# The Soil Profile

Laplace’s Mini World · Plate 76 · Earth & Sky · Earth

> Dig into a forest and the side of the hole shows layers, from leaf litter down to granite. They were not laid down one on another: rock, rain and living things made them in place.

- Interactive model: https://world.lapla.si/en/soil/
- Embed: https://world.lapla.si/embed/en/soil/
- 中文: https://world.lapla.si/zh/soil/
- Updated: 2026-11-01

## 01 A Strip of Soil on a Wall

To take a soil for display, dig a pit 1⁠–⁠1.5 m deep, clean one wall with knives and a spade, nail a metal frame into it, cut a column of soil round the frame and lift it out with a board behind it.

The column’s back is sawn flat and glued, and it is turned over onto a display board. Once it is dry, small knives and picks open up the front to show the soil’s own structure, and it gets a coat of thin glue. The result is called a soil monolith.

ISRIC’s World Soil Reference Collection in Wageningen, the Netherlands, holds over 1,⁠100 monoliths from around 82 countries, most about 1.5 m deep. It began in 1966 as the International Soil Museum.

The monolith in the oak frame on the plinth is the same soil, down to the hard granite: rock like that cannot be dug with a spade.

*A monolith: a strip of soil taken upright from a pit wall · ISRIC holds over 1,⁠100*

## 02 From Leaf Litter to Granite

The block on the plinth is a soil of the Saw series, on the Piedmont uplands of North Carolina and Georgia, weathered in place from porphyritic granite. The US Department of Agriculture’s description gives the depth and colour of each of its layers. Hard bedrock lies 51⁠–⁠102 cm down.

From the top: the O horizon is leaf litter; the A is topsoil, humus mixed with mineral; the E is pale, its clay and iron washed out; the Bt is red clay; the C is rotten granite; the R is hard granite. In the typical profile the Bt runs from 20 to 51 cm, the BC to 66 cm and the C to 74 cm, with rock below.

They look stacked, but they were not laid down one on another. Mud a river lays down in sheets is called strata; the layers of a soil are horizons, and they form in place. Additions, losses, transfers and transformations work down from the surface through the old rock and wipe out its grain.

*O litter · A topsoil · E leached · Bt clay gathered · C saprolite · R bedrock*

## 03 Leaves Become Topsoil

The O horizon at the top is almost all organic: fresh leaves on top, more broken and darker further down. Soil scientists record its layers, from slightly to highly decomposed, as Oi, Oe and Oa.

The A horizon forms at the surface, humus closely mixed with mineral grains, which is why it is usually the darkest layer of a profile. The roots gather here too: in the description of a forest soil from the same region, the A has “many” fine and medium roots and the clay below “common” ones.

The Saw series’ top layer holds only 0.5⁠–⁠2 % organic matter, the Bt 0⁠–⁠0.5 %. Little, yet dark: how dark a soil looks depends mostly on how much dark humus coats its grains.

*Organic matter: top 0.5⁠–⁠2 % · Bt 0⁠–⁠0.5 % (Saw series)*

## 04 Clay Moves Down

The mark of an E horizon is the loss of clay, iron and aluminium to water, leaving sand and silt behind. With their iron coatings gone, the grains show their own colour, mostly pale quartz, so the E is lighter than the layers above and below.

Rain soaking down carries dispersed clay from the topsoil. In drier soil below, the water is drawn away, the clay is filtered out against the faces of the clods and the walls of pores, and it builds up a shiny skin: a clay film. The Saw series’ Bt has common, distinct clay films on its peds.

So the top layer is sandy loam with 5⁠–⁠20 % clay, and the Bt is clay with 35⁠–⁠60 %. Not all of the Bt’s clay came from above: some weathered in place from the minerals there.

*Clay: top 5⁠–⁠20 % · Bt 35⁠–⁠60 % (Saw series)*

## 05 Where the Red Comes From

Soil scientists read a soil’s colour against a Munsell chart and write it as hue, value/chroma: the Saw series’ Bt is 2.5YR 4/8, called “red”. A hue of 2.5YR lies between red and yellow, a value of 4 is on the dark side, and a chroma of 8 is the strongest on a soil chart. The match is made out of direct sunlight, with a broken clod held to the chips.

A soil’s colour comes mostly from very fine coatings on its grains: humus is dark, iron oxides yellow, brown, orange and red. The redder a soil, the more hematite it tends to hold; soils dominated by goethite are yellow. It takes little iron oxide to colour a soil.

Hematite is common in deeply weathered soils of warm, humid climates. In soils that sit in water, iron is reduced and carried off, and they turn grey; the Saw series is well drained, and its profile has no such grey layer. The Bt also holds a few brownish yellow spots (10YR 6/6).

