Lime falls out of soil scientist’s limelight

Continuing long-term research plots dating back to 2003 question the value of liming to enhance crop production in poor soils

Lime, often thought of as a cure for phosphorus deficient soil, may not be as effective as previously assumed, according to researchers.

However, without phosphorus, plants wither and die.

Although plants require phosphorus, there is often a “withdrawal limit” on how much their roots can extract from the soil. Soil-bound phosphorus is often in a form plants can’t absorb. This obviously has a negative impact on the plant.

Soil scientists have long known that a soil’s pH level affects how well the plant can access phosphorus. If soil is too acidic, phosphorus reacts with iron and aluminum and becomes unavailable. On the other hand, if soil is too alkaline, phosphorus reacts with calcium and also becomes inaccessible.

“Phosphorus is most available to plants when soil is at a Goldilocks zone of acidity,” says University of Illinois researcher Andrew Margenot.

There are ways to make soil-bound phosphorus available to plants. For example, American farmers often add calcium hydroxide in the form of lime. This reduces soil acidity and unlocks phosphorus that was previously unavailable.

“Liming is a bread-and-butter tool for agriculture,” Margenot said.

However, liming influences other paths by which phosphorus becomes available. Enzymes called phosphatases also influence the amount of phosphorus that is available to plants.

Margenot, along with his colleagues and predecessors, have been conducting experiments in western Kenya, a region with acidic, worn-out, weathered soil.

The project, which started in 2003, has added varying amounts of lime to the long-term experimental plots. As a check, one set of plots has been unfertilized. Another set received cow manure and a third one received mineral nitrogen and phosphorus.

Twenty-seven days after liming, the researchers measured phosphatase activity and how much phosphorus was available to plants. They were surprised to find no clear relationship between soil acidity levels changed by liming and phosphatase activity.

“This was unexpected,” Margenot said.

“We know that phosphatases are sensitive to soil acidity levels. These findings show that it’s more complicated than just soil acidity when it comes to these enzymes.”

More surprisingly, the researchers saw that changes in phosphatase activities after liming depended on the soil’s history. This suggests that the sources of these enzymes, which are microbes and plant roots, could have responded to different fertilization histories by changing the amount or type of phosphatases that were secreted.

“In all cases, the increases in phosphorus availability were relatively small. Lime alone was not enough to be meaningful to crops and thus to farmers. Lime needs to be combined with added phosphorus to meet crop needs in these soils.”

Margenot is now working to extend this study, according to a news release from the Soil Science Societies of America. With colleagues from the International Center for Tropical Agriculture and the German Society for International Cooperation, he will study western Kenyan farms to see if using lime at rates realistic for growers will have soil health trade-offs in these weathered soils.

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