Table 3.

Procedure of sequential extraction based on Wenzel et al. (2001)

Fraction	Extractant	Extraction conditions	SSR*	Wash step
1	(NH4)2SO4 (0.05 M)	4 h shaking, 20 °C	1:25	–
2	(NH4)H2PO4 (0.05 M)	16 h shaking, 20 °C	1:25	–
3	NH4-oxalate buffer (0.2 M); pH 3.25	4 h shaking in the dark, 20 °C	1:25	NH4-oxalate (0.2 M); pH 3.25 SSR 1:12.5; 10 min shaking in the dark
4	NH4-oxalate buffer (0.2 M); + ascorbic acid (0.1 M) pH 3.25	30 min in a water basin at 96 ± 3 °C in the light	1:25	NH4-oxalate (0.2 M); pH 3.25 SSR 1:12.5; 10 min shaking in the dark
5	HCl:HNO3 (3:1)	Microwave digestion	1:50

*SSR soil to solution ratio