Factors influencing Soil Formation Higher Geography Biosphere Factors influencing Soil Formation

There are six main interacting factors in the formation of soil. Introduction There are six main interacting factors in the formation of soil. Parent Material – Biotic Factors – Climate – Relief – Time – Human Activity

Parent material Parent material is especially significant in the early development of soil and its mineral content. It can vary from solid bedrock to a wide range of unconsolidated deposits including alluvium, wind blown sand and glacial till.

Parent material influences (1): The rate of weathering: hard rocks (eg: schist) weather slowly often leading to thin soils. Softer, sedimentary rocks (eg: shale) weather more quickly. Chemical composition & soil colour: minerals derived from quartzite and granite have a relatively high silica content and are acidic; soils on chalk and limestone are alkaline. The darker the parent material the darker the soil.

Parent material influences (2): Soil texture: is influence by the size of soil material particles. This is important because it helps determine the permeability of the soil and the relative proportions of air (gases) and water. Soil texture = the ‘feel’ of moist soil

Biotic Factors Involves the action of vegetation and a wide range of organisms, from bacteria to vertebrates. They all interact, influenced by climate and the evolving nature of the soil itself, to produce the organic material or humus.

Humus may include: (1) Mor humus - which mainly develops beneath coniferous forest or heather moorland is normally associated with a cooler, wet climate and acidic parent material. The litter layer of pine needles, cones and / or heather shoots further encourages acid soil reactions and discourages the breakdown of plant material. Earthworms are not common and there is a limiting mixing of organic and mineral content.

Humus may include: (2) Mull humus – frequently develops beneath deciduous woodland whose leaves are rich in base materials. With a plentiful supply of litter and a rich soil fauna, there is no clearly defined humus layer. Chemically the soil is almost neutral and is the home to earthworms which are very active decomposers.

Humus may include: (3) Moder: is an intermediate humus between mor and mull.

Climatic Factors (1) Climate is particularly important, especially seasonal and daily variations in temperature and precipitation. At low temperatures the rate of soil formation is slower, especially organic decomposition. Equally, warmer temperatures encourage decomposition.

Climatic Factors (2) The input of water percolating through the soil is not the same as precipitation but, providing precipitation exceeds evaporation, leaching is a particularly important process. This occurs not just vertically but also downslope.

Climatic Factors (3) If evapotranspiration exceeds precipitation, water and minerals may be drawn upwards by the process of capillary movement.

Relief (1) Relief or topography of land influences drainage and soil depth.

The lower receiving site gains water, organic and mineral matter. Relief (2) Upper well drained shedding site from which there is surface run off and through flow. The lower receiving site gains water, organic and mineral matter. If this site is particularly badly drained, excess water accumulates and encourages the formation of gleys and peaty soils.

Relief (3) Relief and aspect can also modify the effect of climate on the soil. Shady north-facing slopes are colder and wetter compared to those with a south-facing aspect. This can slow up organic decomposition and encourage peat formation. eg:

Time Time is critical in the development of soils before they are fully mature. When they are young, soils retain the features of the parent material.

Human Activity This should not be underestimated. It started around 3000 BC ago when humans started to fell woodland and burn peat. More recently, blanket planting of coniferous plantations and modification of soil chemistry by applying fertilisers, lime and pesticides have further altered soil characteristics.