Influence of soil constituents on soil phosphorus sorption and desorption

A study was conducted to examine phosphorus (P) sorption and immediate deso rption in 0.01 M CaCl2 in unmanured and manured grassland and arable soils after chemical treatment designed to remove organic matter [sodium hypochlo rite (NaOCl)], aluminum (Al) and iron (Fe) oxides [dithionite-citrate], and acid soluble materials [hydrochloric acid (HCl)], either in combination or individually. Removal of Al and Fe oxides had the greatest effect in decre asing P sorption and increasing P desorption relative to the fraction of P previously sorbed. Sorption of P was approximately 2 to 5 times greater in soils extracted with HCl than in dithionite-citrate treated soils, while P desorption as a fraction of P sorbed was approximately one-seventh that in dithionite-citrate treated soils. The effect of HCl pretreatment was more p ronounced in arable soils compared to grassland soils, which reflected the influence of Ca on P solubility. Following pretreatment with NaOCl. P sorpt ion was similar in manured and unmanured arable soils. This was not the cas e in the corresponding grassland soils which was attributed to either physi cal occlusion of sorption sites by organic matter and/or differences in the chemical nature and stability of soil organic matter in the grassland comp ared to arable soils.