Biotic and Abiotic factors that control soil development Soil Forming Factors Biotic and Abiotic factors that control soil development

Soil is a function of G + C + O + R + T

PARENT MATERIAL From the rocks Geology BIOTIC or ABIOTIC ? Soil gradually develops from weathered rock called regolith Minerals / nutrients in the soil come largely from the rock below.

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 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

Leaching on well drained slope Peaty gley – on flat summit Poor drainage on flat ground At bottom of slope = waterlogging and gleying

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 = …% 40% 45% interchangeable 10% + 5%