Clay mineralogy effects on sodium fluoride pH of non-allophanic tropical soils

Abstract

Soil pH measured in 1 m NaF (pHNaF) can be a useful tool for soil classification and to provide better advice on the chemical management of agricultural soils in the tropics. In this study, we verified the effects of clay mineralogy on pHNaF values of non-allophanic soils of São Paulo State, Brazil. Fourteen subsurface soil samples were characterised for chemical properties, clay content, clay mineralogy, and for pHNaF values, which were measured in the whole soil and in both natural and deferrified (dithionite-treated) clay fractions. Regression and correlation analyses showed that both ammonium-oxalate-extractable Al (Alo) and gibbsite contents have positive relationships with both clay and soil pHNaF values. On the other hand, kaolinite is inversely related to the pHNaF measured in the clay and has nonsignificant effect on soil pHNaF. X-ray diffraction patterns of dithionite-treated clays did not show disruption of kaolinite or gibbsite after the treatment with 1 m NaF, suggesting that the displacement of surface OH groups by F– seems to be the main mechanism associated with the pH increase verified in the NaF solution after its contact with the deferrified clay fraction. The smaller influence of hematite on pHNaF seems to be due to its correlation to Alo. Goethite and ammonium-oxalate-extractable Fe (Feo) exert no effect on pHNaF. Finally, the relationships observed in the present study strongly suggest that pHNaF values <10.3 measured in non-allophanic kaolinitic soils with low levels of non-humified organic matter are essentially due to their smaller Alo and gibbsite contents, which agrees with the direct correlation verified between pHNaF and soil weathering degree.