED: Charles Shapiro discusses phosphorus management at the recent Fremont Corn Expo. While the critical level — the test level where phosphorus is needed — hasn't changed much in the last 30 years, the amount of phosphorus removed by the crop has changed.

With tight margins and another year of down farm incomes, you may be asking yourself: How can I cut fertilizer rates? But Charles Shapiro, University of Nebraska soil scientist at the Northeast Research and Extension Center, notes that's not the right question to ask. "The question really isn't just cutting — it's making better use of your investment of your fertilizer dollar," Shapiro says.

Farm incomes weren't the only thing to decline in 2016. Fertilizer prices also saw a decline. With that in mind, now may be a good time for some producers to build soil phosphorus levels.

When it comes to phosphorus applications, Shapiro notes two questions to keep in mind: At what soil test level is phosphorus needed (a critical level, or "stop/go sign" for phosphorus applications)? And how much do you need to apply?

While UNL's recommended critical level of phosphorus has remained fairly steady, the amount of phosphorus removed has increased over time with yields — meaning there may be a need for higher phosphorus rates. However, the recommended rates haven't increased over time relative to removal.

"Back in the 1980s, the recommended rate was about the same as crop removal. That's one of the challenges we have is to update those rate recommendations so they more closely reflect what's now being removed," Shapiro says. "I feel pretty comfortable about the stop/go piece. The question of how much we put on is a little bit harder to come up with a correct answer."

So, what level should you build your soil to?

Of course, the best way to ensure a return on investment is soil tests — and determining the yield response based on applied phosphorus at that soil test level.

Yield response curves generally indicate that the higher your soil test phosphorus is, the less likely you'll see a significant yield response to a phosphorus application.

If soil tests are low — UNL's recommended critical level based on Bray-1 soil tests is 15 parts per million — then it makes sense to apply more. This method is known as deficiency correction.

At a higher level — over 25 ppm — you may want to cut back on your rates. However, around 15 to 25 ppm is where the maintenance method should be considered to stabilize phosphorus levels. That's why it's also important to understand crop removal based on yield.

To maintain specific P levels, if Bray tests are 5 to 10 ppm higher than your goal, it may be best to apply half of the rate removed. Within 5 ppm of your goal, it's probably fine to put on the full removal rate. If tests are 10 ppm over your goal, you're probably fine not putting any on this year. While there likely isn't much of a yield increase by raising soil P tests over 20 ppm, it's important to avoid dropping down below 15. So, it's probably best to keep soil tests between 15 and 25 ppm.

"The question is: Can you get by with less or not?" Shapiro says. "That's why it's important to do a good job of sampling, so growers can make sure what they're applying closely takes into consideration what they've got in the soil."

"If you've got money available to purchase fertilizer, and you're not on highly erodible land, phosphorus may be a good investment. You can improve your phosphorus levels on your low-level ground very cheaply right now," Shapiro adds. "Where you have those very low levels, you're going to get by with P that costs less than it did a few years ago — and my guess is the cost is going to increase."



About the Author(s)

Tyler Harris

Editor, Wallaces Farmer

Tyler Harris is the editor for Wallaces Farmer. He started at Farm Progress as a field editor, covering Missouri, Kansas and Iowa. Before joining Farm Progress, Tyler got his feet wet covering agriculture and rural issues while attending the University of Iowa, taking any chance he could to get outside the city limits and get on to the farm. This included working for Kalona News, south of Iowa City in the town of Kalona, followed by an internship at Wallaces Farmer in Des Moines after graduation.

Coming from a farm family in southwest Iowa, Tyler is largely interested in how issues impact people at the producer level. True to the reason he started reporting, he loves getting out of town and meeting with producers on the farm, which also gives him a firsthand look at how agriculture and urban interact.

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