Acid soils are estimated to comprise 20% of the soils in analyzed in soil testing labs in California, with 5% of these having significant quantities of KCl extractable aluminum. The current lime requirement method, utilizing the SMP buffer, was never calibrated for California soils. In 2002 a study was initiated to evaluate lime requirement soil test methods for California using the SMP, Adams Evans, Woodruff and Mehlich buffer pH methods. One hundred twenty soils were collected representing major crops, a range of textures and a wide range of acid pH values. Soils were evaluated for chemical and physical properties which included: pH, by saturated paste, (1:1) H2O and pH (1:1) 0.01 M CaCl2 method; extractable AL and texture. Lime requirement was assessed based on a 5-Day incubation using calcium hydroxide. Results indicate a median saturated pH of 5.10. The critical pH (1:1) 0.01 M CaCl2 at which KCl extractable Al concentrations exceeded 1.0 mg kg-1 was 4.80, with 40 percent of the soils evaluated containing 1.0 - 515 mg kg-1 Al. The SMP method estimated no lime was needed for 46 percent of the soils evaluated. For the Mehlich method lime rates ranged from 541 to 9750 lbs ac-1 lime. A multi-linear regression model composed of soil pH (1:1) 0.01 M CaCl2 and soil saturated paste moisture, explained 78% of the variation in exchangeable acidity. Using these two variables a diagram was developed which estimates lime application rate. In 2004, twenty-four new soils were collected to validate the initial model.
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