Soil and Water Sciences Graduate Student Research Spotlight

“Calibrating Mehlich 3-Phosphorus Recommendations Using Iron Oxide-strip Phosphorus and Mehlich 1-Phosphorus”
Amanda Rodriguez (Soil and Water Sciences M.S. 2021)


In 2013, Florida’s soil test was changed from Mehlich 1 to Mehlich 3. From this point, Mehlich 3-phosphorus fertilizer rate recommendations were based on the linear relationship between these two soil extractants. However, recommendations based on this relationship have not consistently resulted in desirable yields for landowners. Therefore, adjustments to Mehlich 3-phosphorus fertilizer recommendations are needed. Iron oxide strips are an alternative method of measuring bioavailable phosphorus that simulates absorption mechanisms occurring at the soil and root surface interface.

Research Question

We propose that iron oxide strips will provide a better assessment of bioavailable phosphorus as its results are independent of soil type. Our objectives include testing the relationship between iron oxide extractable phosphorus and Mehlich-3 extractable phosphorus for U.S. soils with varying textures to test this calibration.

College student in a lab.
Amanda Rodriguez, SWSD master’s student, preparing the iron oxide (FeO) strips. (photo provided)

“Mehlich 3 uses the acid dissolution of phosphorus, so many people don’t view it as an accurate reading,” said Amanda Rodriguez, Soil and Water Sciences graduate student.

Mehlich 3 for phosphorus often results in lower yields based on current recommended values for “low,” “medium,” and “high” categories. However, the iron oxide method shows this inaccuracy with Mehlich 3 but it is tedious to perform.

“We’re trying to create a calibration between the two methods, so you could perform Mehlich 3 extraction, receive a value for phosphorus, and then plug it into an equation that would hopefully give you a more accurate phosphorus fertilizer recommendation,” Rodriguez explained.

An interesting point is that the Mehlich 3-phosphorus reading was created based on the Mehlich 1 method, so this iron oxide method is updating a previous calibration. The study will also evaluate the Mehlich 1-phosphorus/Mehlich 3-phosphorus relationship using data obtained across soil types under identical lab conditions; results will be related to crop yields in a field setting.

Research and Early Results

Rodriguez is using soil samples taken for an earlier study she did with her advisor, Dr. Vimala Nair, a research professor in the Soil and Water Sciences Department. While that saved some time, making the iron oxide strips was time consuming and a lengthy process. There is not a standardized method to create the strips. That is another goal of her research so that they can be prepared and implemented on a wider scale. The effort put into creating the strips was worth it.

“When we first tested it for the quality control with a known concentration of phosphorus, we found that it extracted very accurately and pretty consistently,” Rodriguez said. “It was a very exciting day in the lab, after a year and a half of researching them.”

With the accuracy of the strips (recovery of phosphorus of known concentration in solution), Rodriguez began testing the soil samples. That included not only Florida soils, but those from Arkansas, Georgia, Maryland, and Pennsylvania.

“It’s very interesting to see how the strips can read the phosphorus and how accurate it could be in these different soils and all these different properties and textures,” Rodriguez said. “So far, they follow the linear relationship that we expected. It does show that this could be an effective way of estimating or providing recommendations for phosphorus application.”


The iron oxide strips are considered by some experts as the gold standard for measuring phosphorus. Unfortunately, their laborious nature hinders widespread use. Another issue is that they only measure phosphorus and not the entire suite of elements measured in a Mehlich 3 solution. Rodriguez hopes her formula based on the readings from iron oxide strips will lead to a better assessment of phosphorus in soil.

“The strips don’t provide a fast turnaround for getting results, so that’s why it would be most effective to create this calibration,” she said. “They don’t actually have to even perform the iron oxide strip test. If our research proves this calibration of Mehlich-3 is accurate enough, hopefully, it can be implemented on a wider scale.”

There is interest in a better method for determining phosphorus in the soil. The Florida Department of Agriculture and Consumer Services (FDACS) is funding the research.

“What we hope to recommend is a better Mehlich 3-phosphorus recommendation based on iron oxide (or Mehlich 1) and Mehlich 3-phosphorus relationship,” Dr. Nair said. “The FDACS contract which supports this research expires in June 2021, so we are moving quickly.”

“If we achieve the calibration, we hope to expand it to other parts of the United States where Mehlich 3 is the test for those soils,” Rodriguez added. “To help farmers reach the desired yields for food and other products they’re trying to grow.”

You can watch Amanda Rodriguez’s presentation on her research for the 2020 ACS Joint Conference here:


Amanda Rodriguez grew up in Fort Lauderdale, Fla., and came to UF as a microbiology major. She switched to environmental science and was introduced to the Soil and Water Sciences Department when her undergraduate advisor suggested she meet with Dr. Vimala Nair, a research professor of environmental soil chemistry. That led to a position in Dr. Nair’s lab over the summer and the opportunity to conduct research on soil phosphorus sorption characteristics.

Female graduate student standing outside
Amanda Rodriguez (SWSD photo)

“I really loved what I did there and she gave me the opportunity to be a master’s student,” Rodriguez remembered. “I very quickly accepted because I really enjoyed the research she was doing.”

By earning her master’s degree, she hopes to combine science and policy with a focus on agronomic and environmental risks.

“I love the concept of science influencing policy,” Rodriguez explained. “That’s what I find very interesting and the research that I’m doing now has the ability to do that.”

“She’s such a sincere person who wants to give back,” Dr. Nair said. “She’s from Florida and she’s excited that this research can help producers in Florida improve their crop yields.”

In addition to her studies, Rodriguez is involved in the SWSD Graduate Student Association. She says fostering a supportive community among the graduate students is important both in the lab and classroom, but also on a social level.

“Being able to bounce ideas off other students, being able to go to lunch with them and discuss what you’re doing and have them share their research with you is helpful,” Rodriguez said. “It’s a very collaborative environment.”


Posted: December 23, 2020

Category: Agriculture, UF/IFAS Research
Tags: Amanda Rodriguez, Iron Oxide Strips, Phosphorus, Soil And Water Sciences, Soil And Water Sciences Department, Soil Water And Ecosystem Sciences

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