Soil-forming factors and processes
How soil forms from rock and sediment: the five soil-forming factors, the processes that build horizons, the parts of a profile, and how slowly it forms.
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Soil is not just broken rock. It is a natural body that develops over centuries to millennia as rock and sediment are weathered, organisms move in, organic matter accumulates, and water carries materials down through the profile. The result is a sequence of layers, horizons, whose character records the conditions under which the soil formed.
How soils form explains why they differ from place to place and why they are, for practical purposes, a non-renewable resource. Pedogenesis, the formation of soil, is the starting point of soil science courses and a regular exam topic.
The five soil-forming factors
The idea that soils are shaped by their environment came from V. V. Dokuchaev, who studied the black earths of the Russian steppe in the 1880s. In 1941, Hans Jenny expressed it in a compact form, the "clorpt" equation:
Soil (S) is a function of climate (cl), organisms (o), relief (r), parent material (p) and time (t). The dots allow for other factors. Studying soils in places where only one factor varies (a climosequence, toposequence, chronosequence and so on) shows each factor's effect.
Climate
Rainfall and temperature control the speed of chemical weathering, leaching and organic matter turnover. Where rainfall exceeds evaporation, bases are leached and soils become acid and, given enough time, highly weathered (as in the humid tropics). Where evaporation exceeds rainfall, carbonates, gypsum and salts accumulate in the profile (as in deserts and steppes). Climate also decides the vegetation.
Organisms
Vegetation supplies organic matter and shapes soils: grasslands, with dense, fibrous, renewing root systems, build thick, dark, fertile topsoils; forests add litter at the surface and often produce more acid, leached profiles. Earthworms, termites, ants and burrowing animals mix soil; microbes drive decomposition and nutrient cycles. People are a powerful organism factor, through clearing, cultivation, terracing, irrigation, drainage and pollution.
Relief (topography)
Slope affects runoff, erosion and drainage. Soils on steep upper slopes tend to be thin and stony; those at the foot of slopes receive eroded material and water and are deeper, wetter and often richer. A sequence of related soils down a slope is called a catena, a concept first developed in East Africa. Aspect (the direction a slope faces) changes temperature and moisture.
Parent material
The rock or sediment the soil forms from sets its starting minerals and texture. Granite weathers to sandy, often acid soils; basalt to clay-rich, base-rich soils; limestone to calcareous or clayey soils. Many soils form not from the rock beneath but from transported material: river alluvium, glacial till, wind-blown loess, volcanic ash, colluvium on slopes.
Time
Soils develop horizons gradually. Young soils on recent alluvium or volcanic deposits show little development; old soils on stable landscapes, such as those of parts of Australia, Africa and South America, can be hundreds of thousands to millions of years old and very strongly weathered.
The processes
Within a profile, four broad groups of processes operate (Simonson, 1959):
- Additions: organic matter, dust, salts, water, fertiliser.
- Losses: leaching of dissolved substances, erosion, gaseous losses.
- Transformations: weathering of minerals into clays and oxides; decomposition of organic matter into humus.
- Translocations: movement of clay, humus, iron, aluminium, carbonates and salts within the profile, by water moving down (and sometimes up).
Specific combinations give named processes that recur in exams:
| Process | What happens | Typical result |
|---|---|---|
| Humification and melanisation | organic matter accumulates and darkens the topsoil | dark, fertile surfaces of grassland soils |
| Eluviation and illuviation (lessivage) | clay is washed out of upper horizons and deposited lower down | clay-enriched (argillic) B horizons |
| Podzolisation | acids from organic matter mobilise aluminium and iron, which move down with humus | bleached E horizon over a dark or reddish spodic B horizon |
| Ferralitisation (laterisation) | intense weathering removes silica and bases, leaving iron and aluminium oxides and kaolinite | deep, red or yellow, highly weathered tropical soils |
| Calcification | carbonates leach only partway and accumulate in the subsoil | calcic horizons in semi-arid soils |
| Salinisation and sodification | soluble salts or sodium accumulate | saline and sodic soils |
| Gleisation | waterlogging removes oxygen; iron is reduced and moved | grey and mottled horizons |
| Pedoturbation | mixing by animals, roots, freezing, or shrinking and swelling | mixed profiles; Vertisols are self-mixing |
Horizons
Master horizons are labelled with capital letters:
- O: organic layers of litter and partly decomposed material.
- A: mineral topsoil darkened by humus; the zone of greatest biological activity. Cultivated topsoil is labelled Ap (plough layer).
- E: eluvial horizon, paler, depleted of clay, iron, aluminium or humus.
- B: subsoil where materials accumulate or where structure and colour have developed; suffixes describe it, such as Bt (clay accumulation), Bk (carbonates), Bs (iron and aluminium oxides), Bw (weak development), Bg (gleying).
- C: weathered but little altered parent material.
- R: hard bedrock.
How long does soil take to form?
Rates of soil formation vary greatly with climate and parent material, but are commonly put in the range of a few hundredths of a millimetre to about a tenth of a millimetre per year on many landscapes.
Taking 0.1 mm a year, forming 30 cm of soil takes 300 mm ÷ 0.1 mm ≈ 3,000 years.
At a bulk density of 1.3 t/m3, 0.1 mm a year over a hectare is 10,000 m2 × 0.0001 m × 1.3 t/m3 = 1.3 t of soil per hectare per year. An unprotected field losing 20 t/ha a year to erosion is therefore losing soil roughly fifteen times faster than it forms, which is why soil is treated as a non-renewable resource on human timescales.
Key points
- Soil forms from parent material through weathering, organic matter accumulation and movement of materials, building horizons.
- Jenny's factors: climate, organisms (including people), relief, parent material and time: S = f(cl, o, r, p, t).
- Processes: additions, losses, transformations and translocations; named processes include humification, lessivage, podzolisation, ferralitisation, calcification, salinisation and gleisation.
- Master horizons: O, A, E, B, C, R; suffixes such as t, k, s, w, g describe B horizons.
- A catena is a sequence of soils down a slope.
- Soil forms at roughly hundredths to a tenth of a millimetre a year, far slower than erosion on unprotected land.
What the exams ask
- School-leaving and entrance papers: the soil-forming factors, the layers of a soil profile and their letters, and weathering.
- Degree and research-fellowship entrances: Jenny's equation, specific pedogenic processes and the soils they produce, horizon suffixes, catenas, and the role of parent material.
- Soil survey and professional examinations: describing profiles in the field, relating soils to landscape and climate, and the implications of slow formation for conservation.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is the soil under my field formed from the rock beneath it?
Not necessarily. Many soils form in transported material such as river alluvium, wind-blown loess, glacial deposits or volcanic ash, which may be quite unlike the bedrock below.
Why are tropical soils often red?
Intense weathering over long periods removes silica and bases and leaves iron oxides, especially haematite, which colour the soil red; goethite gives yellow-brown colours, often in moister soils.
Can soil be "made" faster?
Topsoil quality can be improved within years by adding organic matter and protecting it, but building new soil depth from rock takes centuries to millennia. That is why preventing erosion matters so much.
Sources
- Jenny, H. Factors of Soil Formation: A System of Quantitative Pedology. McGraw-Hill, 1941 (reprinted Dover, 1994).
- Simonson, R. W. "Outline of a generalized theory of soil genesis." Soil Science Society of America Proceedings 23, 152–156 (1959).
- Buol, S. W. et al. Soil Genesis and Classification, 6th edition. Wiley-Blackwell, 2011.
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 2.
- FAO. Guidelines for Soil Description, 4th edition. FAO, 2006.
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