Soil
Soil is the loose surface layer of weathered rock, organic matter, water, air, and living organisms that supports plant growth.
Definition
Soil is the loose top layer of the ground where plants grow, a mix of tiny rock pieces, rotted plant and animal material, water, air, and living things like worms and bacteria. It forms from weathered bedrock or transported sediment (parent material) mixed with organic matter over long periods, and a mature soil has horizons: the O horizon of organic litter, the dark A horizon (topsoil), the B horizon where clay and minerals accumulate, and the C horizon of partly weathered parent material above bedrock. Soil texture is the mix of sand, silt, and clay. Soil development is a function of five factors: parent material, climate, organisms, topography, and time. Clay minerals and humus give soil its cation exchange capacity, holding nutrients like $K^+$, $Ca^{2+}$, and $Mg^{2+}$ against leaching, and soil pH controls nutrient availability. Piedmont Ultisols are acidic and low in bases because millions of years of leaching removed them, leaving iron and aluminum oxides, and soil formation rates are commonly under $0.1\ \text{mm/yr}$, far slower than erosion from tilled fields.
Example
Dig a hole in a yard in Charlotte and you find dark topsoil on top, then sticky red clay underneath. North Carolina's state soil, Cecil, is the red clay soil of the Piedmont formed from weathered granite and gneiss, while Coastal Plain soils are sandier and mountain soils are thin and rocky. Midwestern Mollisols under prairie grass developed thick, dark A horizons rich in organic carbon, which is why the Corn Belt is so productive. In contrast, tropical Oxisols are deeply weathered, nutrient-poor, and rely on rapid recycling of litter.
Key Insight
Soil is not just dirt: a single handful holds more living organisms than there are people on Earth. Soil takes centuries to form but can be lost in a single season, since building a few centimeters of topsoil can take hundreds of years. Soil also stores roughly two to three times as much carbon as the atmosphere. How land is farmed, and whether soil carbon is built or lost, is a direct lever on the global carbon cycle.