Phosphorus status and sorption characteristics of some calcareous soils of Hamadan, western Iran

Phosphorus (P) application in excess of plant requirement may result in contamination of drinking water and eutrophication of surface water bodies. The phosphorous buffer capacity (PBC) of soil is important in plant nutrition and is an important soil property in the determination of the P release potential of soils. Phosphorus sorption greatly affects both plant nutrition and environmental pollution. For better and accurate P fertilizer recommendations, it is necessary to quantify P sorption. This study was conducted to investigate available P and P sorption by calcareous soils in a semi-arid region of Hamadan, western Iran. The soil samples were mainly from cultivated land. Olsen's biocarbonate extractable P (Olsen P) varied among soils and ranged from 10 to 80 mg kg(-1) stop with a mean of 36 mg kg(-1). Half of the soils had an Olsen P > 40 mg kg(-1) and > 70% of them had a concentration > 20 mg kg(-1), whereas the critical concentration for most crops is < 15 mg P kg(-1). Greater average Olsen P in soils occurred under garlic (56 mg kg(-1)) and potato (44 kg(-1)) fields than in dry-land wheat farming (24 mg kg(-1)), pasture (30 mg kg(-1)), and wheat (24 mg P kg(-1)) fields. A marked increase in fertilizer P rates applied to agricultural soils has caused P to be accumulated in the surface soil. Phosphate sorption curves were well fitted to the Freundlich equation. The standard P requirement (SPR) of soils, defined as the amount of P sorbed at an equilibrium concentration of 0.2 mg 1(-1) stop ranged from 4 to 102 mg kg(-1). Phosphorus buffer capacity was relatively high and varied from 16 to 123 l kg(-1) with an average of 58 l kg(-1). In areas of intensive crop production, continual P applications as P fertilizer and farmyard manure have been used at levels exceeding crop requirements. Surface soil accumulations of P are high enough that loss of P in surface runoff and a high risk for P transfer into groundwater have become priority management concerns.