The soil samples were air-dried at room temperature, and sieved through a 2 mm sieve. The soil pH was 6.1, and the OM content determined by the Walkley–Black procedure (Walkley and Black, 1934) and the iron oxide concentration determined by the bicarbonate-citrate-dithionite (DBC) method (Mehra and Jackson, 1958) were 2.1% and 8240 mg kg 1, respectively. The concentrations of available phosphorus and the total nitrogen were determined by using the Bray’s method (Bray and Kurtz, 1945) and the micro-Kjeldahl method (Bremner, 1960), respectively.