Evaluation of lime requirement tests for Northern Idaho soils

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Abstract
Agricultural soils in northern Idaho have become acidified in the last 20 years due to heavy use of ammonium-based N fertilizers. There is, however, a lack of information on the performance of lime requirement tests on these soils. This study was undertaken to evaluate several of the more commonly used lime requirement tests on some northern Idaho soils. Latahco silt loam soil (fine-silty, mixed, frigid Argiaquic Xeric Argialboll) was selected to represent these closely related soils in this study. latahco soils was incubated under controlled environmental conditions with 0, 1, 2, and 4 Mg ha-1 Ca(OH)2 for five weeks. The results were compared to "quick" lime requirement tests using linear regression equations. All lime requirement tests were highly correlated with incubation-indicated lime requirement (r2 0.89 at target pH 6.0). However, only Woodruff and Improved Woodruff buffer methods provided acceptable estimates of the actual lime requirement over the pH range evaluated (5.0 to 5.8). The SMP buffer method overestimated lime requirement over all pH values (5.0 to 5.8). A modification in the Woodruff buffer method was suggested to further improve its predictive ability. The modification was to use a buffer pH adjusted to 6.0 (Woodruff-pH6) rather than 7.0. The Woodruff-pH6 appears to provide the most accurate lime requirement value among methods evaluated in this study.