, Volume 57, Issue 3, pp 225-233

Chemical nature and potential mobility of phosphorus in fertilized grassland soils

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Abstract

This paper describes results from a study of the effects of various applications of phosphorus (P) on the amounts, forms and potential mobility of P in grassland soils (0-7.5 cm) collected from four locations in the United Kingdom (Hertfordshire, Devon) and New Zealand (Taranaki, Canterbury). A sequential extraction scheme (NH4Cl, NH4F, NaOH I, H2SO4, NaOH II, residual P) designed to isolate P associated with aluminium (Al), iron (Fe) and calcium (Ca) was used to characterise P in the grassland soils from each location which had received various quantities of mineral fertilizer, organic manure and lime. Concentrations of total P in the soils ranged from 540 to 3,994 mg P kg-1, and sequential extraction recovered 80–94% of total soil P. Extractable forms of inorganic P and organic P accounted for 40–52% and 31–50% of total soil P respectively. Inorganic and organic P present in the NaOH I fraction (P associated with Fe, Al and organic matter) accounted for most of the P which accumulated in soil from P inputs. Distribution of accumulated soil P between the various inorganic and organic P fractions appeared to be mainly controlled by the nature and availability of sorption surfaces which act as sinks for inorganic P. Phosphate sorption index data for the various soil sets indicated that the mean value of bicarbonate extractable inorganic P (Olsen P) which represented effective P saturation ranged from 61 to 217 mg P kg-1. Potentially mobile soil P as determined by extraction with 0.01M calcium chloride (CaCl2) was found to be most strongly correlated to the NH4F, NaOH I and H2SO4 inorganic P fractions using a Freundlich isotherm.