Table 1.
Most important extraction procedures and Si fraction supposed to be extracted.
| Extractant | Procedure (Soil: Extractant Ratio/Extraction Time/Temperature |
Si Fraction Supposed to Be Extracted | References |
|---|---|---|---|
| H2O | 10 g: 50 mL/21 days/room temperature | Water-soluble Si | [37] |
| H2O | 10 g: 100 mL/4 h/room temperature | Water-soluble Si | [35,36] |
| 0.01 M CaCl2 | 1 g: 20 mL/16 h/room temperature | Readily available + above listed fractions | [40] |
| 0.01 M CaCl2 | 10 g: 100 mL/1 h/room temperature | Readily available + above listed fractions | [41] |
| 0.5 M NH4-acetate; adjusted to pH 4.8 | 1 g: 10 mL/1 h/room temperature | Soluble and some exchangeable Si + above listed fractions | [35] |
| 0.5 M acetic acid | 1 g: 10 mL/1 h at room temperature//--resting | Soluble and some exchangeable Si + above listed fractions | [42] |
| NH4 citrate | 10 g: 25 mL/80 h/room temperature | Soluble, exchangeable and specifically adsorbed Si + above listed fractions | [43] |
| 0.2 M NH4 oxalate; adjusted to pH 3.0 | 2 g: 100 mL/1 h room temperature//dark room | Si bound in amorphous and poorly crystalline pedogenic oxides + above listed fractions | [47] |
| Mehlich-III solution at pH 2 | 2 g: 42 mL/5 min/room temperature | Si bound in amorphous and poorly crystalline pedogenic oxides + above listed fractions potential share of crystalline Si is smaller | [44,45] |
| 0.5 M NaOH | soil: extractant ratio of less than 100 mg: 100 mL/2.5 min/boiling | Si from allophanes and amorphous silica + above listed fractions | [48] |
| 0.2 M NaOH | 1 g: 400 mL/5 h, ~120–168 h/room temperature | Amorphous silica and low amounts of crystalline Si + above listed fractions | [49] |
| 0.1 M Na2CO3 | 30 mg: 40 mL/5 h/85 °C/aliquots after 2, 4 and 5 h/ | Biogenic silica in sediments and water + above listed fractions | [51,52] |
| 0.1 M Tiron, pH 10.5 | 25 mg: 30 mL/1 h/80 °C | Si bound in amorphous silica in soils + above listed fractions | [56] |
| Sequential extraction | [57] | ||
| Step 1: 0.01 M CaCl2 | 1 g: 5 mL/TIME/rinsed with pure water room temperature/ | Readily plant-available Si | |
| Step 2: 0.01 M acetic acid | soil solution ratio = 1:10/24 h/room temperature/ | Adsorbed Si fraction | |
| Step 3: H2O2 (17.5% in water) | soil solution ratio = 1:20/24 h/plus 10 mL 35% H2O2/85 °C until reaction is complete | Si bound to organic matter | |
| Step 4: 0.2 M NH4 oxalate and 0.2 M oxalic acid | 1:50/8 h/overnight treatment with UV light/room temperature/ | Si occluded in pedogenic (hydr)oxides | |
| Step 5a: sodium polytungstate 0.2 M NaOH solution | 50% residue of step 4 pre-preparation with sodium polytungstate, afterwards soil solution ratio = 1:400/168 h/room temperature/ |
Biogenic ASi | |
| Step 5b: 0.2 M NaOH solution | Other 50% residue of step 4 soil solution ratio = 1:400/168 h/room temperature |
Total ASi, also calculate minerogenic ASi (total ASi—biogenic ASi) |