Editors’ Forward: On August 30, ILSoyAdvisor hosted the webinar “Soil sampling is the first step to planning high yield soybeans.” The presenter, an agronomist with a commercial laboratory in Illinois, talked about measuring water and buffer pH to calculate lime requirements. The Illinois Agronomy Handbook references how lime recommendations should be made in the state, including both buffer pH and CEC/organic matter. We asked Dr. Robert Hoeft to explain the difference and why buffer pH isn’t necessarily recommended.

Soil pH values given on a soil test report indicate whether the soil is acid, less than 7.0, or alkaline, greater than 7.0, and whether it needs additional lime—if less than 6.2 for grain crops or 6.5 for legumes. By itself, it does not indicate how much lime is needed.

There are two supplies of acidity in soils, namely active and reserve acidity. The pH value, sometimes referred to as water pH, is a measure of active acidity—the hydrogen in a one-to-one soil-to-water mix. The reserve acidity is the hydrogen held by exchange sites on clay and organic matter that is released into solution when active acidity is neutralized.

Reliable lime recommendations require an estimate of both active and reserve acidity. Active acidity is determined by soil pH; the lower the pH value, the more lime needed. Reserve acidity can be determined by estimating exchange capacity based on organic matter and clay type and content. The higher the exchange capacity, the more lime is needed. Reserve acidity can also be determined by addition of a buffer solution (a solution that resists change in pH) to the water pH sample and measuring change in pH. The higher the buffer pH, the lower the lime needed.

As with any soil test, the testing system must be correlated and calibrated to the soils being tested. To make a lime recommendation using either of the two systems, one must have the calibration chart or table associated with the soil type on your farm. In Illinois, the system that uses active acidity (soil pH) along with an estimate of cation exchange capacity works well on all our soil types. The SMP buffer system does a good job on the medium-to-high cation exchange capacity soils, but overestimates lime needed on the low cation exchange capacity soils. The fact that it does not work well on some soils, and that the initial buffer solution contained a toxic chemical that was difficult to dispose of, led Illinois to recommend the system of active acidity along with cation exchange capacity instead of using the buffer pH.

Whatever system is used, remember that the recommendation is based on a given quality of limestone. If the lime material you purchase is not standard quality, adjust the rate accordingly.

Bob Hoeft, Ph.D., is a soil scientist and professor emeritus at the University of Illinois.