For decades, applying lime to adjust pH has been accepted practice. While a low pH can reduce yields by 40% or more, there’s a surprising shortage of recent research about how lime is tested and graded and what impact it actually has on soil pH and crop yield. These were among the issues raised at a recent conference of lime-industry representatives and USDA-ARS and Iowa State University (ISU) soil researchers. The answers could impact input costs and yields as well as nutrient-management effectiveness.

Particle size impact on reactivity in the soil is one example of current inconsistencies in liming practice. "We have at least 50 particle-size efficiency factors built into the lab process to evaluate limestone in Iowa, but we don't have the research data that were used to determine those factors," says Antonio Mallarino, professor, soils, ISU. "We assume the early Iowa researchers had a basis for them, but we don't have the data to look into it."

Different grading systems

Iowa is not alone. Without data, it is hard to explain why 39 states have different grading systems and 19 offer different definitions of agricultural lime. Even if they were accurate when written, much has changed in cropping systems and inputs, such as anhydrous ammonia and tillage, since most lime-related research was conducted 50 to 60 years ago.

One issue may be the basic test used to evaluate lime sources for their ability to affect pH change. It uses hydrochloric acid to identify the percentage of lime’s calcium carbonate equivalent (CCE). The CCE is used to calculate the ECCE (effective calcium carbonate equivalent), which determines how much of a particular lime product is needed to raise pH to the recommended level.

"Hydrochloric acid may digest large chunks of limestone in the lab, but that doesn't tell us what happens when the organic acids and root exudates hit it," says Andrew Hoiberg, director of research and development, Calcium Products, Gilmore City, Ia. "What impact do particle size and purity of the limestone have on what happens in the soil versus the lab?"