Twenty bulked soil samples will define the average pH of 1 ha of a soil type. Air-drying shortly after collection is needed to prevent development of acidity during moist storage. Determining pH in the clear supernatant of a mixture of 0.01 M CaCI2 and soil shaken for 1 h gives highly reproducible values that are a good estimate of field pH. Threefold changes in sampling volumes had a very small effect on variance, so sample size does not strongly influence the error associated with sample collection.
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