Estimation of the phosphorus sorption capacity of acidic soils in Ireland

The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be alpha (Al-ox + Fe-ox), where Al-ox and Fe-ox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil(-1), and a value of 0.5 is commonly used for the scaling factor a in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (P so that DPS = P-ox/PSC. The relation between PSC and Al-ox, Fe-ox and P-ox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Al-ox and Fe-ox extracted, but there was also a negative correlation with P-ox. When PSC was calculated as the sum of the measured sorption after 252 days and P-ox, the multiple regression of PSC on Al-ox and Fe-ox gave the equation PSC = 36.6 + 0.61 Al-ox + 0.31 Fe-ox with a coefficient of determination (R-2) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Al-ox and Fe-ox did not overlap, indicating a significantly larger regression coefficient of P sorption on Al-ox than on Fe-ox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Al-ox and Fe-ox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R-2 by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0-3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water-soluble P than either the Olsen or Morgan tests for P in soil.