Soil formation and composition
Big idea: Soil is not just dirt — it is a living system that takes thousands of years to form. It is made of minerals, organic matter, water, air, and billions of organisms. Without soil, there would be no terrestrial life!
How soil forms
Soil forms through weathering of parent rock combined with the addition of organic matter over time.
- Physical weathering — rock broken by temperature changes, ice, roots, wind
- Chemical weathering — rock dissolved or altered by water, acids, oxygen
- Biological weathering — organisms (lichens, roots, burrowing animals) break down rock
- Organic matter addition — dead plants and animals decompose, adding humus
Factors affecting soil formation (CLORPT)
- Climate — temperature and rainfall affect weathering rate
- Organisms — plants and animals add organic matter, mix soil
- Relief (topography) — slope affects drainage and erosion
- Parent material — the rock type determines mineral content
- Time — soil development takes hundreds to thousands of years
Soil forms at about 1 cm per 100-1000 years. Once destroyed, it is effectively non-renewable on human timescales!
Soil composition
Healthy soil is roughly:
- 45% minerals — sand, silt, clay (from weathered rock)
- 25% air — in pore spaces, needed for root respiration
- 25% water — in pore spaces, carries dissolved nutrients
- 5% organic matter — humus, living organisms, dead material
Even though organic matter is only ~5%, it is CRITICAL for soil fertility. Humus holds nutrients, improves water retention, and supports soil organisms.