3.1 Soil-forming factors and processes

Soil formation factors

Soil formation results from the interaction of five factors: climate, organisms, topography, parent material, and time. These are often remembered by the acronym CLORPT (after Hans Jenny's classic equation). Understanding how each factor contributes to soil development is foundational for predicting soil properties across different landscapes.

Climate and organisms

Climate drives soil formation primarily through temperature and precipitation. Higher temperatures speed up chemical weathering reactions and accelerate organic matter decomposition. Greater precipitation increases leaching of soluble minerals downward through the profile and promotes clay mineral formation. In contrast, arid climates tend to produce shallow, mineral-rich soils where salts accumulate near the surface rather than being flushed out.

Organisms shape soils from the surface down:

• Plants add organic matter through leaf litter and root turnover. Root networks also improve soil porosity and help bind particles into aggregates, which increases water infiltration and structural stability.
• Soil fauna like earthworms and termites physically mix soil layers (bioturbation), redistribute organic matter, and create macropores that improve drainage. A single hectare of temperate grassland can contain over a million earthworms, each pulling organic material deeper into the profile.
• Microorganisms (bacteria, fungi) decompose organic inputs and drive nutrient cycling, converting complex organic compounds into plant-available forms.

Climate and organisms don't act independently. A warm, wet climate supports dense vegetation, which produces more organic matter, which fuels more microbial activity. This feedback loop is why tropical soils can be deeply weathered yet nutrient-poor: rapid decomposition and intense leaching strip nutrients almost as fast as they're produced.

Topography and parent material

Topography (also called relief) controls how water moves across and through the landscape, which in turn affects soil depth, drainage, and development.

• Slope angle determines erosion rates. Steep slopes lose material faster than soil can form, producing thin, poorly developed profiles. Flat or gently sloping areas accumulate sediment and retain water longer, allowing deeper soil development.
• Slope aspect creates microclimates. In the Northern Hemisphere, south-facing slopes receive more direct sunlight, leading to warmer, drier conditions and faster organic matter decomposition compared to north-facing slopes.
• Landscape position matters too. Soils at the base of a slope (footslope) tend to be wetter and receive nutrients transported from upslope, while summit positions are often better drained but more exposed to erosion.

Parent material is the geological starting point for any soil. It determines the initial mineral composition, texture, and chemistry that pedogenic processes then modify over time.

• Chemical composition directly affects soil pH and nutrient availability. Soils derived from limestone tend to be alkaline and calcium-rich, while those from granite are typically more acidic and lower in base cations.
• Physical properties set the baseline texture. A sandy parent material produces coarse, well-drained soils with low water-holding capacity. A fine-grained parent material like glacial till or shale weathers into clay-rich soils that retain more moisture but may drain poorly.

In young soils, parent material dominates soil properties. Over time, climate and biological processes increasingly overprint the parent material's influence.

Time and soil development

Time is the factor that allows all the others to leave their mark. Without enough time, even favorable climate and parent material conditions produce only weakly developed soils.

• Initial soil development can begin within decades. Pioneer plants colonize bare rock or fresh sediment, organic matter starts to accumulate, and a thin A horizon forms.
• Moderate development (distinct A, B, and sometimes E horizons with measurable clay translocation) typically takes thousands to tens of thousands of years.
• Highly weathered soils, like the Oxisols of tropical regions, may represent millions of years of continuous pedogenesis, with nearly all primary minerals broken down and thick profiles dominated by iron and aluminum oxides.

The rate of development isn't constant. It depends on how the other four factors interact. A soil forming on limestone in a humid tropical climate will develop far faster than one forming on resistant quartzite in a cold, dry environment.

Soil chronosequences are sets of soils that share similar climate, organisms, topography, and parent material but differ in age. By comparing soils along a chronosequence, you can isolate the effect of time. Key properties that change predictably with age include clay content, organic matter accumulation, horizon thickness, and the degree of horizon differentiation.

Soil formation processes

The five factors above set the conditions. The actual transformation of parent material into soil happens through specific pedogenic processes: additions, removals, translocations, and transformations occurring within the profile.

Weathering and leaching

Weathering breaks down rocks and minerals into smaller particles and new mineral phases. It's the most fundamental process in soil formation.

Physical (mechanical) weathering reduces particle size without changing mineral composition:

• Freeze-thaw cycles fracture rock as water expands upon freezing in cracks.
• Root wedging pries apart rock along joints as roots grow and expand.
• Exfoliation (pressure release) causes outer rock layers to peel away as overlying material is removed.

Chemical weathering alters mineral structures and releases ions into soil solution:

• Hydrolysis breaks down silicate minerals (like feldspar) through reaction with water, producing clay minerals and dissolved cations.
• Oxidation transforms iron-bearing minerals (e.g., converting $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$), producing the reddish and yellowish colors common in well-drained soils.
• Carbonation occurs when $\text{CO}_2$ dissolved in water forms carbonic acid ($\text{H}_2\text{CO}_3$), which is especially effective at dissolving carbonate minerals like calcite.

