For each soil, quantification of the INTF adsorbed was as follows. A sample of wet soil equivalent to 1 g of oven-dried (105°C) soil was placed in a test tube and 1.5 ml of 1 M tris–HCl buffer of pH 7.5 was added. To this solution, 2 ml of a solution of INTF (Sigma product No. I 7375) in the extractant (methanol or DMF-ethanol) was added so that the final INTF concentration was 0 (blank), 10, 20, 50, 100 or 200 μg g−1 soil. The resulting mixture was kept at 40°C for 1 h, diluted with 8 mL of the extractant and 2 ml of distilled water, mixed and then stood in the dark for 10 min. The soil was filtered, and the absorbance of the filtrate at 490 nm was measured