Classification of Soils and Soil Formation
Senior 6 · Geography
Classification of Soils and Soil Formation
Lesson 2 of 3
25 min
In the 1880s the Russian scientist Vasily Dokuchaev noticed that as he travelled south across the Russian plain, from forest to grassland to dry steppe, the soils changed in broad belts that followed the climate and vegetation zones. This idea, that the major soils of the world are arranged in zones, is the basis of the zonal, intrazonal and azonal classification still taught in East African schools. Modern soil scientists name soils using the international World Reference Base for Soil Resources (WRB), prepared with the FAO, and this lesson gives the WRB name alongside the traditional one.
Definition
Zonal soils: mature soils whose character is set mainly by the climate and vegetation of a large region. Example: podzols of the cool, wet coniferous forest belt.
Intrazonal soils: soils in which a local factor, such as waterlogging, salt or limestone, overrides the climate, so they can occur within any zone. Example: gley soils of marshes.
Azonal soils: young, immature soils without well-developed horizons, because there has been too little time or because material keeps being added or removed. Example: alluvial soils on a river floodplain.
Figure 3: three zonal soil profiles.
| Traditional name (WRB name) | Climate and vegetation | Main process and features | Distribution |
|---|---|---|---|
| Laterite or ferrallitic soil (Ferralsols) | Hot, wet equatorial and tropical forest and savanna | Laterisation: deep, red or yellow, leached, low in nutrients; iron and aluminium oxides; can harden into ironstone | Amazon and Congo basins, parts of West and East Africa, South-East Asia |
| Podzol (Podzols) | Cool, humid; coniferous forest and heath | Podzolisation: acid, bleached E horizon, iron pan in B | Northern Canada, Scandinavia, northern Russia |
| Brown earth (Cambisols, Luvisols) | Cool, temperate; deciduous forest | Mild leaching, well-mixed, moderately fertile | Western and central Europe, eastern USA |
| Chernozem, black earth (Chernozems) | Semi-arid, continental grassland | Calcification: very deep, dark, humus-rich A; lime layer; very fertile | Ukraine and southern Russia, the North American prairies, the Argentine pampas |
| Desert soils (Calcisols, Gypsisols, Arenosols) | Hot or cold deserts | Little organic matter; salts, lime or gypsum near the surface; often sandy | Sahara, Kalahari, Arabia, central Australia |
| Tundra soils (Cryosols) | Very cold; mosses and lichens | Permafrost below; waterlogged in summer | Arctic fringes of North America and Eurasia |
| Group | Traditional name (WRB name) | Cause | Example of where found |
|---|---|---|---|
| Intrazonal: hydromorphic | Gley soils (Gleysols); peat (Histosols) | Waterlogging; plant remains accumulate where they cannot decay | Valley-bottom marshes; swamps |
| Intrazonal: halomorphic | Saline soils (Solonchaks); sodic soils (Solonetz) | Salt drawn up by evaporation | Dry lands, badly drained irrigated fields, lake basins |
| Intrazonal: calcimorphic | Rendzina (Rendzic Leptosols) | Thin, dark soil on limestone or chalk | Chalk downs of England |
| Azonal | Alluvial soils (Fluvisols) | New layers of silt added by floods | Floodplains and deltas, such as the Nile Delta |
| Azonal | Lithosols (Leptosols) | Thin, stony soil on steep slopes or hard rock | Mountains, steep hill crests |
| Azonal | Regosols; young volcanic soils (Andosols) | Loose, recent sand, dune or volcanic ash with little horizon development | Sand dunes; young volcanic areas |
The traditional scheme and the WRB do not match one-to-one. Andosols, for example, become deep and very fertile as volcanic ash weathers, and soil scientists sometimes treat them as intrazonal rather than azonal. Use the scheme your examination expects, and give the WRB name where you can.
Worked example 1: classifying from evidence
Problem. Classify each soil as zonal, intrazonal or azonal, and name it. (a) A grey soil with rusty mottles, in a valley bottom that is wet all year. (b) A layered silty soil on a floodplain with no clear horizons. (c) A very deep, red soil under rainforest in the Congo Basin.
Solution. (a) Waterlogging overrides climate: intrazonal gley soil (Gleysol). (b) Too young, because floods keep adding silt: azonal alluvial soil (Fluvisol). (c) Formed by the regional hot, wet climate: zonal ferrallitic soil (Ferralsol).
Worked example 2: why rainforest soils are infertile
Problem. A rainforest has huge trees, yet when it is cleared for farming, crop yields fall sharply within two or three years. Explain.
Solution. Under constant heat and heavy rain, weathering and leaching are intense: most nutrients have been washed out of the deep Ferralsol, leaving iron and aluminium oxides. The forest survives by recycling nutrients quickly from leaf litter through a thin, humus-rich topsoil. Clearing removes the litter and exposes the topsoil to rain, which leaches or erodes it, so the small nutrient store is soon exhausted.
Warning
Common mistakes
Tip
Exam technique. Describe a world soil with four points: climate and vegetation, main process, profile features, and two named areas where it is found.
Practice
Summary
Sources and credits
Photos: Podzol (Ailith Stewart, CC BY-SA 2.0); Chornozem typovyi (Serhey0211994, CC BY 4.0); both via Wikimedia Commons.
Figure 3, the worked examples and the exercises were drawn and written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
