Soil is fundamentally defined as a natural body occurring on the land surface that is composed of solids (minerals and organic matter), liquid, and gases. It occupies space and is distinguished by the presence of discernible horizons or layers, which differ from the original parent material due to processes involving additions, losses, and transfers of materials.
According to the Natural Resources Conservation Service (NRCS) - USDA, soil is:
"Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, ..."
Key Components of Soil
Breaking down this definition helps us understand the complexity and significance of soil. It's not just dirt; it's a dynamic mixture essential for life.
Here are the main components:
- Solids: This includes inorganic mineral particles (like sand, silt, and clay) derived from weathered rocks, and organic matter (decomposed plant and animal remains).
- Liquid: Primarily water, which dissolves nutrients and facilitates biological processes.
- Gases: Air found within the pore spaces between solid particles, providing oxygen for roots and soil organisms.
Defining Characteristics
What makes soil different from just a pile of weathered rock or sediment? Its structure and development:
- Occurrence & Space: Soil exists as a distinct layer covering the Earth's land surface and has a measurable volume.
- Horizons (Soil Layers): A key characteristic is the presence of horizons. These are distinct layers (often visible in a soil profile) that form over time. Each horizon develops unique physical, chemical, and biological characteristics due to the ongoing processes of:
- Additions: Materials like organic matter from decaying plants or dust from the atmosphere being added to the surface.
- Losses: Materials like dissolved nutrients or fine particles being washed away (leaching).
- Transfers: Materials moving within the soil profile, such as clay particles or organic compounds shifting from upper to lower layers.
Why Horizons Matter
The development of these horizons is a sign of active soil formation processes. It indicates that the material is not merely parent material (the underlying rock or sediment) but has been transformed into a true soil profile capable of supporting ecosystems.
In summary, soil is a living, evolving system composed of diverse materials, organized into layers, and crucial for supporting plant growth, filtering water, and hosting vast biodiversity.
