Soil Forming Factors

Biotic and Abiotic factors that control soil development

Soil is a function of

G +

C +

R +

O +

T

PARENT MATERIAL

From the rocks

Geology

Soil gradually develops from weathered rock called regolith

Minerals / nutrients in the soil come largely from the rock below.

BIOTIC or ABIOTIC ?

Different Minerals weather at different rates and in different ways.

e.g. Granite

Quartz – hard – physical weathering – sands

Feldspar and Mica – ‘softer’ – chemical weathering - clays

Sand

Coarser texture

Good drainage (‘dries out’)

Shallower soil

Fewer nutrients (‘hungry’ soil)

Clay

Finer texture

Poor drainage (‘heavy soil’)

Deeper soil

More nutrients

Parent Rock affects …

Depth

Texture

Drainage / permeability

Quality / nutrients

Colour

Some rock types are the dominant soil forming factor in the U.K. …

Limestone soils –

Thin

stoney

Alkaline

Much calcium carbonate

CLIMATE – most important soil forming factor at the world scale. BIOTIC or ABIOTIC ?

Wet climate

Much vegetation

More leaf litter

More humus

If Precipitation is heavy

Main soil water movement will be downwards

Leaching nutrients downwards (K, Ca, Mg etc.)

Eluviation of clay minerals

Where evapotranspiration

Exceeds precipitation

Main soil moisture movement is upwards

By capilliary action

Taking nutrients up towards the surface

Giving darker surface layers

RELIEF or TOPOGRAPHY BIOTIC or ABIOTIC?

Higher land is wetter

Higher land is cooler with shorter growing season

Aspect is important – south facing slopes are sunnier and drier in the northern hemisphere

Poor drainage on flat ground

At bottom of slope

= waterlogging and gleying

Peaty gley – on flat summit

Leaching on well drained slope

The catena concept – nature of soil varies with position on a slope

ORGANISMS ( BIOTA)BIOTIC or ABIOTIC Plants, bacteria,

fungi and animals all interact in the nutrient cycle.

Plants take up nutrients from soil water

Plants return nutrients in leaf litter

Decomposers rot the litter and make humus. Others mix it into the soil.

Mull Humus

Deciduous leavesNot too acidicEncourage bacteria and wormsMix quickly into the soil

Crumbly , black, nutrient rich soil e.g Brown Earths

Mor Humus

Acidic pine needlesslow to decomposecold, wet upland areas

Fibrous, acidic and nutrient deficient surface horizon called mor e.g. in Podsols.

TIME BIOTIC or ABIOTIC?

Soils form slowly

400 years for 10 mms.

Upland Northern Britain soils are under 10,000 years old

Parent material and climate influence rate of development

Sand or clay

Wet and hot or cold and dry?

Originally the soil is like its parent material

Over time the soil is less of a regolith

With more organic matter and organisms

Horizons develop as the soil reaches a state of equilibrium with the environment.

A mature soil has four main componentsBiotic = …% Abiotic = …%

45% interchangeable

40%

10% + 5%