Soils Pedology… the scientific study of soils. The surface layer of loose material that covers the earth’s surface containing organic & inorganic matter. Soil Composition: Made up of mineral matter, organic matter, air & water. Mineral Matter: Got from parent material such as bedrock (limestone sandstone) which is broken down by physical & chemical actions. Also got from glacial, river & wind blown deposits. Mineral content refers to calcium, potassium, phosphorus & other such compounds. These are food for plants. Parent material determines the colour, texture, depth and pH value of soil. 7.1 SOILS: Composition Organic Matter Also know as humus. Formed by decaying plants, animal life & through actions of bacteria & micro-organisms. Burrowing animals & worms aerate the soil & mix soil layers. Humus improves soil texture & binds soil particles together. Plant roots also bind soil, take up nutrients & return them when they die. Climate Distribution of soils coincide with distribution of climates. Climate, mainly temp & precipitation, influences the type of veg growth Chemical weathering in hot climates creates great depth of soils. Large the vegetation the greater the amount of humus. Climate influences rate of weathering of soil & decay of plant matter. Heavy rainfall causes leaching. Weathering is the first stage of soil Slope (topography) & water Steep slopes result in removal of fine particles by rainwater but better drained. Flat slope encourage accumulation of fine particles. Aspect (way it faces) creates different soils on north & south slopes. Water moves through the layers & carries nutrients with it…leaching. This can create a hard pan & impedes drainage. Air Vital for oxidation… converts organic matter into oxygen. Vital for presence of bacteria which require oxygen. 7.1 SOILS – Development Profiles General O Organic Layer consists of plant litter and humus Not present in cultivated fields A Topsoil consists of humus and minerals The ploughed layer B Subsoil low in organic material C Transition Area between soil and parent material consists of partially disintegrated bedrock R Bedrock 7.1 SOILS – Development Profiles Brown Earth Source ENCARTA Tropical Red Soil Soil Characteristics Texture, structure, colour, moisture, humus, air, pH Texture Refers to proportion of sand silt & clay. Texture determined by mixing water & soil in a jar Texture influences soils ability *to retain & transmit moisture *retain nutrients *allow roots to penetrate Sandy soils…. 70% or more sand & have few nutrients…feel gritty Clay soils… 50% or more clay & rich in nutrients …easily waterlogged…feel sticky Silty soils intermediate btw sandy & clay… feel smooth Loam soils fertile… mixture of various particle sizes…. well aerated with some moisture & plant food. 7.1 SOILS: Characteristics – Determining Soil Texture Fill a screw topped clear glass jar two thirds full with soil Add water until jar is almost full Shake the jar for 1 minute approx. Leave the jar to settle for 24 hours 100 90 80 70 60 50 Clay Silt Fine Sand 40 30 20 10 0 Coarse Sand Colour Humus soils are dark brown or black. Brown soils get colour from decayed leaves & plants. Dark soils absorb more sunlight & so are warmer than light soils… helps seed germination & longer growing season. Structure Refers to shape of soil particles. Clusters of particles form in undisturbed soil called peds. Shape & size of peds determine the pore spaces for air & water & organisms to pass through. Crumb structure soils give highest agric. yield as it provides a balance btw air, water & nutrients. 7.1 SOILS: Characteristics – Structure Single Grained Structure Associated with sandy soils Water, air and organisms can pass through it easily Single grained Roots can spread out into the soil easily Water retention is poor Nutrient retention is poor Prone to leaching Vulnerable to drought 7.1 SOILS: Characteristics – Structure Crumby Structure Associated with good agricultural land – loam soils Crumb : 1 - 5mm Water, air and organisms can pass through crumby soils easily Roots can spread out into the soil easily Water retention is good as it is soaked up by the crumbs of soil – Nutrient retention good 7.1 SOILS: Characteristics – Structure Massive Structure Massive Usually associated with clay type soils Water, air and organisms can not pass through this type of soil easily Roots have difficulty penetrating the large lumps of clay Soils are often waterlogged after periods of rain Soil dries into hard clumps after a dry period Moisture Content Water moves down by percolation & up by capillary attraction. Carries many mineral compounds…. creates process of solution. Water content varies from nil (arid) to very high (waterlogged). Persistent percolation causes leaching (minerals drawn downwards). Leaching creates podzol soils. Organic Content Formed from decaying plants & animals. Humus gives soil a dark colour…. falling leaves, decaying grass & roots (organic matter) main input for humus… create black earth soils. Bacteria & fungi break down organic matter. Soil organisms will mix plant litter into the A horizon where it will decompose into humus. pH value pH a measure of its acidity or alkalinity. Low pH is acidic soil… happens where minerals like calcium & potassium are leached. Neutralised by adding limestone. Very low acidic soil slows decomposition & creates peat. High pH means alkaline soil… indicate high lime (calcium) & potassium content. Neutral soil most suitable to farming. 