*Bt 2.5YR 4/8 (moist) · dark from humus, red and yellow from iron oxides*

## 06 Granite Rots into Soil

The R horizon at the bottom is hard porphyritic granite: large feldspar crystals set in a finer groundmass. A spade cannot dig it, its cracks are few, and roots hardly get in.

The C horizon above it is saprolite: rock weathered in place, some of its minerals dissolved away, its grain and structure kept and its volume almost unchanged, yet soft enough to crumble in the hand. In the Saw series’ C, granite grit and feldspar crystals make up 20 %, with many small flakes of mica.

Rain takes up carbon dioxide from the air and the soil and turns slightly acid. Albite (NaAlSi₃O₈), a feldspar, reacts with water and hydrogen ions to become kaolinite (Al₂Si₂O₅(OH)₄), a clay mineral, while sodium and silica leave in the water. Iron-⁠bearing minerals oxidise and leave iron oxides behind. Roots widen cracks, and water freezing in them pushes them apart.

*R hard granite · C saprolite: the rock’s structure kept, crumbling in the hand*

## 07 Five Factors

In 1941, in Factors of Soil Formation, Hans Jenny wrote down an equation: s = f(cl, o, r, p, t, …). A soil depends on five factors: climate, organisms, topography, parent material and time. The dots at the end leave room for more. Fix those, he wrote, and only one kind of soil can form.

The idea is older. In 1883, in Russian Chernozem, Dokuchaev already saw soils as the product of climate, living things, parent material, relief and time together.

Change one factor and the soil changes. In Iowa two soils both formed in loess under similar climates: the Fayette, under oak and hickory, has 8 cm of dark topsoil over a pale leached layer; the Tama, under tall-⁠grass prairie, has a dark top 46 cm thick and no leached layer.

The Saw series formed from granite, under a mean temperature of about 15⁠–⁠17 °C and about 990⁠–⁠1,⁠250 mm of rain a year, under pine and oak, on ridges and slopes. How old it is, its description does not say.

*s = f(cl, o, r, p, t, …): climate · organisms · topography · parent material · time*

## 08 How Long a Centimetre Takes

Soil forms slowly, but how slowly varies a great deal. A poster of the UN Food and Agriculture Organization says 1 cm of soil “can take up to 1000 years” to form; its World Soil Day page says 1,⁠000 years can produce “just 2-3 cm”. Both say “up to”, not on average.

A compilation of measurements from around the world puts the median rate of soil production at about 0.017 mm a year and the mean at about 0.036 mm; at those rates a centimetre takes about 590 or about 280 years (computed). The FAO’s 2015 report says soil forms typically well below 1 tonne per hectare a year, with a median of about 0.15 tonnes.

Conventionally ploughed fields erode at a median of about 1.5 mm a year, one to two orders of magnitude faster than soil forms. On a hillside the soil tends towards a balance between forming and wearing away; how rain carries it off is in “How Rain Carves a Mountain”.

*Production: median about 0.017 mm/yr · erosion of ploughed fields: median about 1.5 mm/yr*

## 09 Sand, Silt and Clay

Soil grains under 2 mm across are the fine earth. In the US Department of Agriculture’s scheme, sand is 2⁠–⁠0.05 mm, silt 0.05⁠–⁠0.002 mm and clay under 0.002 mm; coarser grains are gravel. The World Reference Base for Soil Resources puts the line between sand and silt at 0.063 mm.

How much sand, silt and clay a soil holds sets its texture. The USDA has 12 classes: loam, for example, has 7⁠–⁠27 % clay, 28⁠–⁠50 % silt and no more than 52 % sand; a soil over 40 % clay without much sand or silt is clay. The Saw series’ Bt is clay. The settling jar on the plinth is a schematic: shaken up, the sand settles first, the silt next, and the finest clay last.

Clay holds the most water, but grips most of it too tightly for plants; the soils with the most water for plants are silts and silt loams, sand has the least, and clay does not have more. Sand has no more pore space than clay, only bigger pores: saturated, sand drains about 21 cm an hour and clay about 0.06 cm, some 350 times slower (computed).

*Sand 2⁠–⁠0.05 mm · silt 0.05⁠–⁠0.002 mm · clay under 0.002 mm (USDA)*

## 10 What Lives in It

A gram of soil can hold up to 1 billion bacteria and up to 200 m of fungal threads. Nematodes are the most numerous animals in soil, some 4.4 × 10²⁰ of them in the world’s soils; a square metre often holds 10,⁠000 to 100,⁠000 springtails.

Earthworms swallow soil and pass it out again. In 1881 Darwin estimated that in many parts of England more than 10 tons of dry earth an acre passes through worms each year and is brought to the surface, about 2.6 kg per square metre (computed). Fungal threads bind small grains into crumbs, and a sticky protein left by mycorrhizal fungi glues them together.

