SOIL FLUORINE

The development of techniques for measuring soluble and labile soil fluoride using a specific-ion fluoride electrode are described. The methods evolved were used to assess the fluoride status of numerous agricultural soil samples in addition soils which had received known fluoride additions. Two soil profiles were analysed with respect to distribution with depth of soluble and labile fluoride. The equilibrium-fluoride concentration was determined after shaking the soil with a 0.01 M CaCl, solution. Isotopically exchangeable (labile) soil fluoride was determined by use of the radioactive fluorine isotope ¹⁸F. These labile fluoride values were well correlated with the amount of soil fluoride extracted by an anionexchange resin, and the latter method was consequently adopted for further determinations of labile soil fluoride. Nearly half the 1OO agricultural soil samples examined gave fluoride concentrations < 0.2 ppm in 0.01 M CaCl, extracts whereas labile fluoride values were evenly distributed around a mean of 20 ppm. For any given pH value, there appeared be an upper limit to the solubility of soil fluoride. Application to soil of superphosphate (in the field) or of fertilizer containing fluoride (in the laboratory) brought about increases in soluble soil fluoride.