1. Mineral Matter: Rock particles from the bedrock
and weathered rock.
2. Air: found in the pore spaces between rock grains.
3. Water: Also found in pore spaces.
4. Organic Material:
Humus – a black sticky gel produced from decaying
leaves and animals.
Living organisms – earthworms, beetles, fungi,
How are Soils Formed?
1. Solid rock particles
break down from
from the rock grains.
remains of plants to
form humus which
further enriches the
3. Seeds are blown or
carried onto the soil
grains and may grow
into plants that enrich
the soil when they die.
5. The cycle continues
until soils reaches
maximum fertility given
the climate it is in.
Show the type of soil in the region.
Defined as a vertical section of soil from the
ground surface downwards to where soil
meets the underlying rock.
Composed of a number of distinctive layers
The key to understanding the processes
involved in soil development.
O = Organic horizon
This has a high
percentage of organic
matter – leaves, plants
and dead animals.
Humus is formed as the
dead organic matter
Also called the litter
Known as topsoil.
A dark organic material
mixed with mineral soil
It is a source of plant
nutrients and contains
the majority of plant
Also called the subsoil.
The B horizon is the most
widely used to identify
May result purely from
the weathering of
underlying rock or from
of materials from
Lies just above the
The overlying soil
horizons often develop
from the C horizon.
The characteristics of a soil influence the
The colour of a soil indicates the amount of
organic material in it.
Humus-rich soils = black / dark drown.
Leached soils = white.
Dark soils absorb more sunlight and warm
up more quickly, which aids seed
Pale coloured soils reflect sunlight and are
slower to warm up.
2. Soil Structure
soil grains or particles.
This refers to the shape of the
Soil grains or particles cluster together to form small
lumps called peds.
The shape of these peds indicates the structure of the
Common soils structures:
1. Crumb/ granular
Crumb / Granular
Loam soils have this structure.
The peds are small rounded clumps of
soil particles similar to breadcrumbs in
This structure is good for holding air
Sandy soils have this structure.
Peds are closely packed angular blocks.
Well drained but can be compacted
easily - plants have difficulty growing
when this occurs.
Adding humus to this soil can improve
structure for plant growth.
Forms in clay soils.
Soil peds are arranged in thin
Prevents good drainage of water
through the soil.
Plants have difficulty growing in
this soil type.
How a soil feels when you touch it.
Determines whether a soil can support
There are 4 main textures:
Have gritty texture (85 – 100% sand).
Formed from weathered rocks such as limestone
Waterlogging is rare in sandy soils – they are very
Watering and feeding of plants (fertilising) is
needed regularly because the nutrients drain away
Tend not to stick together when wet.
Contain 40-100% clay.
Very sticky and lumpy when wet.
Rock hard when dry.
Made up of very fine particles with few air
Hard to work and often waterlogged.
Rich in nutrients so plants will grow well if
Contain 40-100% silt.
Composed of minerals (mainly quartz)
and fine organic particles.
Have more nutrients than sandy soils
and do not tend to become
Have a smooth and powdery texture
Contain roughly equal amounts of sand, silt
Usually drain well and retain enough
moisture to support roots.
Light and easy to dig.
Most plants will grow in loam soils.
Feels crumbly in texture.
4. Organic / Humus Content
This refers to the amount of dead leaves,
roots, plants and animals that have rotted
away to form humus.
Humus binds the soil together limiting soil
Humus content affects the colour, texture
and structure of soils.
Soils rich in humus tend to be dark with a
good crumb structure and loam texture.
5. pH Value
pH value = a measure of a soil’s acidity or alkalinity.
pH of a soil is affected by the parent material (rock it
Most plants prefer a slightly acidic soil with a pH
value of 6.5.
Very acidic soil discourages the presence of living
things which reduces humus content of soils.
6. Water content
Water is important for soils because it:
Enables plants to absorb nutrients dissolved from the
Enables the survival of micro-organisms responsible for
Reduces soil erosion by wind.
Enables formation of soil horizons.
Binds soil particles together.
6. Water content
Some soils allow percolation and
leaching to occur easily, e.g. sandy
Other soils, like clays, limit these
processes and become waterlogged.