Various soil tests are routinely used in the United States to predict soil lime requirement. New non-hazardous methods have recently been proposed for adoption in Oregon, however the accuracy of these methods for Oregon soils has not been established. The objectives of this investigation were: (a) to document and describe the development of lime requirement estimate (LRE) soil tests from 1900 to 2020, (b) to evaluate the accuracy of five non-hazardous LRE methods for Oregon soils, and (c) to evaluate how laboratory modifications to the Sikora LRE method affect the accuracy of lime rate recommendations.
Four historical periods pertaining to the development of LRE methods were identified. From the 1900s to the 1930s, many wet-chemistry methods were proposed but were not adopted by commercial labs due to lack of method accuracy and to the amount of labor required. From the 1930s to the 1970s, the availability of the pH-sensitive glass electrode led to the development of fast and accurate buffer pH methods that are the foundation for methods that are still used today. From the 2000s to 2020, researchers proposed non-hazardous adaptations of buffer methods as well as the use of other soil test properties. Current research is focused on developing regional calibrations of non-hazardous methods.
Twenty-four Oregon soils were incubated with lime (0 – 22.4 Mg ha-1) to determine their incubation lime requirement. Candidate LRE methods were correlated to incubation lime requirement to evaluate their accuracy. The Sikora (r2 = .90 – .92), Modified Mehlich (r2 = .87 – .89), and Sikora-2 (r2 = .81 – .93) buffer methods showed potential as non-hazardous alternatives to SMP (r2 = .90 – .92) based on accuracy and precision. Combining clay concentration, KCl-extractable Al, soil organic matter (SOM), extractable Mg, and pH1:2 also effectively predicted LRE with r2 = .96 – .97. The Moore-Sikora buffer method (r2 = .89 – .93) was similarly accurate but showed precision issues due to high replication variability. The Single Addition of Ca(OH)2 method (r2 = .69 – .77) was not considered to be an accurate predictor of lime requirement for Oregon soils.
Five modifications to the Sikora buffer pH method were evaluated for five Oregon soils. Modifications related to mixing method, measurement in supernatant instead of slurry, and soil:water:buffer ratio were found to significantly increase the Sikora buffer pH value 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.
The results of these investigations demonstrate the feasibility of adopting at least one non-hazardous method for predicting lime requirement of Oregon soils. They also demonstrate the importance of accounting for method modifications and inter-laboratory variation for the Sikora LRE method. These results are anticipated to be used to help make accurate lime rate recommendations in Oregon
