Classification of Soils and Soil Formation
Senior 6 · Geography
Classification of Soils and Soil Formation
Lesson 1 of 3
25 min
Walk down almost any hill in Rwanda from its crest to the valley floor and the soil changes under your feet: firm and moderately deep on the top, thin and stony on the steepest slope, deep and dark at the foot, and grey and waterlogged in the valley-bottom marsh. The rock beneath may be the same all the way down. What changes is the slope, the drainage and the material washed downhill. Soil is not simply broken rock; it is a living, layered body built by several factors working together over time. This lesson explains those factors and the processes they drive.
Definition
Soil: the thin layer of loose material on the Earth's surface, made of mineral particles, organic matter (humus), water and air, in which plants grow. A typical good loam is roughly 45% minerals, 5% organic matter, 25% water and 25% air by volume.
Humus: dark, decomposed organic matter that holds water and nutrients.
Soil profile: a vertical section through a soil from the surface to the parent rock, showing its horizons (layers).
Figure 1: a soil profile and its horizons.
Definition
The five soil-forming factors, often remembered as CLORPT: CLimate, Organisms, Relief, Parent material and Time.
| Factor | How it shapes the soil |
|---|---|
| Climate | Temperature and rainfall control the rate of weathering and decay and the direction of water movement. Where rainfall exceeds evaporation, water moves down and leaches the soil; where evaporation exceeds rainfall, water moves up and salts collect near the surface. |
| Organisms | Plants supply organic matter; earthworms, termites and burrowing animals mix the soil; bacteria and fungi decompose litter into humus. People change soils by clearing, ploughing, fertilising and draining. |
| Relief (topography) | Steep slopes lose soil to erosion and have thin soils; gentle slopes and foot-slopes collect material and have deep soils; valley bottoms are waterlogged. Slope direction (aspect) affects warmth and moisture. A sequence of soils down one slope is a catena (Figure 2). |
| Parent material | The rock or deposit the soil forms from sets its texture and minerals: granite and quartzite give sandy, acid soils; basalt and volcanic ash give clay-rich, fertile soils; limestone gives thin, alkaline soils. |
| Time | Young soils on new alluvium or lava have few horizons; old soils are deep and well developed, and in the humid tropics may be very old and heavily weathered. |
Figure 2: a soil catena on a hillslope.
Definition
Eluviation: the washing out of fine particles and dissolved substances from an upper horizon. Illuviation: their deposition in a lower horizon. Leaching: the removal of soluble nutrients by water moving down through the soil.
| Process | What happens | Where |
|---|---|---|
| Weathering | Rock breaks down to mineral particles and clay | Everywhere; fastest where hot and wet |
| Humification | Organic matter decays to humus | Fastest in warm, moist soils |
| Podzolisation | Acid water from conifer litter leaches iron, aluminium and humus from the A and E horizons into the B, leaving a bleached E horizon and sometimes an iron pan | Cool, humid climates under coniferous forest or heath |
| Laterisation (ferrallitisation) | Intense weathering and leaching remove silica and bases; iron and aluminium oxides remain, colouring the soil red or yellow | Hot, humid tropics |
| Calcification | Rainfall is too low to leach calcium fully; calcium carbonate collects in the B horizon | Semi-arid grasslands |
| Salinisation | Evaporation draws salty water up; salts crust the surface | Arid lands, badly drained irrigated land |
| Gleying | Waterlogging cuts off oxygen; iron is reduced, giving grey-blue colours with rusty mottles | Valley bottoms, marshes, clay lowlands |
Worked example 1: naming a soil texture
Problem. A 2,000 g soil sample is separated into 900 g of sand, 700 g of silt and 400 g of clay. Find the percentage of each and name the texture. (Loam: about 7–27% clay, 28–50% silt and under 52% sand.)
Solution.
All three fall within the loam ranges, so the soil is a loam: it holds water and nutrients well but still drains and is easy to work, which makes it ideal for farming.
Worked example 2: rainfall, evaporation and the direction of water
Problem. Place A has 1,200 mm of rain and 900 mm of potential evaporation a year; place B has 350 mm of rain and 1,600 mm of potential evaporation. Predict the main soil process at each.
Solution. A: surplus, so water drains down through the soil: leaching dominates, and nutrients are lost. B: deficit, so water is drawn up by evaporation: calcification or, if salts are present, salinisation dominates.
Warning
Common mistakes
Tip
Exam technique. When explaining a soil, link each factor to a process and the process to a feature you can see in the profile, for example: high rainfall → leaching → pale E horizon.
MooMooMath and Science: the soil profile and its horizons. Compare the horizons named with Figure 1.
Practice
Summary
Sources and credits
Figures 1 and 2, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "Soil Profile and Soil Horizons", MooMooMath and Science, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
