Soil

Soil – the loose covering of broken rock particles and decaying organic matter overlying the bedrock of Earth’s surface Humus – decaying organic matter in soil Soil results from mechanical and chemical weathering and biological activity over a long period of time

Soil Formation Weathering breaks rock into smaller and smaller pieces Organisms (bacteria, fungi, insects) begin to live in rock pieces Organisms die, decay, and add nutrients to the rock pieces to form soil

The time it takes to form soil is dependent on type of rock and climate It can take hundreds of years to form one centimeter of soil

Soil Profiles Soil profile – the vertical sequence of soil layers Well developed soils have distinct layers Poorly developed soils show little distinction between layers A distinct layer within a soil profile – soil horizon

3 Major Soil Horizons 1) Horizon A – rich in humus, gray to black 2) Horizon B – sub-soils enriched with clay, contain minerals washed out (leached) from the topsoil Red or brown due to iron oxide

3) Horizon C – weathered parent material directly above the bedrock - All horizons may not be present in all soils

Soil Types Tropical soils Intensely weathered, infertile soil Little humus Thin Horizon A from high rainfall

Temperate soils High weathering Grassland – high humus Forest – less humus, high clay

Desert/Arctic soils Little chemical weathering Very thin soil Mostly Horizon C