Research Highlights
Determining How Soils Store or Supply Phosphorus Will Improve Fertilization Guidelines

Research investigated the concentration and availability of Phosphorus in Alabama soils. Photo: USB director Wendy Yeager

By Laura Temple

Soybeans need phosphorus (P) for photosynthesis throughout the season and especially during pod formation. Growers supply P through both commercial fertilizer and animal manure. However, too much P can pose an environmental risk, prompting guidelines on when it is appropriate to apply P based on soil tests.

“Many Alabama soils are high in aluminum and iron oxides that bind P under acidic conditions, making P unavailable to crops like soybeans,” says Rishi Prasad, assistant professor and Extension specialist, Crop, Soil and Environmental Sciences at Auburn University. “These soils have higher external P requirements, and poultry litter serves as a great external source of P. However, the farmers need to pay attention to the application rate and frequency of applying poultry litter to avoid environmental issues.”

Prasad notes that while soil tests measure plant-available P, they don’t reveal if those soils are capable of storing additional P. This has consequences on risk assessment for environmental purposes, especially when soil test P is used in the P-index for Alabama soils. 

To better assess how soils interact with P, he is leading a study to quantify P storage capacity of Alabama soils to then develop a decision support tool to help determine when P can be applied or stopped to those P-fixing soils. A soy checkoff investment from Alabama Soybean Producers is supporting this work. 

“This study focuses on the concept of soil phosphorus fixation,” Prasad explains. “Some soils are a P ‘source,’ capable of releasing P to make it available to plants or to the environment. Other soils are a P ‘sink,’ capable of retaining or storing additional P without allowing it to move.”

He adds that even if soil tests show that some soils test “high” or “very high” in P, they may still have the capacity to store P if they are also high in soil minerals that bind P. He explains that a field can change from being a source to a sink based on soil type, management history and other factors. Determining if soils are P sources or P sinks has been the first step in this systemic research. 

Soil tests reveal P stratification

During the first year of this research, Prasad and his team collected 850 soil samples, working with about 70 row crop farmers throughout Alabama. Many of the farmers grow soybeans, corn and cotton. Most fields had a history of poultry litter or fertilizer application, allowing P levels to build over time. They took soil core samples at several soil depths all the way to 24 inches. 

Figure 1. Alabama soil tests revealed significant P stratification, with much higher concentrations in the topsoil compared to deeper in the soil profile.

“We found that P is highly stratified,” Prasad says. “The top six inches of soils have high levels of P, whereas soils below six inches run very low in P.”

He believes the distribution of P in the soil, as well as P-fixing minerals, could have implications on yield potential. 

“In soils serving as sinks, P isn’t readily available to the crop,” he explains. “The data from the statewide soil samples showed us that the relationship between P, soils and soybean yields is complicated. It has led us to new questions.”

Implications of P stratification

One of those questions is if deeper soil can serve as P sinks even if basic soil tests rate the topsoil as high or very high in soil test P. 

However, if the topsoil is a P source, but the subsoil is a P sink, Prasad believes options may exist to apply P in a way that is available to deeper roots. 

Another question relates to the availability of P to soybeans due to stratification.

Prasad asks, “In dry years, when the crop is forced to push roots deeper into the soil profile to get water, will low P fertility in the subsoil become a limiting factor?”

To learn more and start to answer both these questions, his team is currently conducting in-field trials. Fertilizer has been injected six inches deep in the soil, and the crop is being monitored to see how it responds. However, he expects differences to be more clear during dry seasons, and much of the 2021 season has had adequate moisture, with rain every couple weeks.

“We won’t know the effects yet, but we are working to confirm how P works in different soils,” he says. “Each year of work is leading to new questions, but eventually we intend to create a decision tool to help farmers address P fertility and environmental stewardship. The available poultry litter in this region offers additional benefits, and with a deeper understanding of P, growers can take advantage of that fertilizer while still protecting their resources, especially water quality.” 

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.