Describe the natural processes of soil formation: weathering of parent rock (physical, chemical and biological), addition of organic matter. Development of main horizons: organic layer, topsoil, subsoil, parent rock.

SOIL Soil is the outer layer of the earth and is built up (accumulated) over a period of geological time. It is a mixture of solid mineral particles in varying sizes/organic matter/water/air.

SOIL-COMPOSITION The 5 basic components of soil are minerals, organic matter, water and air. The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air. These partials give the soil its physical skeleton and some of the dissolved minerals for plant growth.

SOIL-FORMATION see weathering


BEDROCK Bedrock often gives a soil a distinct colour (some red soils in Somerset are derived from the under-lying red sandstone). Chalk in the soil on the downs can look white when dry. In geology, bedrock is the lithified rock that lies under a loose softer material called regolith at the surface of the Earth or other terrestrial planets. The broken and weathered regolith includes soil and subsoil




WEATHERING-(SOIL-FORMATION) Weathering can be: Physical: Temperature – expansion and contraction; Water – freezing and thawing; Water – waves, rivers (tumbling), rain drops; Wind – sand blasting. Chemical: Rain is a mild form of carbonic acid due to the dissolved carbon dioxide it contains. This can quickly react with some rocks dissolving it away (Limestone) or more slowly on other rocks by dissolving away the weaker elements of the rock holding it together. Biological: Burrowing of animals; Plants growing in cracks where the roots can produce physical pressure on the rocks, or some roots can exude chemicals that weaken the rocks; Wear and tear of rock paths by human traffic.



CARBONIC-ACID Contained in rainwater


ORGANIC MATTER Contain carbon. Plant - or animal-based. Organic matter from plant and animal debris starts to accumulate and has both physical and chemical effects on the soil. Over time, bulky organic matter gradually decays and becomes Humus. This colloidal state acts as glue in the soil as well as holding on to plant nutrients.

About 5% of soil content generally (80% hummus, 10%roots, 10% organisms)

Contain carbon Plant - or animal-based Living Organic Matter: Worms/bacteria/fungi/nematodes/moles/plant roots. • Byproduct of living animals or end product of death / decomposition: Dead Organic Matter: Worms/bacteria/fungi/nematodes/moles/plant roots, plus plant remains (natural or added as compost)

Animal by-products (manure/bones/hooves/blood).

*Farmyard manure 😘 • becomes Humus. • physical and chemical effects on the soil • generally acidifying.

Garden compost: • can be richer in plant foods; • unpredictable nutrient content.

Leaf mould • good physical effect on soil structure • limited foods value • time to produce

Quick Breakdown Mulches • Well rotted or coarse compost, strawy manure • breaks down to become Humus over a season • has other benefits eg reduce weeds, retain moisture as with slow breakdown mulches

Slow breakdown mulches: • Un- or incompletely- composted materials eg Bark, wood shaving, shredded paper • Breaks down over several seasons but used mostly for weed control and prevention of soil capping • Uses nitrogen as it breaks down which may affect fertility of soil for surrounding plants.

Humus: • Remains of bulky organic matter in the soil after decompostion • holds plant foods and water • creates soil structure

HUMUS The final colloidal remains of bulky organic matter in the soil – holds plant foods and water and is the major soil glue, thus creating soil structure.

Humus is dark, organic material that forms in soil when plant and animal matter decays. When plants drop leaves, twigs, and other material to the ground, it piles up. … In addition to the plant material in leaf litter, humus is composed of decaying animals, such as insects, and other organisms. humus is not actually type of soil, but is rather a form of mature compost. It can be made either through a composting process or can be found in nature, …

COLLOIDAL STATE In chemistry, a colloid is a mixture in which one substance of microscopically dispersed … Colloidal: State of subdivision such that the molecules or polymolecular particles dispersed in a medium have at least one dimension betwee


The horizons can be seen by digging a hole or a pit.
This will show the depths of each zone in your soil.
This will also show the natural level of water in the soil for the time of year you are in (water table) and the presence of any pans (hard layers stopping the downward movement of water) (chemical (iron) or cultivation plough pans).

There are many illustrations of simple soil horizons, which can become confusing especially when they start to sub-divide each layer.

see also: soil-profile

The graduation between horizons may be very distinct (old soils) or they may merge gradually with little or no line of demarcation (young soils); The depth and components of the profile are used in classifying soils into types and thereby assessing their potential: Organic soil; Calcareous soil; Grey soil; Grey ash soil; Brown earth; Rendzina; Podzol.

SOIL PROFILE organic layer/topsoil/subsoil/parent rock.

WATER TABLE the natural level of water in the soil for the time of year you are in

PAN (SOIL HORIZON) hard layers stopping the downward movement of water)




SEDENTARY SOIL formed from underlying bedrock

TRANSLOCATED SOIL This is where the soil is not related to the under-laying bedrock:

Gravel deposited over chalk, topsoil derived from the gravel not the chalk; River valley soil derived from river deposits; Valley bottoms, where soil has accumulated having drifted downhill.

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