Soil Phosphorus Fractions in Calcareous Vertisols and Aridisols of Northern Jordan

Sequential extraction of the soil by various reagents is designed to remove soil P fractions of varying solubility, with inferences to chemically defined discrete forms of P. This procedure purportedly differentiates between calcium (Ca), iron (Fe), and aluminum (Al) inorganic P fractions, as well as occluded fractions of inorganic P. Vertisols occur in northwestern Jordan, one of the country’s major rainfed agricultural areas with intensive agricultural activities. The arid and semiarid lands of Jordan, locally known as the Badia, are extremely dry and cover a large part of Jordan. Five sites were selected in areas where Vertisols occur to include different precipitation zones and different lithology, while ten sites were selected to represent the Badia in northeastern Jordan, which covers a much larger area. Standard chemical characterization of profile samples was carried out besides determination of P fractions. The Ca-bound P, removed by HCl, was the largest P fraction. The NaHCO3-P, the non-occluded Al and Fe-P, and the occluded P were insignificant fractions. While the dominance of Ca-P was not unexpected given the calcareous nature of the soils, the relatively high proportion of organic P was surprising given the aridity of the environment