Soils

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Soils

• Chapter 12
• By Lori Sedlak

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Soil Minerals

• Soil
• Mixture of minerals, organic material, air,
  water, microorganisms
• Approximately 45 percent is minerals
• Soils lose minerals as well as gain them
• Lost by erosion or an agent of erosion
• Leaching-movement of minerals dissolved in water
• Lose minerals by plant uptake

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Soil Organic Matter

• 5 percent of total bulk of the soil
• Decayed remains of living organisms
• Leaves, roots, twigs, insects, animals, worms,
  and the like
• Also called humus
• Glue that holds mineral particles together
• Helps form pore spaces
• Adding increases fertility

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Soil Water and Air

• Water held in pores called soil moisture
• If contains too much water, soil becomes
  saturated
• Optimum water is called field capacity
• Pores 50 percent air and 50 percent water
• When moisture is gone, called wilting point
• Over an extended period causes death
• See Figure 12-2 on Page 245

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Soil Water and Air (continued)

• Fertile soil also needs to contain air
• Air in soil similar to air in atmosphere
• Oxygen is especially important
• Turning and mixing soil called tilling
• How air is introduced to the soil
• See Figure 12-3 on Page 246

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Soil Organisms

• Earthworms
• Mix and aerate the soil
• Bacteria
• Help add nitrogen to the soil
• Nitrogen-fixing bacteria
• Convert atmospheric nitrogen to plant-available
  forms
• Such as nitrates

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Soil Organisms (continued)

• Fungi and insects
• Improve structure and add organic matter
• Rodents-mix the soil
• Lichen-composed of algae and fungi
• Algae share food with fungi
• Fungi secrete an acid
• Symbiotic relationship breaks down rock forms
  minerals

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Soil Structure

• Arrangement of mineral particles and organic
  matter
• Loam equal parts sand, silt, and clay
• Sandy loamexcessive drainage
• If high in clay, opposite condition
• Prevents water infiltration
• Does not support crop growth and highly erodible

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Parent Material

• Rock from which soil minerals were derived
• Two main types
• Residual-parent material located directly below
• Transported-moved parent material
• Glacial transported-by ice
• Water transported-called alluvial soils
• Wind transported-called loess

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Soil Horizons

• Soil profilecross-sectional view of soil
• Identifies unique horizons
• Soil horizon
• Layer with similar physical and chemical features
• Five horizons
• Ocalled the organic layer
• Layer of decaying organic matter

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Soil Horizons (continued)

• A-horizontopsoil
• Darker, rich in minerals, air, and water
• Zone where most roots grow
• Basis for all productive agriculture
• B-horizonsubsoil
• Light tan/reddish brown, low organic matter
• Very dense and high in iron

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Soil Horizons (continued)

• C-horizonunconsolidated parent material
• Broken-up fractured rock
• D-horizonbedrock
• Igneous metamorphic or sedimentary rock
• Some soils do not have all horizons

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Soil Classification

• Soil is classified into groups called orders
• Ten soil orders
• Alfisolsdeciduous soils of the northeast
• Extensive O-horizons
• Dark, rich A-horizons
• C-horizons formed from glacial deposits

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Soil Classification (continued)

• Mollisolsgrasslands of the Midwest
• No O-horizons
• Deep and rich A-horizons
• C-horizonloess, alluvial, or glacial till
• Oxisoltropical rainforest
• Thin O-horizon barely an A-horizon
• B-horizon dense and high in iron

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Soil Classification (continued)

• Exposed heavy rains and tropical sun
• Bakes into hard, bricklike material
• Do not support long-term crop growth