Phosphorus sorption of coal-mine soils in Galicia, NW Spain

Phosphorus sorption index (PSI) values for 142 samples of coal mine spoils from Galicia, NW Spain, ranged between 2.4 and 55.1. pH values (1:2.5 soil/water) varied from 2.7 to 8.4, with a mean value of 4.9, and pH values in KCl (1:2.5) varied from 2.1 to 8.1, with a mean value of 4.4. Olsen P values ranged between 1 and 54 mg kg-1. Clays and shales, often in conjunction with pyrite and other sulphides, were the main constituents of the spoils; many of them are extremely acid as a result of oxidation of sulphides. Reclamation tasks consisted of either topsoiling, chemical and organic fertilizing or various amendments. Two depths, 0 to 15 and 15 to 30 cm, were sampled and analyzed at each sampling site. Field experiments suggested strong phosphorus adsorption processes at some sites. Of 11 soil properties examined, four (viz. organic carbon, pH in water, Al extracted by Mehlich-3 and clay + fine silt) accounted for 69% of the variance in log PSI. The fitted multiple regression predicts that PSI decreases with increasing pH; at pH 5, PSI ranged between 6.7 and 44.2, whilst at pH 7, PSI is below 25.9.