Quantifying the Effects of Modifications to the Sikora Buffer pH Method for Oregon Soils

The Sikora buffer pH method is a non-hazardous alternative to the Shoemaker, McLean, and Pratt (SMP) method for lime requirement estimation (LRE) for Oregon soils. Commercial soil testing laboratories may implement method modifications in order to streamline the method for large-scale testing, however the potential effects of these modifications on buffer pH results are currently unknown. Modifications that artificially increase buffer pH are of particular concern, as they could potentially result in under-application of lime and crop damage. The goal of this project was to evaluate and quantify five Sikora buffer pH method modifications for five Oregon agricultural soils. Soil type did not show a significant interaction effect with method modifications. Modifications related to mixing method, measurement in supernatant instead of slurry, and soil:water:buffer ratio were found to significantly increase Sikora buffer pH results by 0.063, 0.065, and 0.058 units, respectively, in comparison to the control. Mean Sikora buffer pH decreased by 0.02-0.05 units when equilibration time was increased from 0 to 180 min. Our findings show how even minor modifications can alter Sikora buffer pH results, illustrating the importance of avoiding deviations from the original method. Additionally, comparing results among labs may help minimize over- or under-recommending of lime to agricultural fields.