Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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