Various soil biological and chemical tests were compared in respect to their suitability for predicting deficiency of potassium for red pine grown on soils of Wisconsin and New York. The biological tests were accomplished by the Azotobacter and Aspergillus niger methods. The chemical tests included extraction of soil by N NH 4 OAc of pH 6.9 and 4.6, extraction by N NH 4 NO 3 of pH 6.9 and 4.0, extraction by 0.4 N HCl, ignition of ammonium‐saturated soil followed by extraction with neutral N NH 4 OAc, and extraction with boiling N HNO 3 . The last method provided the sharpest distinction between potassium‐deficient and nondeficient forest soils as detected by the rate of growth and response of plantations to application of potash fertilizers.
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Albert L. Leaf (1958) studied this question.