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Soils and Minerals

Soils and Minerals

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Soils and Minerals

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  1. Unit 5 Soils and Minerals

  2. What is soil? • Relatively thin surface layer of the Earth’s crust • Consists of mineral and organic matter that has been modified by natural actions of agents such as weather, wind, water, and organisms • VERY important  many organisms live there and terrestrial plants could not survive without it

  3. How is soil formed? • From parent material (usually rock) that is slowly fragmented (broken down) into smaller and smaller particles • Weathering processes = biological, chemical, or physical processes that break down rocks  can take a very long time, sometimes thousands of years

  4. What affects weathering processes? • Organisms = Plant roots, insects, bacteria, etc. • Respiration produces CO2, which diffuses into the soil and reacts with soil water to form carbonic acid (H2CO3) • Other acids form cracks in rock, which fill with water  freezing and thawing • Climate = precipitation, temperature (freezing and thawing), wind currents, etc.

  5. Topography • A region’s surface features  mountains, valleys, etc. • Steeper slopes have little or no soil on them, more runoff, and more erosion • Moderate slopes and valleys tend to have deep soils

  6. Typical Soil Composition • 45% mineral particles – from weathered rock • 5% organic matter • 25% water • 25% air

  7. Terms to know… • Humus = Partly decomposed organic material in the soil; brown or black in color • Leaching = Movement of water and dissolved material downward through the soil • Illuviation = Deposition of a material into a lower soil layer from a higher layer as a result of leaching

  8. Terms to know… • Sand = Large (0.05 – 2 mm in diameter) inorganic particles in the soil; coarse enough to feel gritty • Silt = Medium sized (0.002 – 0.05 mm in diameter) inorganic particles in the soil; too small to feel gritty • Clay = Small (<0.002 mm in diameter) inorganic partilces in the soil; too small to settle out of suspension

  9. Terms to know… • Soil water = water that fills the pores (spaces) between soil particles; can originate as precipitation or groundwater • Soil air = air that fills the pores between soil particles; usually has the same gases as atmospheric air, but with more carbon dioxide and less oxygen

  10. Soil Horizons • Distinctive horizontal layers in soil • Soil profile = a vertical section from surface to parent material that shows the soil horizons

  11. Soil Horizons • O horizon = uppermost layer of soil, rich in organic material • A horizon = topsoil, dark and rich in organic material and humus, tends to be granular and nutrient poor due to leaching

  12. Soil Horizons • E horizon = sometimes occurs between A and B horizons, very heavily leached, not always present • B horizon = light colored subsoil, often a zone of illuviation where minerals and nutrients accumulate, usually rich in iron, aluminum, and clay

  13. Soil Horizons • C horizon = weathered pieces of rock, below most roots, usually saturated with groundwater • Bedrock = original parent material that the soil rests on

  14. Soil Properties • Texture = determined by proportions of sand, silt and clay • Loam = ideal agricultural soil, 40% sand, 40% silt, 20% clay • Soil triangle = tool used to classify soil

  15. Soil Properties • Charge = some are positively charged (K+) or negatively charged (NO-3) • Clay particles are usually negatively charged

  16. Soil Properties • Soil acidity = measured on the pH scale (1-14) • 1-6.9 = Acid • 7 = Neutral • 7.1-14 = Base • Close to 7 = weak • Far from 7 = strong • pH affects what minerals remain in the soil • Optimum pH for plant growth = 6.0-7.0

  17. Major Soil Groups • Spodosols = form in colder climates with a lot of precipitation and good drainage (ex. Coniferous forest) • O horizon = acidic litter (mostly needles) • E horizon = ash-grey, acidic • B horizon = dark brown • Too acidic and nutrient-poor to be good for agriculture

  18. Major Soil Groups • Alfisols = form in temperate deciduous forests • A horizon = brown to grey-brown • B horizon = contains most of the clay and nutrients washed out of A and E horizons • If the deciduous forest is intact, soil fertility is maintained by continuous supply of plant litter • If soil is cleared for farmland, fertilizers must be used