Soil also stores a great deal of carbon. On the IPCC’s carbon-⁠cycle figure, soils outside the permafrost region hold about 1,⁠700 billion tonnes of carbon, about twice as much as the air (computed). How carbon moves between air, plants and soil is in “The Greenhouse Effect”.

*A gram of soil: up to 1 billion bacteria · up to 200 m of fungal threads (FAO 2020)*

## 11 Horizon by Horizon

The model comes apart into six blocks along its horizons: O, A, E, B, C and R. Apart, each shows its top and bottom faces. The boundaries wave up and down, some sharp and some gradual, not flat as layers of sediment are.

The roots belong to the A: lift the topsoil and they pull out from underneath. Hover over a horizon and it lifts on its own, with its name.

*O · A · E · B (Bt and BC) · C · R*

## 12 Set the Texture

This is the page’s model: set the sand and the clay; silt is the rest. The page finds the texture class by the USDA’s rules and reads the tables: how much water a metre of such a soil holds at field capacity and how much of it plants can use, from the typical ranges of FAO-⁠56 Table 19, and how fast it drains when saturated, from the means by class of Rawls and others (1982).

Table 19 has no row for sandy clay loam, clay loam or sandy clay, and Rawls’s table none for silt; for those the reading says “not in the table”. These are typical values for a class, not a measurement of any one soil. “Start again” returns to loam.

*The page’s model · texture by the USDA’s rules · water from the table’s typical ranges*

**Try this**: Set the sand to 85 % and the clay to 5 %: which texture class is it, and how much water can a metre of it keep for plants? Then take the clay to 45 %: how do the water kept and the drainage change?

## What the model leaves out

The block on the plinth is 1 m wide, 0.7 m deep and 1 m from the top of the litter to its base, at a quarter of its size (computed). The mineral horizons’ depths and colours follow the Saw series’ typical profile, which was described in a field; the forest litter (3 cm), the A (8 cm) and the E (to 20 cm) are thicknesses the model chose, in colours inside the description’s ranges. Colours are drawn near their Munsell notation, not measured. The structure, roots, burrows, stones and spots are the page’s drawing, not measurements of where or how many. The monolith, the colour book, the trowel and the jar are at the same scale; the chips in the book are drawn, not the printed chart, and the jar’s three layers are drawn in proportion to the sand, silt and clay, with no allowance for how differently they settle and pack. The lab reads typical ranges for a class; it computes nothing about any one soil.

## Easy to get wrong

A soil’s horizons were not laid down one on another: they form in place, from the surface down, unlike river sediments. Not every soil has all six: many have no E horizon, the Saw series’ typical profile among them, and some have bedrock far down, such as the Cecil series of the same region, which reaches bedrock below 152 cm. A soil’s colour does not come only from its minerals: the dark is mostly humus, the red and yellow thin coatings of iron oxide. Clay holds the most water, but not the most water for plants. A figure such as “100 years for each centimetre” is wrong without a range: rates differ by a hundredfold and more. Soil is the largest carbon store on land; the deep ocean holds far more.

## Common questions

### Were a soil’s layers laid down one on another?

They look stacked, but they were not laid down one on another.

See: [02 From Leaf Litter to Granite](https://world.lapla.si/en/soil/#profile)

### What are the five soil-forming factors?

A soil depends on five factors: climate, organisms, topography, parent material and time.

See: [07 Five Factors](https://world.lapla.si/en/soil/#factors)

### Why are some soils red and some dark?

A soil’s colour comes mostly from very fine coatings on its grains: humus is dark, iron oxides yellow, brown, orange and red.

See: [05 Where the Red Comes From](https://world.lapla.si/en/soil/#colour)

### Does soil wash away faster than it forms?

Conventionally ploughed fields erode at a median of about 1.5 mm a year, one to two orders of magnitude faster than soil forms.

See: [08 How Long a Centimetre Takes](https://world.lapla.si/en/soil/#time)

### Does clay hold the most water for plants?

Clay holds the most water, but grips most of it too tightly for plants; the soils with the most water for plants are silts and silt loams, sand has the least, and clay does not have more.

See: [09 Sand, Silt and Clay](https://world.lapla.si/en/soil/#texture)

