5.1 INTRODUCTION TO SOIL SYSTEMS
Significant ideas
● The soil system is a dynamic ecosystem that has inputs, outputs, storages and flows.
● The quality of soil influences the primary productivity of an area.
Applications and skills
● Outline the transfers, transformations, inputs, outputs, flows and storages within soil systems.
● Explain how soil can be viewed as an ecosystem.
● Compare and contrast the structure and properties of sand, clay and loam soils, with reference to a
soil texture diagram, including their effect on primary productivity.
Understandings
1. The soil system may be illustrated by a soil profile that has a layered structure (horizons).
2. Soil system storages include organic matter, organisms, nutrients, minerals, air and water.
3. Transfers of material within the soil, including biological mixing and leaching (minerals dissolved in
water moving through soil), contribute to the organization of the soil.
4. There are inputs of organic material including leaf litter and inorganic matter from parent material,
precipitation and energy. Outputs include uptake by plants and soil erosion.
5. Transformations include decomposition, weathering and nutrient cycling.
6. The structure and properties of sand, clay and loam soils differ in many ways, including mineral and
nutrient content, drainage, water-holding capacity, air spaces, biota and potential to hold organic
matter. Each of these variables is linked to the ability of the soil to promote primary productivity.
7. A soil texture triangle illustrates the differences in the composition of soils.
Soil
Soil is a complex ecosystem made up of minerals, organic material, gases and liquids which forms the habitat
for many animals and plants.
● all the food that we consume depends on soil.
● holds water & mineral nutrients that plants depend upon.
● plants grow in soil and we eat plants that grow directly in the soil or animals that have eaten plants.
● habitat for many organisms in some ecosystems, below-ground biomass > above-ground biomass.
● enormous filter for any water that passes through it, often altering the chemistry of that water.
, ● store and transfer heat so affect atmospheric temperature, which in turn affect the interactions between
soil and atmospheric moisture.
● part of the lithosphere where life processes and soil-forming processes both take place.
○ five spheres of the Earth: atmosphere, hydrosphere, lithosphere, biosphere, plus pedosphere
(soil sphere).
○ pedosphere links biosphere & lithosphere; influenced by atmosphere, hydrosphere, lithosphere.
Components
The exact mix of these four portions give soil its character:
● mineral particles mainly from the underlying rock.
● organic remains that have come from the plants and animals.
● water within spaces between soil grains.
● air also within the soil grains.
It is also a habitat for plants and animals. Soil is a highly porous medium typically with a 50:50 mix of solids and
pore spaces. The pore spaces contain variable amounts of water and air.
Soil profile
A record of the processes that have created the soil, its mineral composition, organic content, and chemical and
physical characteristics such as pH and moisture.
Horizons
Soil cross-section profile is changed over time as organic material leaches downwards and mineral materials
move upwards.
● top layer is often rich in organic material while the lower layers consist of inorganic material.
● inorganic material is derived from the weathering of rocks.
● materials are sorted and layers are formed by water carrying particles either up or down –
translocation.
● in hot, dry climates (P<E) water is evaporating at the soil surface and water from lower soil layers
moves upwards.
○ it dissolves minerals and takes them to the surface, where the minerals are left behind when the
water evaporates.
○ this also happens in irrigation and is called salinisation.
● in colder and wetter climates (P>E) water flows down in the soil.
, ○ it dissolves minerals and transports them downwards, causing leaching to occur.
P: precipitation E: evaporation
O Horizon
● uppermost layer of newly added organic material – comes from organisms that die on top of the soil.
● fungi, bacteria and other animals will start to decompose the dead material.
A Horizon:
● upper layer where humus builds up.
● humus forms from partially decomposed organic matter and is often mixed with fine mineral particles.
● incomplete decomposition forming a layer of dark brown/black organic material – the humus layer.
● in normal conditions, organic matter decomposes rapidly, releasing soluble minerals that are then taken
up by plant roots.
● waterlogging reduces the number of soil organisms, resulting in a build-up of organic matter and can
lead to peat formation.
B Horizon:
● where soluble minerals and organic matter tends to be deposited from the layer above.
● in particular, clay and iron salts can be deposited in this horizon.
