| - | Class | Description | Ranking |
| Organic matter (%) | a | <1 (mineral soil, very low organic-matter content) | Very low |
| b | 1 - 3 (mineral soil, moderate organic-matter content) | Low | |
| c | 3.1 - 15 (mineral soil, high organic-matter content) | Moderate | |
| d | 15 - 30 (organic soil) | High | |
| e | >30 (organic carbon soil) | Very high |