7.1 SOILS: Characteristics – pH value pH = potential Hydrogen ions pH value of 7 = neutral 7.1 SOILS: Characteristics Texture Particle Size Clay: Less than 0.002mm Silt: 0.002 – 0.02mm Sand: 0.02 – 2.0mm 7.1 SOILS: Characteristics Texture Sample Sand 70% Clay 20% Silt 10% 7.2 SOILS – Influences Affected by: • Immediate Environment • Processes Including Human Interference Soil Formation Factors that affect soil formation: Parent Material Soil formation occurs more rapidly if its composed of boulder clay deposits rather than bedrock. Soils inherit characteristics of parent material. Soils from limestone will have calcium & become alkaline. Soils from sandstone will be sandy, easily drained & heat quickly. Climate Temp & precipitation effects include: * Heat & moisture encourages growth. * Hot wet regions have lots of veg. covers… more humus * Decomposition occurs quickly * Heavy rainfall causes leaching * Spells of drought increases minerals like calcium due to evaporation. Topography Level surfaces.. deeper soils. Steep slopes… thin soils, erosion but well drained. South facing slopes warmer… encourage growth. Living Organisms Roots bind soil & absorb moisture & nutrients. Insects break down & aerate soil. Time Soils develop over thousands of years..speed depends on parent material. Can be washed away in floods or eroded quickly like Dust Bowl In North America. Precipitation Water essential for growth….absence leads plants to die or adapt (desert) Water retention determined by soil structure & texture. Clays hold more water than sandy soils. 7.2 SOILS – Global Climate Types 7.2 SOILS – Global Patterns 7.2 SOILS – European Patterns Factors influencing Soil Characteristics Processes Weathering Physical weathering breaks down the parent rock… soil retains these characteristics. Chemical weathering breaks down soil by carbonation, hydration, hydrolysis & oxidation. Weathering release nutrients into soil. Humification Breakdown of plant matter into humus. Faster in hot, humid regions. Leaching Heavy rainfall carries minerals to lower horizons. Upper soil becomes acidic & mineral deficient. 7.2 SOILS – Influences Processes - Weathering Podzolisation Rainwater becomes acidic in coniferous or acid bog soils & removes minerals & soil becomes acidic. A horizon becomes greyish … drained of all colouring materials & B horizon reddish….has many of the dissolved minerals. Hard pan develops causing waterlogging. Infertile podzol soil. Laterisation Intense rainfall & rapid decay means little acid in soil. Leaching removes minerals except iron & aluminium…..due to large concentration of these minerals. These build up close to surface creating bauxite (alum) & red laterite (iron) soils. Occurs in tropics where rainfall & temp are high. Salinisation Ground water rising to surface by capillary action carries salt deposits… create a hard toxic crust. Occurs where rainfall is low & evaporation high 7.2 SOILS – Influences Processes - Erosion Calcification Calcium remains close to surface in low rainfall areas due to evaporation & capillary action. Very little leaching due to low rainfall. Gleying Waterlogged & poorly aerated soils where lack of oxygen retards decomposition…. Drumlin belt. 7.2 SOILS – Influences Human Interference Human • Over-cropping • Over-grazing • Desertification • Conservation 7.2 SOILS – Human Interference Clearing Bush-land in Australia 7.2 SOILS – Human Interference Erosion caused by raindrops on bare land 7.2 SOILS – Human Interference Erosion caused by tilling dry land 7.2 SOILS – Human Interference Erosion caused by wind Source: Seafriends.org.nz 7.2 SOILS – Human Interference Livestock Damage Source: Seafriends.org.nz 7.2 SOILS – Human Interference Source: Soil Protection Seafriends.org.nz 7.2 SOILS – Human Interference Erosion caused by tilling hill slopes Source: Seafriends.org.nz 7.2 SOILS – Human Interference Contour ploughing Strip cropping Source: Seafriends.org.nz Terracing Paddy (Padi) fields 7.2 SOILS – Human Interference Soil Protection Source: Seafriends.org.nz 7.2 SOILS – Human Interference 7.2 SOILS – Human Interference Desertification – Europe Classification of Soils Three basic groups: Zonal, Intrazonal, Azonal. Zonal Soils Classified according to climate zones in which they occur. They are mature soils with distinctive profiles & mature horizons. Tundra: Climatic Zone Tundra Vegetation zone Tundra. Artic regions of Canada, Russia & Scandanavia. Vegetation…. Lichens, shrubs & mosses. Shallow, brown to dark grey A Horizon…. permafrost. Latosols: Climatic zone Tropical Vegetation zone Tropical rainforest Found in tropics & rainforests like Indonesia & Brazil. High rainfall has leached out most of the minerals except iron & aluminium…. very little acid left in soil. Iron oxide builds up in a layer due to leaching… called lateralisation. Chemical weathering is dominant & decay is rapid. Iron oxide tints the soil a red colour (as it is exposed to oxygen) forming laterite while the aluminium tends to form bauxite. Intrazonal soils Individual soils that develop within zonal soil regions due to local factors such as parent material or drainage having more influence than climate. Terra Rossa Climatic zone warm temperate maritime Vegetation zone Mediterranean. Mature limestone based soil. Iron minerals are oxidised by chemical weathering creating a red soil. Found in Mezzogiorno, Italy & coffee-growing regions of Brazil. Azonal Soils Soils with an immature profile not yet fully developed. Parent material is weathered rock or glacial deposition. Location not confined to any one climatic