(Dirt is soil out of place and not performing ecological functions!)
Updated: 8 November 2024
What is soil? Soil is/has
- a 3-dimensional body at the earth’s (or planetary) surface that has length, width and depth (see profiles below);
- a three-phase, porous media, heterogenous, dynamic;
- 3-phase: always contains solids, liquids (water) and gases;
- solids include
- mineral particles of different sizes separated by voids (pores);
- varying amounts of organic matter – partially to mostly decomposed remains of plant and animal parts and wastes;
- porous media – has space between the particles;
- water and gases fill the pores;
- heterogenous – contains varying-sized particles of different types of minerals and organic materials, with different sizes of pores and aggregates (clumps of particles, clods);
- dynamic – always changing – water content changes as the soil wets and dries, pore and aggregate sizes change every time it is plowed or trafficked (feet and tires), nutrients added and lost, etc.;
- biologically active, host to macro- and micro-organisms (billions upon billions in a handful);
- a negative charge (usually) allowing it to retain many plant nutrients;
- used to grow plants to support human and animal life.
Perspectives
- Soil scientists – Soil is a natural body consisting of layers (horizons) of mineral and/or organic constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical properties and their biological characteristics;
- Agronomy – Soil is the outer layer of the earth’s crust capable of supporting plant growth;
- Geology – Soil is
- unconsolidated, surficial material (weathered rocks);
- the weathered and fragmented outer layer of the earth’s terrestrial surface;
- Ecology – Soil is
- any part of the earth’s crust in which plants are anchored: the muddy bottoms of ponds, porous rock surfaces into which cryptogams send their rhiziods, peat, raw gravel deposited by glaciers, etc.;
- the weathered superficial layer of the earth’s crust with which are mingled living organisms and products of their decay
- Engineering – Soils are natural materials
- which occur in infinite variety over the earth and whose engineering properties may vary widely from place to place within the relatively small confines of a single engineering project;
- whose properties of soils are continuously changing as the amount of moisture fluctuates and other environmental influences vary… and may change dramatically under load;
- used as construction material;
- and the list goes on (dictionary and web search finds more than 20 definitions).
Soil Profiles
Soil profiles are vertical cross-sections of a soil that can only be seen in a hole or soil core.
Color, texture and structure are the main features used to identify horizons. Only color and structure (shape of the clumps identified by the pores between them) can be seen in a photo; texture (combination of sand, silt and clay particles – the smallest silt and clay particles are microscopic) almost always requires touching the soil.
Watch Dr. Dirt discuss soil horizons looking at a profile from a soil pit (YouTube short).
Examine the profiles below; how many horizons can you see?
Starting at the top,
- Left photo:
- Color changes twice: dark, lighter and mottled (mixed);
- Aggregate shape changes twice in the dark part at the top and is somewhat similar in the bottom parts.
- Center photo:
- Color is light at the top, slightly yellow, red, mixed red and yellow, and yellow at the bottom;
- Aggregate shapes change slightly above and below the cracks sloping down from the left between the light and slightly yellow areas and between the red and mixed red identify changes in structure.
- Right photo:
- Color is lightest at the top, darker in the middle, then lighter;
- Aggregate shapes in this one are easier to see than the color differences:
- Top – lot of small blocky looking clumps separated by sometimes large pores;
- Middle – lots of cracks, both vertical (longer) and horizontal;
- Bottom – wide vertical crack forming wide column-like aggregates (prisms to soil scientists); smaller vertical and horizontal cracks are visible.
Surface 35 cm of 3 soils: L to R, intensive tillage, 5 years recovery in grass, forested soil plowed and used for crop production
Starting at the top,
- Left photo:
- Color gets a little lighter;
- Aggregate shapes change slightly above and below the horizontal cracks near the top, in the middle and near the bottom.
- Center photo:
- Color gets a little lighter;
- Aggregates shapes are difficult to see because they are smaller and less blocky/chunky than in the profiles on either side.
- Right photo:
- Color changes are hard to see;
- Aggregate shapes change slightly near the top and dramatically at the bottom:
- Top – difficult to see, probably small;
- Middle – larger with an obvious vertical crack (longer) and column-like aggregates with some small horizontal cracks;
- Bottom – blocky chunks separated by obvious cracks.
SSSA K-12 Outreach, Soils4Teachers – Horizons page
Updated: 8 November 2024