Comparison of Soil Buffer pH Methods to Determine Lime Requirement in Iowa.

Soil buffer pH methods are commonly used in Iowa to determine the rate of lime to be applied in acidic agricultural soils. This study compared the Shoemaker-McLean-Pratt (SMP), modified SMP (i.e, Sikora), and Mehlich buffer methods on the basis of laboratory analysis and simple strip trials. An advantage of the Sikora and Mehlich buffers over the SMP method is that no hazardous chemicals are used. Ten field trials were established in Iowa farmers’ fields managed with corn-soybean rotations and tillage during 2007-08 using a dense grid soil sampling approach and harvest with GPS and yield monitors. Replicated treatments were a control and 3 ton of effective calcium carbonate equivalent (ECCE)/acre). Soil samples were collected from a 15-cm depth before applying lime and after crop harvest. Buffer pH values for the Sikora and Mehlich methods correlated well with the currently used SMP method (r = 0.93 and 0.92, respectively). However, the Mehlich buffer pH values were significantly lower than those for SMP and Sikora. Analyses of results by field, soil series, or organic matter ranges did not show significant deviations from the general relationship. Correlations of buffer pH values with pH change due to lime application at the field confirmed that the Mehlich method requires a different calibration concerning interpretation of results for lime recommendations. Therefore, the Sikora method can be immediately implemented in the region using existing calibrations for the SMP method. The study will continue by conducting multiple-rate trials to develop new field calibrations for the three buffer pH methods.