Determining Lime Requirements Using Buffer Tests in Eastern Washington and Northern Idaho.

Soil acidification has been occurring at an accelerated rate in Eastern Washington and Northern Idaho, primarily due to long-term inputs of ammoniacal fertilizers. It is well known that liming ameliorates acidic soils; however, past research indicates that the heterogeneous soils of this region may respond differently to liming. This leaves growers uncertain as to how much lime to apply to achieve a target soil pH. Lime requirement is determined in other parts of the U.S. by buffer tests that measure exchangeable acidity. Though several buffer tests have been developed in other parts of the U.S., the most appropriate test for this region has not been established. We identified ten agriculturally significant soils in the region, with 1:1 water pH values below 5.25 in the top 15 cm. These soils were tested using five common buffer test methods: SMP, Sikora, Woodruff, Woodruff pH 6, and Modified Mehlich. Buffer tests were compared to a 90- day incubation of the ten soils with increasing rates of calcium carbonate (CaCO₃). The results of this comparison identify a buffer test method that is most appropriate for deriving lime recommendations for soils in Eastern Washington and Northern Idaho.