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 TOPOGRAPHYBIOTIC 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 leaves Not too acidic Encourage bacteria and worms Mix quickly into the soil Crumbly, black, nutrient rich soil e.g Brown Earths Mor Humus Acidic pine needles slow to decompose cold, 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 components Biotic = …% Abiotic = …% 45% interchangeable 40% 10% + 5%