The Dirt of Forestry
SOIL FORMATION
SOIL

Soil is very important to us on this planet.

We would not be able to survive without it.

It is widely distributed, but it only forms a very
thin veneer on the Earth’s surface, compared
to the thick rock layers within the Earth

Soil is important in a multirole nature which
means it has a variety of definitions.
 Engineers–would
define it as the material upon
which structures are built
 Farmers – would define it as material that supports
plant growth
 Geology – however define it as material that can
support vegetation
COMPOSITION OF SOIL

Soil is a mixture of mineral grains, organic
material, water, and air.

Mineral Grains
 These
include clay, sand, silt, and rock fragments created
by the weathering (both mechanical and chemical) of
underlying parent rock.

Organic Materials
 Is
in the form of humus, which is unrecognizable
decaying organic material, i.e. when a leaf ceases to look
like a leaf because of decomposition
 Living
organisms are also part of the organic component
of soils
 There are as many as 3 billion organisms in one gram of
soil which include bacteria, fungi, insects worms, etc.

Water
 This
enters the soil from infiltration of surface water or
precipitation

Air
 It
enters the soil from diffusion from the atmosphere and
the work of organisms.
 The gases present in soil include carbon dioxide, oxygen
and hydrogen
FORMATION OF SOIL

This is an extremely slow process as on
average, 2.5cm of soil can form in 100 years

It all begins with the weathering of rocks where
a thin layer of sediment (regolith) can form on
the surface of the rock. Organic material, water
and air are added to the regolith, producing
soil.
Soil Formation
TYPES OF SOIL

A soil can be classified as either:
 Residual
- soils that develop in place on the
bedrock from which it was formed.
 Transported
– soils that have been moved (by
glaciers, streams, wind, and/or gravity) and lie on
bedrock other than the one from which they formed
RESIDUAL SOIL

Residual soils develop distinct layers or horizons.

transported soils may not form horizons because of
the transportation that was involved.

Soil horizons differ from one another in texture,
structure, composition and colour.

Water filtration is what causes the horizons to
develop.
HORIZONS

The soil horizons together comprise what is called the
soil profile.

Soil profiles vary with the type of environment and soilforming conditions present.

Most soil profile would contain four distinct horizons,
the O-horizon, the A-horizon, the B-horizon, and Chorizon
O-HORIZON




This is the upper most
zone and is not present in
all soils.
It is dominated by organic
matter and is the most
biologically active.
It contains little in the way
of minerals being that it is
the furthest from the
bedrock.
It is around a few
centimetres thick.
A-HORIZON



It is commonly called ‘topsoil’.
This horizon is the zone of
leaching. Water (which is
slightly acidic) dissolves ions
like calcium, iron, and
aluminum and carries them
away.
There is an amount of organic
matter in the soil giving it a
grey to black colour.
B-HORIZON


Also called the zone of
accumulation, as this is
where the ions taken
from the A-horizon end
up.
There are few organisms
and less organic matter
in this horizon
C-HORIZON

This horizon is of partially
disintegrated and
decomposed parent rock
material.
CLORPT



Characteristics of soils can differ dramatically from
one location to another.
These characteristics include colour, fertility, and
thickness.
The environmental factors affecting a soil’s
characteristics are; climate, organic activity, relief,
parent material, and time. Giving the acronym
CLORPT.
CLIMATE



This influence is the most influential factor, as rainfall
and temperature very important.
The precipitation affects the degree of leaching of
soluble ions, which in turn affects fertility
Temperature and water dictate the weathering and
amount of organic material present.
ORGANIC ACTIVITY

The amount of organic activity
affects the fertility of the soil.

Generally the more organic activity
present, the more fertile the soil,
with one exception.
RELIEF



The slope of the land on
which soil develops is called
relief.
Generally, the greater the
relief (the steeper the slope)
the thinner the soil profile.
Valleys floors are usually
given thick soils, while
hillsides are eroded.
PARENT MATERIAL

The rock from which the soil originated will determine
the mineral matter and texture of the soil.

Examples:

Granite-Contains feldspars that are converted to clays during weathering

Quartzite- consists of quartz, and upon weathering remains as sand,
therefore sandy soil.
TIME

The effect of time given
that all other factors are
equal would be that more
time would result in more
weathering , and organic
activity which would give
the thicker and more
fertile soil.