Soil-Forming Processes – Soils in Africa and the World (Senior 3 Geography) | Ganzaa

Soils in Africa and the World

Lessons

Senior 3 · Geography

Soils in Africa and the World

4 lessons

Lesson 2 of 4

Soil-Forming Processes

25 min

Let's begin

The five factors decide which soil forms; the soil-forming processes are how it forms. They are the things that actually happen inside the soil: rock breaking down, dead leaves turning into humus, water carrying minerals down or drawing them up. Most of them depend on one simple question you can answer from a climate table: does more water fall on the soil than evaporates from it?

Three basic processes

Weathering (you studied it in S2) breaks rock into smaller pieces and new minerals such as clay. Physical weathering cracks rock without changing it; chemical weathering changes its minerals, and is fastest where it is hot and wet; biological weathering is done by roots and living things.

Definition

Humification: the breaking down of dead plant and animal matter by bacteria, fungi and soil animals into humus, a dark, soft material that holds water and plant nutrients.

Leaching: the washing of soluble minerals, such as calcium, potassium and sodium, down through the soil and out of reach of plant roots by water soaking downwards. It happens where rainfall is greater than evaporation.

Eluviation and illuviation: the washing of fine clay, humus and iron out of an upper horizon (eluviation) and their deposition in a lower horizon (illuviation).

Figure 3. Leaching and capillary rise. Drawn by Ganzaa.

Compare the yearly rainfall (P) with the water that could evaporate (E). Where $P > E$ , water moves down and leaching dominates. Where $E > P$ , water is drawn up through tiny spaces between soil particles (capillary action) and evaporates at the surface, leaving dissolved minerals behind.

Processes that make the great soil types

Process	Where	What happens	Result
Laterization (ferralitization)	Hot, wet tropics	Very fast chemical weathering and strong leaching remove silica and plant nutrients; iron and aluminium oxides stay behind	Deep red or yellow latosols; a layer rich in iron may harden into laterite
Podzolization	Cool, wet coniferous forest	Acid water from needle litter dissolves iron, aluminium and humus from the upper soil and deposits them lower down	Podzol: an ash-grey leached horizon over a red-brown, sometimes cemented, iron pan
Calcification	Dry grasslands where evaporation is slightly greater than rainfall	Weak leaching; calcium carbonate collects in the lower soil; grass roots add much humus	Chernozem (black earth) and chestnut soils, among the most fertile soils on Earth
Salinization	Hot, dry areas; badly drained irrigated land	Capillary rise brings salty water up; it evaporates and leaves salt at the surface	Salty crusts; few crops can grow
Gleying	Waterlogged ground, e.g. valley floors and swamps	Lack of oxygen changes iron compounds	Grey or blue-grey soil with rusty orange spots (mottles)

Figure 4. Three typical soil profiles and the climates that make them. Drawn by Ganzaa.

A podzol uncovered by a digger in Scotland, 2006. Below the dark top layer you can see the pale, leached horizon and, beneath it, the orange-brown layer where iron has been deposited. Photo: Ailith Stewart, CC BY-SA 2.0, via Wikimedia Commons (Geograph).

Worked example

Worked example 1: which way will water move? Station A has a yearly rainfall of 2,000 mm and potential evaporation of 1,300 mm. Station B has 450 mm of rain and potential evaporation of 1,800 mm. Which process is likely to dominate in each soil?

Station A: $P - E = 2000 - 1300 = +700\,\text{mm}$ . Water surplus, so water moves down: leaching; if it is also hot, laterization.
Station B: $P - E = 450 - 1800 = -1350\,\text{mm}$ . Water deficit, so water is drawn up: capillary rise, giving calcification or, if very dry or badly irrigated, salinization.

Worked example

Worked example 2: why laterite matters. A farmer clears a patch of tropical forest. After a few years the crops fail and the ground becomes hard. Explain.

In the forest, most nutrients are held in living plants and in the thin humus layer, which is renewed by falling leaves.
After clearing, heavy rain falls straight on the bare soil and leaches the remaining nutrients away; there are no new leaves to make humus.
The soil left is mostly iron and aluminium oxides. Exposed to sun and air, an iron-rich layer can harden into laterite, which roots and hoes cannot break.

Warning

Leaching is not erosion. Erosion removes the soil itself, washing or blowing it away from the surface. Leaching removes dissolved minerals through the soil, leaving the soil in place but poorer. Mixing them up is a common exam error.

Tip

Rule of thumb: P bigger than E, water goes down (leaching, laterization, podzolization). E bigger than P, water comes up (calcification, salinization). Waterlogged, water stays (gleying).

Practice

What is humus, and how is it formed?
Define leaching and state the climatic condition under which it happens.
Explain the difference between leaching and soil erosion.
Name the process that forms (a) a podzol, (b) a chernozem, (c) a latosol.
A place has 1,600 mm of rain and potential evaporation of 2,100 mm a year. Calculate P − E and state which way water moves through the soil.
Why does salinization often happen on irrigated land in hot, dry countries?
Describe the profile of a podzol, naming its horizons.
Explain why tropical rainforest soils lose their fertility quickly when the forest is cleared.

Show answersHide answers

A dark, soft organic material in soil, formed when bacteria, fungi and soil animals break down dead plants and animals (humification).
The washing of soluble minerals down through the soil, out of reach of roots, by water soaking downwards. It happens where rainfall is greater than evaporation.
Leaching removes dissolved minerals through the soil and leaves the soil in place; erosion removes the soil itself from the surface, by water or wind.
(a) Podzolization; (b) calcification; (c) laterization (ferralitization).
$1600 - 2100 = -500\,\text{mm}$ : a deficit, so on balance water is drawn upwards by capillary action.
Irrigation water raises the water table; in the heat, water is drawn up and evaporates fast, leaving dissolved salts behind at the surface, especially where drainage is poor.
Top: O horizon of needle litter and A horizon of dark humus; then a pale, ash-grey leached E horizon; then a red-brown B horizon where iron and humus are deposited, sometimes as a hard iron pan; then C, the parent rock.
Most nutrients are in the living forest and the thin humus layer. Clearing stops the supply of leaves; heavy rain leaches the remaining nutrients from the bare soil; the leftover iron- and aluminium-rich soil is poor and may harden into laterite.

Sources and credits

Leaching (agriculture), Laterite, Podzol, Chernozem and Soil salinity: Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
Photo: Podzol, Ailith Stewart, Geograph Britain and Ireland, via Wikimedia Commons, CC BY-SA 2.0.

Figures 3 and 4, the worked examples and the practice exercises are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three basic processes

2Processes that make the great soil types

Three basic processes

Processes that make the great soil types