| Quartz (SiO2) | Mineral (sand/silt fraction) | SiO2 | Physical matrix; drainage and aeration | Weathering of silicate rocks | Soil texture structure |
| Feldspar minerals | Aluminosilicate minerals | KAlSi3O8, NaAlSi3O8, CaAl2Si2O8 | Source of K+, Na+, Ca2+, Al, Si | Rock weathering | Clay minerals, ion release |
| Clay minerals | Aluminosilicate phyllosilicates | Variable (kaolinite, illite, smectite) | High cation exchange capacity (CEC) | Weathering of feldspars/micas | Nutrient retention and exchange |
| Calcium carbonate | Carbonate mineral | CaCO3 | pH buffer, Ca2+ source | Weathering of limestone | Dissolved Ca2+, CO2 |
| Iron oxides | Mineral | Fe2O3, FeOOH | Color, redox chemistry | Oxidation of Fe2+ minerals | Electron acceptors/donors |
| Organic matter (humus) | Complex organic polymer | Lignin-derived polyphenolics, polysaccharides, proteins | Nutrient reservoir, soil structure, C sequestration | Plant/animal residue decomposition | Mineralized nutrients, stable humus |
| Lignin | Aromatic polymer | C9H10O2 units | Structural plant polymer in residues | Plant litter input | Phenolic breakdown products |
| Cellulose | Polysaccharide | (C6H10O5)n | Major plant litter carbohydrate | Plant litter input | Glucose, microbial biomass |
| Hemicellulose | Heteropolysaccharide | Variable | Plant structural carbohydrate | Plant litter input | Monosaccharides, microbial biomass |
| Glomalin | Glycoprotein | Variable | Produced by arbuscular mycorrhizal fungi; soil aggregation | Fungal metabolism | Soil aggregates, C storage |
| Chitin | Structural polysaccharide | (C8H13O5N)n | Fungal cell wall polymer in soil biota | Fungal biomass | N-acetylglucosamine |
| Amino acids | Organic molecules | Variable | N source for microbes/plants | Protein decomposition | Ammonium, microbial biomass |
| Ammonium ion | Inorganic ion | NH4+ | Plant-available N; microbial substrate | Organic N mineralization | Nitrification to NO3- |
| Nitrate ion | Inorganic ion | NO3- | Plant-available N; leaching loss | Nitrification | Denitrification to N2O/N2 |
| Phosphate ion | Inorganic ion | H2PO4-, HPO4^2- | Plant-available P | Mineral weathering, OM mineralization | Biomass P, precipitation with cations |
| Potassium ion | Inorganic ion | K+ | Plant nutrient; osmotic regulation | Mineral weathering | Biomass K, leaching |
| Sulfate ion | Inorganic ion | SO4^2- | Plant-available S | Mineral weathering, OM mineralization | Biomass S, reduction to sulfide |
| Soil lipids | Organic molecules | Fatty acids, waxes | Hydrophobic coatings, microbial membranes | Plant/animal residues, microbial biosynthesis | SOM fraction, microbial biomass |
| Extracellular enzymes | Proteins | Variable | Depolymerize SOM for microbial uptake | Microbial gene expression | Monomers for assimilation/mineralization |
| Polyphenols | Organic molecules | C6–C3 phenolics | Antimicrobial activity, C complexation | Plant litter | Humic substances |
| Humic acids | Organic macromolecules | Aromatic/aliphatic C with carboxyls | C sequestration, nutrient chelation | OM humification | Stable SOM fraction |
| Fulvic acids | Organic macromolecules | Aromatic/aliphatic C with more O groups | Mobile nutrient chelators | OM humification | Complexed micronutrients |
| Black carbon (biochar) | Aromatic C | Polycondensed aromatics | Stable C pool, sorption surface | Biomass burning | Long-term SOM fraction |
