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4 ways to optimize phosphorus applications

Crops require phosphorus (P), which is critical in building the structural components needed for plants to grow.

Here are four ways to ensure optimum P levels in fields.

1. Regularly test soils.

Soil test for P about every two years, says Lindsay Pease, University of Minnesota (U of M) Extension specialist in nutrient and water management.

“Generally, a postharvest soil test that mimics a plant’s ability to extract P from the soil lets you know how much P will be plant-available in the next season,” Pease says. “Plant tissue tests are also available.”

Knowing field P levels also helps reduce the possibility of environmental issues if excessive P is present. P applications exceeding what plants use can build up in the soil and potentially become a water-quality risk because of surface runoff carrying soil and P into waterways.

2. Match P applications with crop.

“When soil tests indicate your range for P is high, apply at the removal rate of your intended crop,” says Paulo Pagliari, U of M nutrient management specialist. “If the P range is on the lower end, apply about 20% to 30% more than the crop needs until your soil-test P level reaches a high level. Then, apply it at the plant removal rate. Many Extension offices have calculated crop removal rates and will gladly share the information.

“If your soil has a high clay content, with elevated iron or aluminum content, try banding the P as close to the seed as you can,” he adds. “This creates a higher P concentration band compared with broadcasting P over the entire field and maximizes plant fertilizer use.”

3. Know your soil type.

“At most, P moves through the soil slowly in the range of a few millimeters per year” says Pagliari.

“Soils with a lot of clay tend to bind P tightly, especially in the Midwest,” he says. “Aluminum and iron in more southern soils cause P retention. These bonds between soil particles and P are strong and make it difficult for P to become available again.”

P is even less mobile in topsoil. Pease recommends farmers place P by dropping it into a small trench or a small slice of soil. Light tillage can also be used to incorporate surface applications, she adds.

“For those with reduced and no-till operations, the subsurface placement may be time-consuming, but it’s better than leaving it on the surface where it’s at risk of being carried away by a rain- storm,” Pease says. “The goal is to prevent a high concentration of P in the top couple of inches of the soil. You can also try applying P at planting and pop-up fertilizers can be helpful.”

Avoid applying P to topsoil on frozen ground or if frost is imminent, says Pagliari. Instead, he advises incorporating broadcast P into the top 6 inches of soil before it freezes. This enables P to start its slow movement into the soil when rains come.

Spring applications that match crop use are ideal, he adds.

Because of runoff concerns, do not broadcast P applications without incorporation if heavy rains are predicted, adds Pagliari.

4. Let the 4Rs guide P application.

These include:

  • Right rate
  • Right source
  • Right time
  • Right placement

“University recommendations are always based on the probability of getting a good yield response and getting the most bang for your buck,” Pease says. “If your soil test indicates you won’t get much P response, reduce the application rate. Plus, that soil test will guide the rate, source, time, and placement of all remaining nutrients so your crop reaches maturity as healthy as possible.”

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