Soil Formation Factors

Why This Matters

Soil formation sits at the intersection of geology, climate science, and biology—making it a favorite testing ground for questions about Earth system interactions. When you understand the five soil-forming factors (remembered by the acronym CLORPT: climate, organisms, relief/topography, parent material, and time), you're not just memorizing a list. You're building a framework for explaining why soils vary dramatically across landscapes, why some regions support agriculture while others don't, and how human activities can accelerate or disrupt pedogenesis.

Exam questions rarely ask you to simply name the factors. Instead, you're being tested on how these factors interact—why a tropical climate produces deeply weathered laterites while arid regions develop calcic horizons, or how slope position controls soil thickness. Don't just memorize what each factor is; know what processes each factor drives and how changing one factor cascades through the system.

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The Starting Point: Parent Material

Parent material establishes the baseline chemistry and texture from which all soil development proceeds. Think of it as the raw ingredients—everything else modifies what's already there.

Parent Material

• Determines initial mineralogy and texture—soils derived from granite tend to be sandy and acidite, while those from limestone are often clayey and calcium-rich
• Residual vs. transported origins affect soil uniformity; residual soils weather in place from bedrock, while transported materials (alluvium, loess, glacial till) may mix multiple sources
• Weathering resistance of parent minerals controls how quickly soil develops; quartz persists while feldspars break down into clays relatively fast

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The Energy Drivers: Climate

Climate provides the energy and moisture that power chemical weathering, leaching, and biological activity. Temperature and precipitation together determine the intensity and direction of soil-forming processes.

Climate

• Temperature accelerates reaction rates—chemical weathering roughly doubles with every $10°C$ increase, explaining why tropical soils weather far deeper than arctic ones
• Precipitation controls leaching intensity; high rainfall moves soluble ions downward, creating distinct horizons like the eluviated E horizon in humid forest soils
• Climate determines soil order—wet tropics produce iron-rich Oxisols, arid regions develop calcium-carbonate-rich Aridisols, and cold regions preserve organic matter in Histosols and Gelisols

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The Landscape Control: Topography

Topography redistributes water and sediment across the landscape, creating soil catenas—predictable sequences of soil types from hilltop to valley bottom.

Topography (Relief)

• Slope angle controls erosion vs. accumulation—steep slopes lose material faster than pedogenesis can build soil, while flat lowlands accumulate both sediment and organic matter
• Aspect influences microclimate; in the Northern Hemisphere, south-facing slopes receive more solar radiation, leading to drier, thinner soils compared to cooler, moister north-facing slopes
• Drainage position determines oxidation state; well-drained uplands develop red/yellow oxidized colors, while poorly drained lowlands show gray gleyed horizons from anaerobic conditions

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The Biological Engine: Organisms

Living organisms transform parent material into true soil through organic matter addition, bioturbation, and nutrient cycling. Without biology, you'd have weathered rock—not soil.

Organisms

• Vegetation type shapes the O and A horizons—grasslands produce thick, dark mollic epipedons from dense root turnover, while forests create thin litter layers with distinct organic horizons
• Soil fauna drive bioturbation; earthworms alone can move 10-50 tons of soil per hectare per year, mixing horizons and improving structure
• Microbial communities control decomposition rates and nutrient mineralization; mycorrhizal fungi extend root networks while nitrogen-fixing bacteria add essential nutrients to the system

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The Master Variable: Time

Time integrates all other factors, allowing processes to progressively differentiate horizons and transform mineralogy. Young soils reflect their parent material; old soils reflect their climate.

Time

• Soil development follows predictable stages—young soils (Entisols, Inceptisols) show weak horizonation, while ancient soils display thick, strongly differentiated profiles
• Horizon development requires centuries to millennia; a distinct Bt horizon (clay accumulation) typically needs 10,000+ years to form under temperate conditions
• Old soils aren't always fertile—extreme weathering over millions of years depletes base cations and leaves behind residual iron and aluminum oxides, as seen in ancient tropical Oxisols

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Quick Reference Table

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Self-Check Questions

1. Which two soil-forming factors most directly control the rate of chemical weathering, and how do they interact?

2. A soil scientist finds thin, rocky soils on a steep north-facing slope and thick, organic-rich soils in the adjacent valley bottom. Which factor best explains this pattern, and what specific processes are responsible?

3. Compare and contrast how organisms influence soil development in grassland versus forest ecosystems. Which soil orders typically result from each?

4. An ancient tropical soil and a young floodplain soil both exist in the same climate zone. Explain why the younger soil might actually be more fertile for agriculture.

5. If an FRQ presents two soil profiles—one with strong horizon differentiation and one with almost none—what questions should you ask about each of the five CLORPT factors to explain the difference?