PEDOLOGY

The Study of Soil

PEDOLOGY

Factors that influence the formation of

SOIL

Soil Forming Factors

Parent Rock

Climate

Vegetation & Living Organisms

Relief/Topography

Time

Humans

Climate:

Moisture

Heavy rain

Temperature

Wind

Sunshine

Vegetation & Living Organisms

Vegetation influences runoff and the

amount of groundwater

Influences the types of nutrients in humus

Vegetation influences animals living in an

area

Animals influence type of humus found in

the soil

Relief

Slope direction influences temperatures

and moisture

Gradient influences drainage – steep

slopes drier with less groundwater than

gentle slopes

Valleys: frost pocket

Time

The prolonged action of the soil-forming

processes of weathering, climate,

vegetation, living organisms and good soil

drainage result in mature soils with clear

profiles (Zonal soils)

Insufficient of any of the above = Azonal

soils (immature soils with an unclear

profile)

Humans

Farming practices i.t.o. irrigation,

fertilizing, ploughing, clearing and re-

claiming land all cause changes in soil

conditions

Over-grazing, over-cropping,

deforestation and burning of the land all

result in soil erosion and desertification

SOIL PROPERTIES

1. MINERALS

2. SOIL TEXTURE

i.e.. Coarseness or fineness of soil

Particle size ranges for sand, silt, and clay.

Type of Mineral Particle

Size Range

Sand 2.0 - 0.06 millimetres

Silt 0.06 - 0.002 millimetres

Clay less than 0.002 millimeters

Texture (size of particles)

Texture determines drainage quality of the soil:

Clay has fine texture and poor drainage

Silt has medium texture and some water drains through

it

Sand has rough texture and drains easily

A loam has equal proportions of clay, silt and sand.

(clay loam; silt loam & sand loam)

Soil Characteristics

TEXTURE TRIANGLE

3.SOIL STRUCTURE

Structure (how individual soil particles are grouped)

Depends on the shape of the individual soil

particles.

Particles may be crumb, prism, plate or block-

shaped

Structure influences:

Rate of water absorption

Susceptibility to erosion

Ease of cultivation

Soil Characteristics

4. Soil Characteristics

Colour

Indicates:

conditions of soil

formation

Type of vegetation

Type of parent rock

Grey: arid areas; low

humus content

Red: high iron content

Dark Brown: grasslands;

high humus content

Whitish: excessive

calcification

Bluish: marshland; weak

drainage

COLOUR

SOIL HIGH IN IRONSOIL HIGH IN ACIDITY

5. ACIDITY

NEUTRAL = A BALANCE BETWEEN PRECIPITATION AND EVAPORATION

ACIDIC = HIGH RAINFALL AREAS – LEACHING OCCURS

pH index

Acid soils have pH of less than 7 and are caused by

a lot of leaching

Alkaline soils have a pH of more than 7

Soil Characteristics

SOIL PROFILES

A soil profile

is a vertical

column or

section of soil

that shows

the horizons

of soil

THE LAYERS ARE CALLED HORIZONS

SOIL PROFILE

Soil Profile

O: humus is the decayed organic material (plant & animal matter) that forms

the top layer of the soil. This layer provides the nutrients to the soil

A: true soil is the fertile topsoil made up of finer soil particles and nutrients.

This layer is important for farming.

B: subsoil is the coarser soil particles and is usually less fertile

C: disintegrated parent material (rock), which is rock that has been partially

broken down through weathering processes.

R: regolith or parent material (rock), which is the solid rock that has not

been broken up by weathering processes (often not considered part of the

Horizon)

LEACHING: occurs when excessive

rainfall washes nutrients from A horizon

towards the bottom of the soil

Eluviation:

The downward movement of nutrients

Illuviation:

The deposition of minerals

SALINISATION:

(calcification) due to

excessive evaporation;

evaporation causes lime

or calcium to move

upwards as a result of

capillary action (against

gravity). A hard

crust/layer (calcrete)

forms and restricts

infiltration

Soil Profile: Definitions

1.Humus: organic matter found in soils resulting from

decay/decomposition of plant and animal remains

2.Solum: true soil/topsoil consisting of both A and B

horizons

3.Leaching: downward movement of minerals and

soluble materials with water infiltrating into the soil

4.Eluviation: washing down of material or removal of

organic and mineral matter from the A horizon

Soil Profile: definitions (cont.)

5. Illuviation: the process of deposition of organic

and mineral matter (from eluviation) in the B horizon

6. Regolith: recently weathered material and un-

weathered parent rock – not considered to be part

of the soil

Soil Erosion:

Soil Erosion on steep slopes removes

some of the topsoil, which results in a

thin A horizon.

The soil eroded from steep slopes is

deposited at the base of the hill and

so soils found at the bottom of the hill

generally have a thicker A horizon