Four parts, mixed together: Soil is not just dirt. It is a mixture of four parts: inorganic (mineral) matter, organic matter, water and air. Living things make it an ecosystem of its own.
The points to remember
- Inorganic (mineral) matter: rock fragments, sand, silt and clay, about 45% of the volume.
- Organic matter: living things and dead, decaying matter (humus), about 5%.
- Water, about 25%, and air, about 25%, fill the pore spaces.
- The parts interact: a soil is a mixture with its own ecosystem and its own soil organisms.
- Soils are named and sorted with keys, many published online.
Remember it as: Rock, rot, rain and air.
Real example: the ground on Mars, studied by NASA's Curiosity rover, is broken rock and dust with no humus and no living things. It has the minerals, but not the other parts of a fertile soil.
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The inorganic part of soil comes from the rock beneath it, the parent rock. Weathering breaks it into ever smaller pieces.
Where the inorganic part comes from
- The inorganic part comes from weathering of the parent rock below or nearby.
- Physical weathering: ice in cracks (freeze-thaw), heating and cooling, roots wedging rock apart.
- Chemical weathering: rainwater, weakly acid, dissolves or changes minerals; iron rusts.
- Biological weathering: lichens, roots and burrowing animals break rock up.
- From biggest to smallest: rock fragments, sand, silt, clay.
- The parent rock decides which minerals the soil holds.
| Weathering | What does it | Example |
|---|---|---|
| Physical | Ice, heat and cold, roots | Water freezes in a crack, expands and splits the rock |
| Chemical | Rainwater, weak acids, oxygen | Rain slowly dissolves limestone; iron in rock rusts |
| Biological | Lichens, roots, animals | Lichens release acids that eat into rock |
Real example: on Hawaii, the soils formed from dark volcanic basalt. Weathering lets the iron in the basalt rust, so many Hawaiian soils are bright red.
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Living and dead: The organic part of soil is the living organisms in it and the dead material they break down. Decay turns dead leaves into humus.
The organic part
- Living organisms: bacteria, fungi, earthworms, insects, mites and plant roots.
- Dead material: fallen leaves, dead roots and animals, dung.
- Decomposers break the dead material into dark, crumbly humus.
- Only about 5% of the volume, but vital: humus holds nutrients and water and feeds soil life.
- Without organic matter there is only broken rock, not a fertile soil.
Small but vital: Organic matter is only about 5% of the volume, yet it is critical for fertility: humus holds nutrients and water, and it feeds the soil organisms.
Real example: the United Nations' food agency (FAO) says one teaspoon of healthy soil can hold more living microorganisms than there are people on Earth.
About half the volume of a good soil is not solid at all: it is pore space. Water and air share those gaps, and both are needed for life in the soil.
Water and air
- Water fills some pore spaces and carries dissolved nutrients to roots.
- Air fills the rest and gives oxygen to roots and soil organisms for respiration.
- Water and air share the same spaces: more of one means less of the other.
- A waterlogged soil has almost no air, so roots and many organisms cannot respire.
- A very dry soil has plenty of air but too little water for plants.
Remember it as: Water feeds the roots; air lets them breathe.
Well-drained soil
- Water and air both in the pores
- Roots get water, nutrients and oxygen
- Earthworms and bacteria thrive
Waterlogged soil
- Water fills almost every pore
- Little oxygen: roots struggle to respire
- Dead matter decays very slowly
Real example: in the Mekong Delta in Vietnam, rice fields are flooded for months, so water pushes the air out of the soil. Rice copes because air reaches its roots through hollow channels in its stems; most crops would die.
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How fertile soil forms from bare rock: Bare rock becomes fertile soil when weathering supplies the mineral part and living things add the organic part, over hundreds of years.
The processes, in order
- Weathering breaks the parent rock into small particles.
- Wind and water deposit more sediment, making the soil deeper.
- Particles dissolve, releasing soluble minerals; rain adds water.
- Pioneer species colonise: lichens and mosses; moss mats hold the soil in place.
- Nitrogen fixation by bacteria adds nitrates the plants can use.
- Decomposition of dead organisms and leaf litter adds minerals and humus.
- Earthworms and burrowing insects mix the soil and open up pores.
1963
- A volcano erupts under the sea off Iceland, building the bare island of Surtsey from lava and ash.
Weathering
- Waves, frost and rain break the lava and ash into sand and smaller particles.
Pioneers
- In 1965 the first plant, sea rocket, grows from a seed washed ashore; mosses and lichens follow.
Seabirds
- From 1986 a gull colony nests there; their droppings add nitrogen and dead matter, and many more plants take hold.
Soil
- Where the gulls nest, a thin soil with humus now covers the ash.
Processes, not a list of things: 'Lichens, mosses, worms' scores little. Say what each one does: 'lichens release acids that weather the rock', 'earthworms mix the humus into the mineral particles'.
How this comes up: Paper 2, Section B (a): the processes that turn bare rock into fertile soil, one per point.
In 2021, lava from the Cumbre Vieja volcano on La Palma, in the Canary Islands, covered more than 1,000 hectares of farmland and villages with bare rock.
Outline the processes involved in the formation of fertile soil on this bare rock.
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