Comparison of Two Fractionation Methods for Determination of Distribution of Chemical Forms of Manganese in Highly Calcareous Soils.
Najafali Karimian and Seyed Ali Ghaffari-Nejad. College of Agriculture, Shiraz University, Shiraz, 71441-65186, Iran
Manganese (Mn) is an essential plant nutrient. Knowledge about its distribution among different chemical forms is useful in understanding the chemistry, bioavailability, and mobility of this nutrient in soil. There is no universal procedure for determining the chemical forms of soil Mn. Different workers, therefore, use different procedures. This experiment was conducted to compare two procedures for determining the distribution of chemical forms of Mn in selected highly calcareous soils. Twenty-two surface (0-20 cm) samples representing the cultivated calcareous soils of Fars, a southern province of Iran, were analyzed in triplicates for Mn by two different Mn fractionation methods. One procedure, referred to hereafter as Sposito method, fractionates soil Mn into exchangeable, sorbed, organic, carbonate, and residual forms. Whereas the other one, referred to hereafter as Warden method, fractionates it into readily soluble, weakly adsorbed, carbonate, and oxide forms. The soils used in this study had never received Mn fertilizer and had a pH of 7.8-8.2; clay of 160-630 g kg-1; organic matter of 11-49 g kg-1; cation exchange capacity of 9-27 cmol(+) kg-1; and calcium carbonate equivalent of 280-630 g kg-1. Both fractionation methods show that a major proportion of Mn in highly calcareous soils occur in the less soluble forms (i.e., carbonate and residual by Sposito method or carbonate and oxide by Warden method). Warden method designates a larger portion of Mn as carbonate. This is due to the fact that it uses 1.6M HNO3 to extract carbonate form, whereas, Sposito method uses 0.05M Na2EDTA for this purpose. Warden method also shows a larger sorbed form of Mn when compared to Sposito method. This is, presumably, because the former uses 0.025M CaDTPA+0.025M Na2B4O7 for extraction of weakly adsorbed Mn whereas the latter uses three time rinsing with plain deionized water for this purpose. Warden procedure is believed to use more specific extractants for Mn. It is recommended that further studies be conducted with calcareous soils in which plant uptake and the mobility of Mn in soils are also studied to investigate the correlation of different Mn forms (extracted by different procedures) with plant availability and mobility in soil in order to be able to select the best suitable methods of fractionation for calcareous soils.