Comparison of Two Sequential Extraction Procedures for Copper Fractionation in Some Soils of Golestan Province

The different fractions of soil metals vary considerably in their chemical reactivity and bioavailability. The distribution of metals among soil components is important for assessing the soils potential to supply sufficient micronutrients for the growth of plant. The sequential extraction procedures have been used to identify and evaluate the mobility and the availability of the metals in soil samples. Ten soil samples with a wide range of chemical and physical properties were collected from agricultural soils of Golestan province. The extractability and solid-phase fractionation of copper (Cu) in the soils was determined by two different sequential extraction procedures and then studied relationship between this fractions together and with plant uptake. Results indicated that the amounts of Cu present in exchangeable form and associated with carbonates were very small (0.3 and 0.4%, respectively). Copper in these soils was dominantly associated with mineral lattices (Tessier procedure: 80%; Singh procedure: 37%). In the remaining fractions i.e. (associated with organic matter and associated with Mn and Fe oxides), the results obtained by Tessier et al. procedure were differed considerably from those obtained by employing Singh et al. procedure. This study demonstrated that dissolution of iron oxides was incomplete during the reductive step of Tessier’s scheme, leading to an overestimation of the residual fraction. Strong and negative correlation (r=-0.692*) was found between plant Cu concentration and the Sodium hypochlorite-extractable Cu (associated with organic matter, Singh procedure) in the 10 calcareous soils, suggesting a prominent role of organic matter in the retention and bioavailability of Cu. Such relation was not observed when employing Tessier’s scheme, suggesting suitability of NaOCl for use in the sequential extraction procedures.