Biosphere

Soil

What is soil?

Soil is a mixture of particles of weathered rock, decayed organic matter, water and gases in which living organisms are present.

Occupy the spaces in the soils

Water

Minerals from the physical / chemical weathering of the parent rock

MineralsAir

Lacks light, Varies in proportions

Organisms

Decaying roots, leaves, animals broken down by organisms and micro-organisms

Mineral matter

Minerals derived from parent material by physical and chemical weathering

Largest component in terms of volume 45% in a typical topsoil

Organic Material

Mainly derived from decaying roots, leaves, needles and remains of dead organisms

Broken down by micro-organisms, worms and moles

5% volume in a typical topsoil

Air and water

Ever changing volumes which fill the voids - the spaces in the soil

Soil Profile

Soil Profiles

A soil profile is a vertical section through the soil from surface vegetation to the bedrock

Soils can be identified by looking at distinct layers or horizons

Usually 4 distinct horizons can be identified in a soil

Soil Profile - Ao Horizon Surface organic layer - decaying vegetation.

Subdivided into 3 layers L(litter) - leaves , pine needles, cones or dead

heather shoots F(fermentation layer) - organic material starts

to decompose H(humus) - decomposed remnants of

vegetation, animals and bacteria. Important source of nutrients for soil below

Soil profile - A Horizon Main top layer Consists of a mixture of organic and

inorganic material. Organic material is introduced from Ao

layer Usually nutrient rich and fine textured Eluviation - washing out of minerals The topsoil

Soil profile - B Horizon The subsoil less organic material and courser in

texture due to importance of weathering Soluble material can be leached out of

A horizon into B horizon Leaching is the removal of soluble

minerals Illuviation - washing in of minerals

Soil profile - C Horizon

Zone of the regolith large particles sit upon the bedrock Physical and chemical weathering of

parent material is source of nutrients

Soil Formation - Factors

5/6 factors are seen to be important in influencing the formation of a soil

Soil Formation – 1)Parent material (13 bullet points)

Significant in early stage of soil development

can vary from solid bedrock to deposits such as alluvium and glacial till

Soil Formation - Parent material

Rate of weathering Hard rocks such schist (metamorphic

rocks) weather slowly leading to thin soils

Softer sedimentary rocks such as shale weather more quickly

Soil Formation - Parent material Chemical composition and soil colour soils from granite have high silica

content and are acidic Soils on chalk and limestone are

alkaline Silica rich soils are light in colour Rocks such as basalt have high iron

content and form dark soils

Soil Formation - Parent material

Soil texture the feel factor is influenced by the size

of the soil particles determines permeability of soil

Soil formation – 2) Biotic factors (1 bullet point)

Involve the action of vegetation and organisms. They interact, influenced by climate to produce humus. This may lie below the L and F layers of the Ao horizon or mixed through the whole A horizon

Soil formation - 3) Climate(5 bullet points)

At low temperatures soil formation is slower particularly organic decomposition

Warm temperatures encourage decomposition and production of organic material in the soil

Soil formation - Climate

Water percolation also affects soil Where precipitation exceeds

evaporation, leaching is an important process

Where evaporation exceeds precipitation water and minerals are drawn to the surface by capillary movement

Soil formation – 4) Relief(3 bullet points)

Upper slopes - runoff and through flow of water

Lower slopes gain water organic and mineral material

Shady north facing slopes are colder and wetter than south facing slopes

Soil formation – 5) Time(3 bullet points)

Critical to development of soil When soils are young they retain

features of parent material Scottish soils are young mostly as a

result of last glaciation

Soil formation – 6) Human Activity(3 bullet points)

Prehistoric people began to fell woodland around 3000 BC

Accelerated naturally occurring soil erosion

More recently planting blanket coniferous forest and the application of fertilisers have altered soil characteristics