  19. Major Soil Groups • Mollisols = primarily found in temperate, semiarid grasslands; very fertile • A horizon = thick, dark brown to black, rich in humus • Precipitation is not enough to leach nutrients to lower layers • Most of worlds grain crops are grown on mollisols

  20. Major Soil Groups • Aridisols = found in arid regions; lack of precipitation and rich vegetation results in a lack of leaching and illuviation or distinct layers • Crops can be grown with irrigation • Rangeland for grazing animals

  21. Major Soil Groups • Oxisols = primarily found in tropical and subtropical areas with a lot of precipitation, low in nutrient minerals • O horizon = little organic material because leaves and twigs are rapidly decomposed • A horizon = rich in humus • B horizon = thick, highly leached, acidic, nutrient-poor

  22. Major Soil Groups • Pedalfers = found around here and in other temperate regions that get significant rain; B horizons contain iron- and aluminum-rich materials  red and orange • Pedocal = typical of dry climates like those found n the western United States; chemical weathering is slower.  less clay; rich in calcium based minerals (calcite, limestone) • Laterite = form in hot, wet tropical areas; large amount of precipitation leaches out much of the silicate material making these soils orange and organically poor. One major reason for rainforest deforestation is the periodic need to replace the land exhausted by farming.

  23. Major Soil Groups

  24. Who lives in the soil? • Millions of microorganisms (bacteria, fungi, algae, microscopic worms, protozoa, etc.) can occupy 1teaspoon of fertile agricultural soil • Bigger organisms live there too  plant roots, insects, earthworms, moles, snakes, groundhogs, etc.

  25. Why are soil organisms important? • They provide various ecosystem services such as: • Decaying and cycling organic material • Preventing erosion • Breaking down toxic materials • Cleansing water • Affecting the atmosphere • Castings = bits of soil that have passed through the gut of an earthworm, moving nutrients from deeper layers to the surface

  26. Why are soil organisms important? • Ants make tunnels and chambers that aerate the soil • They also bring food from the surface down into the soil  some of it decomposes and adds nutrients to the soil • Also bury seeds in soil  grow into plants

  27. Why are soil organisms important? • Plants are affected by the properties of soil, and the soil is affected by the plants that grow in it • Mycorrhizae = associations between plants and fungi that enable the plants to absorb enough nutrients and minerals • Mycelium = tiny thread-like extensions of fungi that go deeper into the soil than the roots of a plant, allowing the plant more access to nutrients and minerals and the fungi access to the food produced by photosynthesis

  28. Nutrient Cycling • Nutrients (like nitrogen and phosphorus) cycle between soil and organisms • Decomposition = bacteria and fungi • Reabsorption = mainly plants • Leaching • Runoff • Dust storms •

  29. Soil Problems • Human activities • Erosion • Mineral depletion • Sustainable soil use = using soil resources wisely without a reduction in the amount or fertility of the soil so that it is productive for future generations

  30. Erosion • The wearing away or removal of soil from the land • Caused by water, wind, ice, etc. • A natural process that can be accelerated by human activity • About 4.6 billion tons of soil are eroded from US croplans and rangelands each year

  31. Major Types of Erosion • Surface erosion = rain, wind, frost, etc. detach soil particles from the surface that are washed or blown away • Fluvial erosion = water gouges shallow channels or deep gullies in soil • Mass-movement erosion = gravity combines with heavy rain or earthquakes; whole slopes can slump, slip or slide • Stream bank erosion = not dramatic, can occur in regions where there is frequent flooding

  32. Surface erosion • Sheet erosion = rain falls on bare or sparsely covered soil and moves loose material (silt, clay, humus) downhill  soil loses fertility • Wind erosion = soil on surface is blown away, can form dunes, more severe if vegetation has been grazed or disturbed