Leaching is the downward removal of dissolved materials by percolating water. In humid climates, leaching strips soluble ions ($\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{K}^{+}$) from upper horizons, often creating a pale eluvial (E) horizon that has lost clay, iron oxides, and organic matter. The intensity of leaching depends on rainfall amount, soil texture (coarse soils leach faster), and the solubility of the compounds involved.

Together, weathering and leaching drive horizon differentiation by selectively removing and transforming materials at different depths.

Accumulation and pedoturbation

Accumulation concentrates materials within specific zones of the soil profile, often working in tandem with leaching.

• Illuviation is the deposition of material that was translocated from upper horizons. Clay particles, iron oxides, and organic compounds leached from the A or E horizon accumulate in the B horizon (sometimes called the illuvial horizon). You can often see this as clay coatings (called argillans or clay films) on ped surfaces in the B horizon.
• Organic matter accumulation in surface horizons (O and A) reflects the balance between organic inputs (litter, root turnover) and decomposition losses. In cold or waterlogged environments, decomposition slows and thick organic horizons develop.

Pedoturbation refers to physical mixing processes that disrupt or homogenize soil horizons:

• Bioturbation: Burrowing animals, root growth, and tree throw mix soil material across horizons. This is the most widespread form of pedoturbation.
• Cryoturbation: In permafrost and periglacial environments, repeated freeze-thaw cycles churn soil, creating irregular, contorted horizon boundaries and patterned ground features.
• Argilliturbation (also called vertic mixing): Soils rich in shrink-swell clays (like smectite) crack deeply when dry and swell shut when wet. Surface material falls into cracks during dry periods, and the swelling pressure upon rewetting pushes material upward, creating a self-mixing cycle. This is the defining process in Vertisols.

Pedoturbation can work against horizon development by blending distinct layers back together, which is why heavily bioturbated or cryoturbated soils often have weak or irregular horizonation.

Redoximorphic processes

When soils are saturated with water for extended periods, oxygen is depleted and anaerobic conditions develop. Microorganisms switch to using oxidized forms of iron and manganese as electron acceptors, reducing $\text{Fe}^{3+}$ (insoluble, rust-colored) to $\text{Fe}^{2+}$ (soluble, grayish-blue). This process is called gleization.

The visual result is distinctive:

• Gleyed horizons display low-chroma gray or bluish-gray colors, indicating prolonged saturation and reduction.
• Redoximorphic features (mottles, concentrations, depletions) form where the water table fluctuates. Zones that are periodically aerated show orange or reddish spots (oxidized iron), while zones that remain saturated longer appear gray (reduced or iron-depleted).

These color patterns are not just cosmetic. They're reliable indicators of a soil's drainage status and are used extensively in wetland delineation and soil classification. Soils dominated by these processes are classified as Gleysols (in the WRB system) or placed in aquic suborders (in USDA Soil Taxonomy).

Redoximorphic conditions also affect nutrient availability. Reduced iron and manganese become more soluble and can reach toxic concentrations for some plants, while phosphorus availability may actually increase as iron-phosphate compounds dissolve under reducing conditions.

Time and soil development

Progressive and regressive pedogenesis

Soil development isn't a one-way street. Progressive pedogenesis is the "forward" trajectory: horizons become more distinct, clay accumulates in the B horizon, weathering deepens the profile, and the soil becomes increasingly differentiated from its parent material. This is what you see along most chronosequences under stable conditions.

Regressive pedogenesis moves in the opposite direction, simplifying or degrading the soil profile. Causes include:

• Erosion stripping upper horizons faster than they can reform, exposing less-developed material at the surface.
• Climate shifts that alter the moisture or temperature regime, potentially reversing leaching patterns or changing vegetation cover.
• Human activities like deforestation, intensive tillage, or overgrazing, which accelerate erosion and organic matter loss.
• Depositional events like volcanic ash falls or flood sediments that bury existing horizons and reset development at the surface.

A soil's current state reflects the net balance of progressive and regressive processes over its history. Many real-world soils show evidence of both, with well-developed lower horizons overlain by younger, less-developed material.

Soil chronosequences and paleosols

Chronosequences are powerful tools for isolating the effect of time on soil development. Classic examples include soils formed on a series of dated glacial moraines or river terraces of known age. By holding the other four factors roughly constant, researchers can track how properties like clay content, cation exchange capacity, and horizon thickness change with age.

Paleosols are ancient soils that have been preserved, either by burial under younger sediments or by surviving at the surface as relict features in a changed environment.

• Buried paleosols are found in stratigraphic sequences and can record past climate, vegetation, and landscape conditions. For example, a deeply weathered paleosol within a loess sequence suggests a prolonged warm, humid interval between periods of dust deposition.
• Relict paleosols remain at the surface but formed under conditions different from the present. Deeply weathered laterite soils in currently semi-arid parts of Australia, for instance, are relicts of wetter Tertiary climates.

Both chronosequences and paleosols help connect soil science to broader questions about landscape evolution, climate change, and long-term ecosystem dynamics. They also provide a basis for predicting how soils might respond to future environmental shifts.