Published on March 2, 2014
SOILS SOILS DIFFER BECAUSE OF A COMBINATION OF FACTORS AND PROCESSES THAT ARE AT WORK IN AN AREA. Geography
PROCESSES AFFECTING SOIL CHARACTERISTICS Recap: What are the characteristics of soils?
CHARACTERISTICS OF SOILS Colour Structure Texture Organic content pH Value Water content
PROCESSES AFFECTING SOIL CHARACTERISTICS Eight major processes af fect soil characteristics: 1. Weathering 2. Humification 3. Leaching 4. Podzolisation 5. Gleying 6. Laterisation 7. Salinisation 8. Calcification The factors of climate, relief, sock type, living things and time all influence which of these processes occur in a region.
EXAM BRIEF You are expected to know the natural processes that affect soil characteristics.
PROCESS 1: WEATHERING The breakdown of rocks due to mechanical and chemical weathering. Rates of weathering are greatest in the world’s hot and humid areas. Soil grains produced by weathering keep the characteristics of the parent rock such as pH, texture and colour. Chemical weathering processes such as carbonation and oxidation can release nutrients such as calcium and iron from the mineral grains. E.g. The chemical weathering of limestone by carbonation leads to soils rich in calcium.
PROCESS 2: HUMIFICATION • Method by which dead organic matter is converted into humus by oxygen. • Humification releases nutrients into the soil. • Climate: Humification is accelerated by high temperatures and high rainfall. • In tropical regions, humification is very fast. • In temperate climates such as Ireland humificaiton is slow in the winter. • In cold climates humification may stop completely.
PROCESS 3: LEACHING • The washing of minerals through soils. • When rainwater falls on soil it dissolves the minerals and plant matter in it. • As the water soaks down through the soil, it carries these dissolved minerals and nutrients with it. A certain amount of leaching is needed to wash humus into soil. • Excessive leaching can rob the upper parts of the soil of the nutrients that plants need to grow. This soil will become infertile. A hardpan can form.
PROCESS 4: PODZOLISATION A type of leaching where rainwater is more acidic. Podzol soils form under coniferous forests. As these forests die and decompose they add to the acidity of the rainwater. The water seeping through the soil beneath this dead vegetation becomes acidic and dissolves soil minerals such as iron and aluminium as it passes downwards. In time a hardpan layer forms and the soil becomes waterlogged.
PROCESS 5: GLEYING The soil is waterlogged and lacks oxygen. The pores between the soil peds are filled with water for all or part of the year. This prevents living things from receiving oxygen. Little can grow in such wet, oxygen poor (anaerobic) conditions. Heavy rainfall can lead to gleying. Due to lack of oxygen, gley soils have patches of blue/grey colouration.
PROCESS 6: LATERISATION A layer of iron oxides build up as a result of rapid weathering and leaching. This process is most active in areas of high rainfall and high temperatures. The heavy rainfall dissolves and leaches most minerals except for iron and aluminium oxides. Oxidation folllows as the iron is exposed to oxygen in the air and latosols or red soils develop. If the soil dries out it turns into a hard laterite. Occurs in tropical and equatorial regions.
PROCESS 7: SALINISATION Occurs when mineral salts move up through the soil instead of down into the soil. Can happen in hot desert areas where rainfall is low and the amount of water evaporating out of the soil is greater than rain falling onto it. Evaporation causes salts in ground water to rise through the soil and collect in the upper layers. Salt is deposited on the surface as a hard white crust.
PROCESS 8: CALCIFICATION Process where calcium carbonate is concentrated near the surface of the soil. Occurs in regions of low rainfall. The amount of water drawn up through the soil by plants (transpiration) may be greater than the precipitation falling on the soil. As a result, calcium carbonate builds up in the upper layer of the soil. Calcium carbonate is useful for plants and these soils often have lush grass growth.
EXAM BRIEF A typical exam question: “Examine the factors/processes that influence soil characteristics.”
FACTORS AFFECTING SOIL FORMATION 1. Climate 2. Relief 3. Parent Material 4. Living Things 5. Time
FACTORS AFFECTING SOIL FORMATION: 1. CLIMATE Climate is the single most important factor in soil formation. Climate influences: The rate of weathering which in turn controls soil thickness. The rate of humification – the breakdown of organic matter. The amount of leaching – the process where minerals are washed downwards leaving top layers of soil without minerals. Soils that have developed in response to particular climate conditions are called zonal soils.
FACTORS AFFECTING SOIL FORMATION: 2. RELIEF Relief can influence the depth and drainage of a soil. In general, sloping land is well drained and soils are quite dry. However, mountainous terrain tends to have less soil because: Gravity removes soil from steep slopes in mass movements. Areas of high relief have higher precipitation rates and lower temperatures, hence there is more leaching and slower rates of weathering. Lowlands are usually warmer and flatter which encourages a build up of soil.
FACTORS AFFECTING SOIL FORMATION: 3. PARENT MATERIAL The type of bedrock, be it granite or limestone, determines the amount and type of minerals present in soil. These minerals such as calcium and potassium provide nutrients and food for plants. The parent material also controls a soil’s depth, colour, texture and pH value.
FACTORS AFFECTING SOIL FORMATION 4. LIVING THINGS Living things influence soil fertility. Living things within soil can increase fertility and aeration of a soil and prevent soil erosion. Earthworms help to aerate soil by moving about in it. Insect larvae burrow through soil and create air spaces for plant roots. Water can collect in these burrows and keep soil moist. Fungi and bacteria add nutrients by decomposing dead material.
FACTORS AFFECTING SOIL FORMATION 5. TIME It takes about 400 years for 1 cm of soil to form. It takes time for soil to develop fully in response to the processes that occur due to climate, relief, parent material and living things.
EXAM BRIEF A typical exam question: “Examine the factors that influence the development of soils.”
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