## Sources

1. [USDA NRCS: Official Series Description of the Saw series (rev. 1999)](https://soilseries.sc.egov.usda.gov/OSD_Docs/S/SAW.html)
2. [USDA NRCS: Official Series Description of the Pacolet series (a forested typical pedon)](https://soilseries.sc.egov.usda.gov/OSD_Docs/P/PACOLET.html)
3. [USDA NRCS: Official Series Description of the Cecil series](https://soilseries.sc.egov.usda.gov/OSD_Docs/C/CECIL.html)
4. [USDA NRCS: Official Series Descriptions of the Fayette (forest, loess) and Tama (prairie, loess) series](https://soilseries.sc.egov.usda.gov/OSD_Docs/F/FAYETTE.html)
5. [USDA NRCS: Official Series Description of the Tama series](https://soilseries.sc.egov.usda.gov/OSD_Docs/T/TAMA.html)
6. [USDA Soil Science Division Staff (2017): Soil Survey Manual (Handbook 18)](https://www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Survey-Manual.pdf)
7. [USDA Soil Survey Staff (2022): Keys to Soil Taxonomy, 13th edition](https://www.nrcs.usda.gov/resources/guides-and-instructions/keys-to-soil-taxonomy)
8. [FAO (2006): Guidelines for Soil Description, 4th edition](https://www.fao.org/3/a0541e/a0541e.pdf)
9. [IUSS Working Group WRB (2022): World Reference Base for Soil Resources, 4th edition](https://www.isric.org/sites/default/files/WRB_fourth_edition_2022-12-18.pdf)
10. [P. Driessen et al., eds. (2001): Lecture Notes on the Major Soils of the World (FAO; how clay moves down and settles)](https://www.fao.org/3/y1899e/y1899e.pdf)
11. [R. G. Allen et al. (1998): Crop Evapotranspiration, FAO Irrigation and Drainage Paper 56, Table 19 (field capacity and wilting point by texture)](https://www.fao.org/3/x0490e/x0490e0c.htm)
12. [J. A. Kozak et al. (2005): Vadose Zone J. 4, 418 (Table 1 reprints the means by texture of Rawls, Brakensiek and Saxton, 1982)](https://www.ars.usda.gov/ARSUserFiles/3495/79.%20kozak-vzj05.pdf)
13. [USDA NRCS (2008): Soil Quality Indicators: Bulk Density](https://www.nrcs.usda.gov/sites/default/files/2023-01/Soil%20Quality-Indicators-Bulk%20Density.pdf)
14. [FAO et al. (2020): State of Knowledge of Soil Biodiversity](https://doi.org/10.4060/cb1928en)
15. [C. Darwin (1881): The Formation of Vegetable Mould through the Action of Worms](https://darwin-online.org.uk/converted/published/1881_Worms_F1357/1881_Worms_F1357.html)
16. [J. G. Canadell et al. (2021): IPCC AR6 WGI Chapter 5 (Figure 5.12, carbon stocks)](https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf)
17. [Hans Jenny (1941): Factors of Soil Formation](https://archive.org/details/dli.ernet.5806)
18. [D. R. Montgomery (2007): PNAS 104, 13268 (rates of soil erosion and soil production)](https://pmc.ncbi.nlm.nih.gov/articles/PMC1948917/)
19. [FAO and ITPS (2015): Status of the World’s Soil Resources](https://www.fao.org/3/i5199e/i5199e.pdf)
20. [FAO (2015): International Year of Soils poster; the World Soil Day page](https://www.fao.org/world-soil-day/en)
21. [USGS (2017): SIR 2017-5118, geochemical and mineralogical maps for soils of the United States (kaolinite, goethite, hematite)](https://pubs.usgs.gov/sir/2017/5118/sir20175118_element.php?el=908)
22. [J. D. Hem (1985): USGS Water-⁠Supply Paper 2254 (albite weathering to kaolinite)](https://pubs.usgs.gov/wsp/wsp2254/)
23. [British Geological Survey: Weathering](https://www.bgs.ac.uk/discovering-geology/geological-processes/weathering/)
24. [ISRIC World Soil Information: World Soil Reference Collection](https://isric.org/explore/world-soil-reference-collection/)
25. [UC Davis Soil Monolith Collection: How is a soil monolith made?](https://soilmonolith.ucdavis.edu/what-soil-monolith)

## Data

The block’s horizons, depths and colours follow the USDA Natural Resources Conservation Service’s Official Series Description of the Saw series (typical pedon, Rev. 07/1999); the litter, the A and the E of the forested block, their thicknesses, and every root, burrow, stone and mottle are this page’s model. Colours are drawn near their Munsell notation, not measured. The lab’s texture classes follow the rules of the USDA Soil Survey Manual (2017); its water figures are the typical ranges of FAO Irrigation and Drainage Paper 56, Table 19 (Allen et al., 1998), and its drainage the means of Rawls, Brakensiek and Saxton (1982) as reprinted by Kozak et al. (2005). The scale, the conversions and the ratios are computed here (docs/plates/soil/computed.py). The monolith, the Munsell booklet (its chips drawn here, not the printed chart), the trowel and the settling jar are this page’s own drawing. Material presets and the model kit as on the site’s other model plates (public/looks/CREDITS.md).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

https://creativecommons.org/licenses/by-nc/4.0/deed.en

## Cite this plate

Laplace’s Mini World, Plate 76, “The Soil Profile”, https://world.lapla.si/en/soil/, CC BY-NC 4.0, updated 2026-11-01.