Relationships between a P-sorption index, extractable Fe and Al and fluoride reactivity in the soils of an area of mid-Wales

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Abstract
SUMMARY: The soils of on area around Llangadog (mid-Wales) have been systematically sampled on a grid basis at 0–8 cm and 30–35 cm depths.A P-sorption index was determined for these samples as were Fe and Al extractable by both 0·1 M K-pyrophosphate solution and 0·2 M-NH4-oxalate solution at pH 3. The reactivities of the soils with 1 M-NaF solution at pH 6·8 and pH 8 were also measured.The results were stratified in terms of parent material, soil classification at subgroup level and soil series, and correlation coefficients calculated between the P-sorption index and the other variables (singly and in combination).Only pyrophosphate-extractable Fe, i.e. ‘organically bound’, correlated reasonably well with the P-sorption index. However, the correlation was such that tolerably good estimates of P-sorption in an agronomic context could be made from pyrophosphate-extractable Fe values only for previously uncultivated brown podzolic soils (Manod series).Fluoride reactivities at both pH values were poor predictors of P-sorption in these soils. Fluoride reactivity at pH 6·8 and oxalate-extractable Al, which have both been proposed as indices of the amounts of ‘poorly-ordered’ Al compounds in soils, failed to group these soils in similar ways. Doubt exists therefore as to whether these variables are measures of the same (or similar) property in these soils.