Relationship of Fluoride Content with Physico-Chemical and Chemical Properties of Soil

Abstract

This paper analyses fluoride content in 82 soil samples and correlated with physico-chemical and chemical properties of soil. The results indicated that, available F concentration in soils positively correlated with pH and total F content of the soil whereas the correlation coefficient between available F and EC was negative. The relationship between fluoride and CEC, fluoride and P2O5 was negative whereas the correlation coefficient between fluoride and other ions is very poor during both seasons. Increasing fluoride content in soil decreases the available P2O5 content of the soil. The reduction of P2O5 content in soil due to F addition can be attributed to formation of insoluble phosphorus compounds in the soils. Fluoride is negatively correlated with clay percent of soils and positively correlated with sand percent of soils during both seasons but no significant correlation was observed. Clay, soil pH and P2O5 in soil control fluoride content in the soil solution. Soil with high pH and low clay and P2O5 favours greater solubilization of the fluoride bearing minerals leads to leaching th most abundant element of the earth's crust representing about 0.3 g kg -1 . It occurs naturally in the combined state as fluorite (fluorospar), apatite, fluorapatite, topaz and cryolite (Rakshit, 2004). The world average of fluoride concentration in granitic rocks was found to be 810 ppm (Wedepohl 1969), while fluoride content of granitic rocks from Nalgonda district was found to be in the range of 325 to 3,200 ppm, with a mean of 1,440 ppm (Rao et al., 1993). Thus, the granitic rocks of Nalgonda possess the highest fluoride content than in any other parts of the world. Assessment of the granite gneisses from Nalgonda showed the presence of fluoride containing minerals such as fluorite (0-3.3%), biotite (0.1-1.7%), and hornblende (0.1-1.1%) (Reddy et al., 2009). The clay content and pH of soil are primarily responsible for the origin and/or retention of F in soils. F in soil is primarily associated with the soil colloid or clay (Omueti and Jones, 1977). Correlation coefficients between fluorine content (total and available) and some important chemical and physical soil properties were studied.