SummaryThe reaction of sodium fluoride solution with soils and soil minerals at constant pH has been assessed as a possible single‐value characteristic in the classification of soils, particularly those containing significant amounts of poorly ordered inorganic material. A suitable method involves reaction of the soil or clay at 25°C with 0·85 M sodium fluoride at pH 6·8, the amount of OH−released after 25 min being taken as the single‐value characteristic.There was a statistically significant correlation between the amount of OH−released and the amount of alumina extracted with cold 5 per cent sodium carbonate solution from both freely drained and poorly drained Scottish soils. The multiple correlation coefficients were improved for freely drained soils by including the dithionite‐extracted ferric oxide and the silica soluble in 5 per cent sodium carbonate solution. The procedure is not suitable for surface soils, because of interference by organic matter, or for carbonate‐containing soils unless carbon