Soils were extracted by hot water, dilute CaCl 2 , and by mannitol-CaCl 2 solutions and boron was determined by either azomethine-H or the curcumin method. Results were strongly method dependent; in particular results by the simplest method, mannitol-CaCl 2 , were generally lower and not highly correlated (r = 0.64) with those by the recommended procedure. This procedure involves extraction with hot distilled H 2 O (2:1 solution:soil), centrifugation, filtration, color development with azomethine-H and correction for background color. Extractable boron values by this procedure ranged from 0.1 to 1.4 mg kg −1 . Background color can be further reduced by the substitution of 0.02 M CaCl 2 for distilled water.
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P. A. Schuppli (1986) studied this question.