Soil
Weathered rock, organic matter, water and air — and the life in it.
Soil is not dirt. It is a structured mixture of weathered mineral particles, organic matter, water, air and living organisms, and it takes centuries to form a few centimetres. That formation rate is why soil is treated as effectively non-renewable on human timescales.
Horizons
A soil profile shows layers formed by weathering and the downward movement of material.
| Horizon | Name | Contents |
|---|---|---|
| O | Organic | Leaf litter, decomposing matter |
| A | Topsoil | Humus mixed with minerals; most biological activity |
| E | Eluviation | Leached of clay and minerals, paler |
| B | Subsoil | Accumulates what washed down from above |
| C | Parent material | Weathered bedrock fragments |
| R | Bedrock | Unweathered rock |
Texture
Texture is the proportion of sand, silt and clay. Sand drains fast and holds few nutrients; clay holds water and nutrients but drains poorly and compacts. Loam, a balance of all three, is the agricultural ideal. Texture can be estimated by feel — sand grits, silt feels floury, wet clay ribbons between the fingers.
Soil life
A teaspoon of healthy topsoil holds billions of bacteria plus fungi, protists and invertebrates. Earthworms mix layers and open drainage channels; mycorrhizal fungi extend root reach and trade nutrients for sugars. Soil is a habitat, not a substrate.