Biosphere Soil

What is soil? n 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 Minerals Air Lacks light, Varies in proportions Organisms Decaying roots, leaves, animals broken down by organisms and micro-organisms

Mineral matter n Minerals derived from parent material by physical and chemical weathering n Largest component in terms of volume n 45% in a typical topsoil

Organic Material n Mainly derived from decaying roots, leaves, needles and remains of dead organisms n Broken down by micro-organisms, worms and moles n 5% volume in a typical topsoil

Air and water n Ever changing volumes which fill the voids - the spaces in the soil

Soil Profile

Soil Profiles n A soil profile is a vertical section through the soil from surface vegetation to the bedrock n Soils can be identified by looking at distinct layers or horizons n Usually 4 distinct horizons can be identified in a soil

Soil Profile - Ao Horizon n Surface organic layer - decaying vegetation. Subdivided into 3 layers n L(litter) - leaves, pine needles, cones or dead heather shoots n F(fermentation layer) - organic material starts to decompose n H(humus) - decomposed remnants of vegetation, animals and bacteria. Important source of nutrients for soil below

Soil profile - A Horizon n Main top layer n Consists of a mixture of organic and inorganic material. n Organic material is introduced from Ao layer n Usually nutrient rich and fine textured n Eluviation - washing out of minerals n The topsoil

Soil profile - B Horizon n The subsoil n less organic material and courser in texture due to importance of weathering n Soluble material can be leached out of A horizon into B horizon n Leaching is the removal of soluble minerals n Illuviation - washing in of minerals

Soil profile - C Horizon n Zone of the regolith n large particles sit upon the bedrock n Physical and chemical weathering of parent material is source of nutrients

Soil Formation - Factors n 5/6 factors are seen to be important in influencing the formation of a soil

Soil Formation – 1)Parent material (13 bullet points) n Significant in early stage of soil development n can vary from solid bedrock to deposits such as alluvium and glacial till

Soil Formation - Parent material n Rate of weathering n Hard rocks such schist (metamorphic rocks) weather slowly leading to thin soils n Softer sedimentary rocks such as shale weather more quickly

Soil Formation - Parent material n Chemical composition and soil colour n soils from granite have high silica content and are acidic n Soils on chalk and limestone are alkaline n Silica rich soils are light in colour n Rocks such as basalt have high iron content and form dark soils

Soil Formation - Parent material n Soil texture n the feel factor is influenced by the size of the soil particles n determines permeability of soil

Soil formation – 2) Biotic factors (1 bullet point) n 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) n At low temperatures soil formation is slower particularly organic decomposition n Warm temperatures encourage decomposition and production of organic material in the soil

Soil formation - Climate n Water percolation also affects soil n Where precipitation exceeds evaporation, leaching is an important process n Where evaporation exceeds precipitation water and minerals are drawn to the surface by capillary movement

Soil formation – 4) Relief (3 bullet points) n Upper slopes - runoff and through flow of water n Lower slopes gain water organic and mineral material n Shady north facing slopes are colder and wetter than south facing slopes

Soil formation – 5) Time (3 bullet points) n Critical to development of soil n When soils are young they retain features of parent material n Scottish soils are young mostly as a result of last glaciation

Soil formation – 6) Human Activity (3 bullet points) n Prehistoric people began to fell woodland around 3000 BC n Accelerated naturally occurring soil erosion n More recently planting blanket coniferous forest and the application of fertilisers have altered soil characteristics