Influence of fluoride, sulfate and acidity on extractable phosphorus, calcium, magnesium and potassium

Abstract

In an attempt to develop an improved extractant for the soil test evaluation of plant available nutrients the influence of fluoride, sulfate and acidity on extractable P and Ca was investigated with 12 taxonomically different soils, (sequentially fractionated for Ca, Al and Fe forms of P). The soils were extracted with 0.025, 0.05, 0.075 and 0.1 N HCl alone and in combination with 0.0, 0.005, 0.01, 0.02 and 0.03 N NH4F and in part in the presence of 0.025 N (NH4)2SO4 or 0.025 N H2SO4. A 1:10 soil:solution ratio with 5 min shaking at 200 reciprocations per min was used for extraction. In Ultisols where Fe-P was predominant, extractable P increased with increasing concentrations of NH4F and increased further in the presence of sulfate, while in 1 Ultisol and 1 Terric Medisaprist where Al-P was predominant, extractable P increased with increasing concentrations of NH4F but with no further increases due to sulfate. Where Ca-P was predominant (Alfisol, Inceptisol, Mollisols, Typic Medisaprist) extractable P increased greatly with increasing concentrations of acid, whereas NH4F had either little influence or decreased extractable P above 0.01 N NH4F, particularly above 0.05 N HCl. Extractable Ca, Mg and K increased with increasing concentrations of acid while NH4F had little influence on extractable K, decreased Mg slightly and greatly reduced extractable Ca due to precipitation as CaF2. Soil factors influencing formation of CaF2 were largely related to pH and concentration of Ca in solution. Bray 1 and a new extractant by Mehlich (0.2 N NH4Cl-0.2 N HOAc-0.015 N NH4F-0.012 N HCl, pH 2.5) extracted comparable quantities of P from di- and tri-calcium phosphate and 2 sources of phosphate rock. Extractable Ca was however, lower with Bray 1 due to higher precipitation of CaF2.