Soils: Processes affecting soil characteristics and factors that affect soil formation.

1. SOILS
SOILS DIFFER BECAUSE OF A
COMBINATION OF FACTORS AND
PROCESSES THAT ARE AT WORK IN
AN AREA.
Geography

2. PROCESSES AFFECTING SOIL
CHARACTERISTICS
Recap:
What are the
characteristics of soils?

3. CHARACTERISTICS OF SOILS
Colour
Structure
Texture
Organic content
pH Value
Water content

4. PROCESSES AFFECTING SOIL
CHARACTERISTICS
 Eight major processes af fect soil characteristics:
1. Weathering
2. Humification
3. Leaching
4. Podzolisation
5. Gleying
6. Laterisation
7. Salinisation
8. Calcification
The factors of
climate, relief,
sock type, living
things and time
all influence
which of these
processes occur
in a region.

5. EXAM BRIEF
You are expected to know the
natural processes that affect soil
characteristics.

6. PROCESS 1: WEATHERING
 The breakdown of rocks due to mechanical and
chemical weathering.
 Rates of weathering are greatest in the world’s hot
and humid areas.
 Soil grains produced by weathering keep the
characteristics of the parent rock such as pH, texture
and colour.
 Chemical weathering processes such as carbonation
and oxidation can release nutrients such as calcium
and iron from the mineral grains. E.g. The chemical
weathering of limestone by carbonation leads to
soils rich in calcium.

7. PROCESS 2:
HUMIFICATION
• Method by which dead organic matter is converted
into humus by oxygen.
• Humification releases nutrients into the soil.
• Climate: Humification is accelerated by high
temperatures and high rainfall.
• In tropical regions, humification is very fast.
• In temperate climates such as Ireland
humificaiton is slow in the winter.
• In cold climates humification may stop
completely.

8. PROCESS 3: LEACHING
• The washing of minerals through soils.
• When rainwater falls on soil it dissolves the minerals
and plant matter in it.
• As the water soaks down through the soil, it carries
these dissolved minerals and nutrients with it. A
certain amount of leaching is needed to wash
humus into soil.
• Excessive leaching can rob the upper parts of the
soil of the nutrients that plants need to grow. This
soil will become infertile. A hardpan can form.

9. PROCESS 4: PODZOLISATION
 A type of leaching where rainwater is more acidic.
 Podzol soils form under coniferous forests. As these
forests die and decompose they add to the acidity of
the rainwater.
 The water seeping through the soil beneath this dead
vegetation becomes acidic and dissolves soil
minerals such as iron and aluminium as it passes
downwards.
 In time a hardpan layer forms and the soil becomes
waterlogged.

10. PROCESS 5: GLEYING
The soil is waterlogged and lacks
oxygen.
The pores between the soil peds are
filled with water for all or part of the
year. This prevents living things from
receiving oxygen. Little can grow in
such wet, oxygen poor (anaerobic)
conditions.
Heavy rainfall can lead to gleying.
Due to lack of oxygen, gley soils have
patches of blue/grey colouration.

11. PROCESS 6: LATERISATION
 A layer of iron oxides build up as a result
of rapid weathering and leaching.
 This process is most active in areas of
high rainfall and high temperatures. The
heavy rainfall dissolves and leaches
most minerals except for iron and
aluminium oxides. Oxidation folllows as
the iron is exposed to oxygen in the air
and latosols or red soils develop. If the
soil dries out it turns into a hard laterite.
 Occurs in tropical and equatorial
regions.

12. PROCESS 7: SALINISATION
 Occurs when mineral salts move up through the soil instead of
down into the soil.
 Can happen in hot desert areas where rainfall is low and the
amount of water evaporating out of the soil is greater than
rain falling onto it.
 Evaporation causes salts in ground water to rise through the
soil and collect in the upper layers. Salt is deposited on the
surface as a hard white crust.

13. PROCESS 8: CALCIFICATION
 Process where calcium carbonate is concentrated near the
surface of the soil.
 Occurs in regions of low rainfall.
 The amount of water drawn up through the soil by plants
(transpiration) may be greater than the precipitation falling
on the soil. As a result, calcium carbonate builds up in the
upper layer of the soil.
 Calcium carbonate is useful for plants and these soils often
have lush grass growth.

14. EXAM BRIEF
A typical exam question:
“Examine the factors/processes that
influence soil characteristics.”

15. FACTORS AFFECTING SOIL FORMATION
1. Climate
2. Relief
3. Parent Material
4. Living Things
5. Time

16. FACTORS AFFECTING SOIL FORMATION:
1. CLIMATE
 Climate is the single most important factor in soil
formation.
 Climate influences:
 The rate of weathering which in turn controls soil
thickness.
 The rate of humification – the breakdown of organic
matter.
 The amount of leaching – the process where minerals are
washed downwards leaving top layers of soil without
minerals.
 Soils that have developed in response to
particular climate conditions are called zonal
soils.

17. FACTORS AFFECTING SOIL
FORMATION:
2. RELIEF
 Relief can influence the depth and drainage of a soil.
 In general, sloping land is well drained and soils are
quite dry.
 However, mountainous terrain tends to have less soil
because:
 Gravity removes soil from steep slopes in mass
movements.
 Areas of high relief have higher precipitation rates and
lower temperatures, hence there is more leaching and
slower rates of weathering.
 Lowlands are usually warmer and flatter which
encourages a build up of soil.

18. FACTORS AFFECTING SOIL FORMATION:
3. PARENT MATERIAL
 The type of bedrock, be it granite or
limestone, determines the amount and
type of minerals present in soil.
 These minerals such as calcium and
potassium provide nutrients and food
for plants.
 The parent material also controls a
soil’s depth, colour, texture and pH
value.

19. FACTORS AFFECTING SOIL FORMATION
4. LIVING THINGS
 Living things influence soil fertility.
 Living things within soil can increase fertility and aeration of
a soil and prevent soil erosion.
 Earthworms help to aerate soil by moving about in it.
 Insect larvae burrow through soil and create air spaces for
plant roots. Water can collect in these burrows and keep soil
moist.
 Fungi and bacteria add nutrients by decomposing dead
material.

20. FACTORS AFFECTING SOIL FORMATION
5. TIME
It takes about 400 years for 1 cm of soil to
form. It takes time for soil to develop fully in
response to the processes that occur due to
climate, relief, parent material and living
things.

21. EXAM BRIEF
A typical exam question:
“Examine the factors that influence
the development of soils.”