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Bedrock: This is solid rock that formed before the soil above it. It will wait until erosion or an
earthquake exposes it to the surface. Then it will be weathered to become parent material.
Decompose: To be digested and broken down into simpler substances, making nutrients more
available to plants.
Dirt: Lacks all characteristics that would allow it to support life.
Humus: Dark, organic material that forms in soil when plant and animal matter decay.
Loam: A fertile soil of clay and sand containing humus.
Organic Material: Anything that was once living, or produced by a living organism that will
continue to decompose. Examples: Dead leaves, twigs, dead bugs, etc.
Organic Matter: Material that was once alive, but has decomposed until stable (resists
decomposition). Organic material that created it is no longer identifiable.
Percolate: To pass slowly through something that has many small holes in it. Water percolates
through soil, recharging groundwater.
Silt: A material similar in size to sand and clay whose mineral origin is quartz and feldspar.
Soil: A complex mixture of minerals, air, water, and microorganisms that supports life on Earth.
Soils modify the atmosphere by emitting and absorbing dust and gases. They process and
recycle nutrients, including carbon, so that living things can use them over and over again.
Soil Profile: Distinct layers of soil make up a soil profile. They include (listed from the layer
closest to your feet to the furthest): Topsoil, Subsoil, Weathered Parent Material, and Bedrock.
Subsoil: A mixture of mineral particles and some humus near the top. It is very low in
inorganic matter. This is where most of the soil’s nutrients are found.
Topsoil: Plants grow and animals live on top of the soil. Plants keep the soil cool and from
drying out. Decomposers recycle dead plants and animals into humus.
Weathered Parent Material: There is no organic matter here at all. It is all rock particles, full
of minerals. This is what the entire soil profile used to look like on the surface before
weathering broke the parent material into small pieces.