5.2 Soil Horizons and Properties

Notes

A vertical cross-section of soil, called a soil profile, shows layers called horizons.

• O horizon: top layer of organic matter (leaf litter, decaying material).
• A horizon (topsoil): mixture of humus and minerals; most roots and organisms; most fertile.
• E horizon: zone of leaching, where water carries minerals downward (not in all soils).
• B horizon (subsoil): accumulates minerals and clay leached from above.
• C horizon: partly weathered parent material.
• R: unweathered bedrock.
• Soil pH affects nutrient availability. Most crops prefer pH of about 6 to 7. Cation exchange capacity is a soil's ability to hold nutrients. Permeability is how fast water moves through; porosity is the share of pore space.
• Example: a gardener who digs down finds dark crumbly soil rich in roots in the A horizon, and lighter, clay-rich soil below in the B horizon.