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Abstract
A study was conducted to examine phosphorus (P) sorption and immediate desorption in 0.01 M CaCl2 in unmanured and manured grassland and arable soils after chemical treatment designed to remove organic matter [sodium hypochlorite (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 decreasing 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 pronounced in arable soils compared to grassland soils, which reflected the influence of Ca on P solubility. Following pretreatment with NaOCl, P sorption was similar in manured and unmanured arable soils. This was not the case in the corresponding grassland soils which was attributed to either physical occlusion of sorption sites by organic matter and/or differences in the chemical nature and stability of soil organic matter in the grassland compared to arable soils.
Acknowledgments
Financial assistance was provided by St John's College, Cambridge (UK), the Agricultural and Marketing Research and Development Trust (New Zealand), and Lincoln University (New Zealand). We also wish to thank AgResearch (New Zealand), the Primary Production Co-operative Society (New Zealand) and L Raith & Sons (UK) for providing soils for this study.