  33. Fluvial erosion • Rill erosion = water runoff from shallow slopes and gathers is small V-shaped channels or rills • Gully erosion = occurs on loose subsoils, they are deep and generate a lot of sediment, which is usually deposited into rivers • Tunnel gullying = water enters on a slope through cracks in the soil after a dry period, forming a tunnel that collapses on itself, forming a gully

  34. Mass-movement erosion • Slips = common where rock is mudstone or siltstone, topsoil and subsoil becomes saturated and slides downhill • Earth flows = saturated soils move downhill, but vegetable mats are left behind • Scree erosion = occurs in steep mountainous areas, soil erodes in fan shapes

  35. The American Dust Bowl • The semi-arid prairie soil of the American mid-west were cleared for crops and overgrazed by animals • The disturbance of the native vegetation left the soil vulnerable to erosion  the fertile topsoil was removed by wind and water

  36. The American Dust Bowl • Dust could be seen as far as the east coast and the Atlantic Ocean

  37. Nutrient Mineral Depletion • In a natural ecosystem, essential nutrient minerals cycle between soil and organisms (especially plants) • Nutrients and minerals usually return to soil through decomposition • Harvesting crops stops this cycle  soil is depleted of nutrients • Especially severe in rainforests

  38. Soil Problems in the USA • Rates of erosion measured every 5 years • Good news: erosion is declining (since 1982) • Bad news: erosion is still happening at significant levels  approximately 25% of agricultural lands in US are losing topsoil faster than it is being replaced

  39. World Soil Problems • 75 billion tons of topsoil lost each year, especially in Asia, Africa, and Central and South America • Caused by the amount of agriculture needed to feed densely populated areas  over-exploitation of soil

  40. Solving Soil Problems… • Conservation tillage = residues from previous crops are left in the soil, covering it and keeping it in place • No-tillage = soil is undisturbed over the winter • During planting, a narrow furrow is cut for seeds • Traditionally, soil would be cut and turned in the spring, weeds would be removed, and seeds would be covered • Plant cover removed = more risk for erosion •

  41. Solving Soil Problems… • Crop rotation = planting a series of different crops in the same field over a period of years • Same crop over and over = more pests accumulate, same nutrients get depleted

  42. Solving Soil Problems… • Contour plowing = plowing in curved lines that conform to shape of land instead of straight lines • Strip cropping = alternating strips of different crops on natural contours • Terracing = making level areas on steep slopes for crops

  43. Solving Soil Problems… • Organic fertilizers = animal manure, crop residue, bone meal, compost, etc. Slow acting, long lasting • Instead of… • Commercial inorganic fertilizers = manufactured from chemical compounds, exact chemical composition is known, but they are only available for a short time before they wash away

  44. Solving Soil Problems… • Shelter-belts = rows of trees between fields that lessen the impact of wind

  45. Soil Conservation Act of 1935 • Authorized formation of Soil Conservation Service • Works with US citizens to conserve natural resources on private lands • Farmers tend to use less soil when prices are lower, but more when prices are high

  46. Food Security Act (Farm Bill) of 1985 • Provisions for two main soil conservation programs: • Conservation compliance program • Conservation Reserve Program • Farmers with highly erodible land must develop a 5-year conservation plan, or lose federal subsidies

  47. Conservation Reserve Program (CRP) • Voluntary program that pays farmers $50 per acre per year to stop producing crops on erodible farmland • Planting native grasses and “retiring” the land for 10 years • Very beneficial and effective

  48. Grasslands Reserve Program (2002) • Pays farmers to protect up to 2 million acres of virgin and improved pastureland for at least 10 years

  49. General Mining Law of 1872 • Passed to encourage settlement in the western states • Allows companies or individuals (of any origin) to stake mining claims on federal land, purchase the land for $2.50-$5 per acre, extract all valuable minerals from the land, and keep all the profits • In 1995, ASARCO obtained land with $2.9 billion worth of copper and silver for $1,745 • Congress has since put a hold on all such sales (1996)