Soils

• Regolith vs. Soil
• Regolith - loose rock and mineral debris
• Soil - rock and mineral matter, organic matter (humus), water, and air
• Paleosol - ancient (fossil) soil

Soil Formation

• Controls on Soil Formation
• Parent Formation - type of bedrock (granite? limestone? sandstone?)
• Residual Soil vs. Transported Soil
• Climate - considered most important
• weathering rates faster in warm, moist climate
• more rainfall, more leaching and more vegetation
• Plants and Animals
• plants are a source of organic matter (humus)
• organic acids enhance weathering
• micro-organisms (bacteria) break down organic matter
• Slope - soil poorly developed on steep slope
• Time - soils generally take 1000’s of years to develop, but can be eroded in a few decades

• Soils usually consist of fairly distinct layers - horizons; Ex: soil from granitic rock in humid region
• A-horizon - “top soil”; high organic content; zone of leaching
• B-horizon - clay-rich, Fe-rich zone; little organic matter; soluble minerals present; zone of accumulation
• C-horizon - partially altered parent rock mixed with clays

• Soil Types - Classification is based on chemistry.  Assumes soil is well developed.
• Pedocal
• CaCO3 (caliche); arid grass, brush; Western US
• basic soil
•  Pedalfer
• Organic rich in A horizon, Fe oxides and Al in B horizon (red brown)
• Forest, temperate humid; Eastern US.
• acidic soil
• Laterite
• intensly leached, practically no humus
• Enriched in Fe and Al hydroxide (bauxite)
• tropical humid
• Brazil