C Horizon:
● mainly weathered rock from which the soil forms.
Significant ideas
● The soil system is a dynamic ecosystem that has inputs, outputs, storages and flows.
● The quality of soil influences the primary productivity of an area.
Applications and skills
● Outline the transfers, transformations, inputs, outputs, flows and storages within soil systems.
● Explain how soil can be viewed as an ecosystem.
● Compare and contrast the structure and properties of sand, clay and loam soils, with reference to a
soil texture diagram, including their effect on primary productivity.
Understandings
1. The soil system may be illustrated by a soil profile that has a layered structure (horizons).
2. Soil system storages include organic matter, organisms, nutrients, minerals, air and water.
3. Transfers of material within the soil, including biological mixing and leaching (minerals dissolved in
water moving through soil), contribute to the organization of the soil.
4. There are inputs of organic material including leaf litter and inorganic matter from parent material,
precipitation and energy. Outputs include uptake by plants and soil erosion.
5. Transformations include decomposition, weathering and nutrient cycling.
6. The structure and properties of sand, clay and loam soils differ in many ways, including mineral and
nutrient content, drainage, water-holding capacity, air spaces, biota and potential to hold organic
matter. Each of these variables is linked to the ability of the soil to promote primary productivity.
7. A soil texture triangle illustrates the differences in the composition of soils.
Soil
Soil is a complex ecosystem made up of minerals, organic material, gases and liquids which forms the habitat
for many animals and plants.
● all the food that we consume depends on soil.
● holds water & mineral nutrients that plants depend upon.
● plants grow in soil and we eat plants that grow directly in the soil or animals that have eaten plants.
● habitat for many organisms in some ecosystems, below-ground biomass > above-ground biomass.
● enormous filter for any water that passes through it, often altering the chemistry of that water.
, ● store and transfer heat so affect atmospheric temperature, which in turn affect the interactions between
soil and atmospheric moisture.
● part of the lithosphere where life processes and soil-forming processes both take place.
○ five spheres of the Earth: atmosphere, hydrosphere, lithosphere, biosphere, plus pedosphere
(soil sphere).
○ pedosphere links biosphere & lithosphere; influenced by atmosphere, hydrosphere, lithosphere.
Components
The exact mix of these four portions give soil its character:
● mineral particles mainly from the underlying rock.
● organic remains that have come from the plants and animals.
● water within spaces between soil grains.
● air also within the soil grains.
It is also a habitat for plants and animals. Soil is a highly porous medium typically with a 50:50 mix of solids and
pore spaces. The pore spaces contain variable amounts of water and air.
Soil profile
A record of the processes that have created the soil, its mineral composition, organic content, and chemical and
physical characteristics such as pH and moisture.
Horizons
Soil cross-section profile is changed over time as organic material leaches downwards and mineral materials
move upwards.
● top layer is often rich in organic material while the lower layers consist of inorganic material.
● inorganic material is derived from the weathering of rocks.
● materials are sorted and layers are formed by water carrying particles either up or down –
translocation.
● in hot, dry climates (P<E) water is evaporating at the soil surface and water from lower soil layers
moves upwards.
○ it dissolves minerals and takes them to the surface, where the minerals are left behind when the
water evaporates.
○ this also happens in irrigation and is called salinisation.
● in colder and wetter climates (P>E) water flows down in the soil.
, ○ it dissolves minerals and transports them downwards, causing leaching to occur.
P: precipitation E: evaporation
O Horizon
● uppermost layer of newly added organic material – comes from organisms that die on top of the soil.
● fungi, bacteria and other animals will start to decompose the dead material.
A Horizon:
● upper layer where humus builds up.
● humus forms from partially decomposed organic matter and is often mixed with fine mineral particles.
● incomplete decomposition forming a layer of dark brown/black organic material – the humus layer.
● in normal conditions, organic matter decomposes rapidly, releasing soluble minerals that are then taken
up by plant roots.
● waterlogging reduces the number of soil organisms, resulting in a build-up of organic matter and can
lead to peat formation.
B Horizon:
● where soluble minerals and organic matter tends to be deposited from the layer above.
● in particular, clay and iron salts can be deposited in this horizon.
C Horizon:
● mainly weathered rock from which the soil forms.
