Step

Fractions

Reagent

Shaking time and temperature

1

Exchangeable (EXC)

8 ml of 1 M MgCl2 (pH 7) in 1 g soil

2 h at 25˚C

2

Bound to carbonates (CAR)

8 ml of NaOAc (pH 5.0 with HOAc)

5 h at 25˚C

3

Amorphous iron-manganese oxides (OXI)

20 ml of 0.04 M hydroxylamine hydrochloride in 25% acetic acid (pH 2 with HNO3)

6 h at 96˚C

4

Organic-bound (ORG)

5 ml of 30% H2O2 (pH 2), plus 3 ml of 0.02 M HNO3 and 3 ml of 30% H2O2 (pH 2) Cool, add 20 ml of a mixture of 3.2 M NH4Ac and 20% HNO3

2 h at 85˚C

5

Residual (RES)

The residual fraction was digested by 8 ml of a mixture of HCl and HNO3Volume ratio of 3 to 1, therefore they were reached by 0.5 M HNO3

30 min at 25˚C