Methods

Extraction procedure

Bray No. 2

0.03 mol/L NH₄F + 0.1 mol/L HCl,

1 min shaking by hand,

soil:extractant = 1:20 (w/v)

Truog

1 mmol/L H₂SO₄,

30 min shaking (165 rpm),

soil:extractant = 1:200 (w/v)