zone. Regosols: Derived from volcanic, sand or alluvial deposits. A Horizon is light in colour and B horizon does not exist. C horizon a mixture of sand & silt. Form alluvial deposits on floodplains like the Tigris-Euphrates rivers in Iraq & the Indus in India. Profile of an Irish soil Brown earth: Most common Irish type. Well drained, medium textured with a uniform dark brown profile. pH within 5 to 7 range & a crumb structure. Parent material is boulder clay from ice age. Highly productive, though quality varies, & easily adapted to pasture & tillage. Develop in cool temperate climates covered with deciduous forests. Heavy leaf fall & rapid decay resulted in thick layers of dark humus…. gives it the brown colour & makes it crumbly. Tree roots absorb large amounts of nutrients but return them to the soil when leaves fall off. Temp suitable for earthworms & organisms & these with burrowing animals mix up soil…. less distinct boundary between A & B Horizons. Well drained soil. 3 main types of Brown earth soils found in Ireland. 1. Podzol… Rainwater becomes acidic & removes minerals… on lowland limestone areas of central & eastern Ireland. Slightly leached. 2. Acid brown earth… dev. from parent material poor in lime like granite & sandstone….. acidic soils… found in South East. 3. Shallow brown earth… fertile but shallow & stony with parent rock too near the surface for cultivation…. Burren & east Galway. 7.3 BIOMES – World Regions Taiga Desert Deciduous Forest Tropical Grassland Rain Forest Tundra Human Interference with soil characteristics… Desertification Desertification: The reduction in veg. cover thus exposing the soil to erosion, making a region unable to provide for its natural wildlife or human pop. Desertification does not just refer to moving forward of deserts but to formation of degraded patches of soil & veg cover. Two main factors 1. Climatic Change 2. Human Interference. E.g. Sahel…. narrow band of Africa that lies between the Sahara to the north & savannah (grass & open forest) & equatorial forest to the south. Area receives rain during a short active wet season (late June to Sept.) It supports a nomadic, pastoral based society. Sahara desert advancing southwards into Sahel by between 5 & 10km per year. Causes of Desertification: Climatic Change 1. Decrease in rainfall… fell by 30% in last ten years. Rain arrives late & wet season shorter… effects wildlife & veg. 2. Increased drought… below average rainfall & periods of total drought 1968-1973 & 1983-1985. 3. Global warming… higher world temp.-- increased evaporation & reduced condensation. Factors resulted in : Less rainfall wells, rivers & water holes drying up Water table fell. Overall result: Vegetation dies Lack of veg. cover Soil erosion Desertification Causes of desertification… a) Human interference Increased animal numbers…cattle ownership = wealth & status - led to increased herd numbers. …. overgrazing. Increased number of boreholes dug---- nomads stayed longer - pressure on grasslands. Boreholes drained water table & promoted soil salinisation. Intensive land use led to overcropping & overgrazing….increased tillage for cash crops like groundnuts so no more fallow year. Reduced yields led to increased tillage area on marginal lands. Trees & bushes cut down for firewood….led to increases wind effect & soil erosion. Result: Veg. lost by grazing & trampling. Soil depleted of nutrients. Moisture evaporated from soil…. erosion & desertification. 7.4 BIOMES – Human Activities Intensive Agricultural Practices Bolivia Source: UNEP Causes of desertification b) Population Growth: Higher birth rates… meant more cash crops & higher demand for food so more overcropping leading to reduced yields & infertile soil…. move to new lands & cleared it of its existing veg. Wood cut for heating & cooking (90% is wood) & buildings in urban areas…. deforestation as tress, shrubs & small bushes cut down. Result: Less humus, infertile soil & lack of wind block by veg. Soil erosion & desertification. Soil conservation: 1. Stone lines…. placing small stones on slopes to reduce run-off…. aligned with the contours so they trap water & give it time to infiltrate into the soil. Benefit: replenish water table; reduce soil erosion; traps soil & seed so plants begin to grow. Method takes up 2% of land but can increase yields by up to 50%. 2. Contour ploughing: Soil is ploughed across the slope. Furrows act like dams holding water. Traps water & reduces soil erosion by 50%. On steeper slopes terracing… slope cut into a series of steps & front of each terrace is edged with stones or mud to trap water. Crop rotation used to replace nutrients into the soil e.g. alfalfa grasses replace nitrogen. Improved farming methods: Strip farming… planting crops in widely spaced rows, filling the space with a different type & height crop, that mature at different times…. Allows water to soak into ground, different nutrients required so soil not exhausted & soil not exposed to wind as a crops protect it. Shelter belts: trees & shrubs planted to reduce wind erosion & bind soil. Some are fruit producing trees which provides food. New animal breeds: smaller better quality cattle that fattened quickly or produced more milk. 7.4 BIOMES – Human Activities Industrial Development – Acid Rain Effects of Acid Rain Source: angelfire.com New animal breeds: smaller herds, better quality cattle that fattened quickly or produced more milk…. reduced grazing needs & increased milk & meat production. Sheep & goats introduced to areas of poor quality scrub.