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Environmental Factors
Soils
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Earth’s Surface 70 % Water 30 % Land Only 10 % of land is arable (suitable for
cultivation) Of this arable land, 50 % is under cultivation,
therefore, Only 1.5 % of the Earth’s surface is in cultivated
food production
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Earth’s Surface
30 % land
70 % water 70%
30%
LandWater
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SOIL PROVIDES
Physical support
Water
Nutrients
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5 Components of Soil
Mineral Particles
Organic Matter
Water
Atmosphere
Organisms/Microorganisms
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5 Soil Forming Factors
Parent Material
Climate
Organic Material/Organisms
Topography
Time
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5 Soil Forming Factors
Parent Material
unconsolidated accumulation forming from the breakdown of parent rocks by chemical and physical weathering
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Parent Material Chemical Weathering
Four processes:
Carbonation – reactions with carbonic acid
Hydration – adds molecular water
Hydrolysis – reactions with water
Oxidation – reactions with oxygen
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Parent Material Physical Weathering
Examples:
Exfoliation by temperature changes
Varying coefficients of expansion
Expansion of freezing water
Glacial grinding
Moving water
Wind “sandblasting”
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Climate
Rainfall and Temperature affect formation:
High Rainfall – leaching, acid reaction, low fertility, red or yellow colors, rapid chemical weathering
note: higher temperatures accelerate formation
Arid Climate – low leaching, Ca & Mg accumulate, basic reaction, excess salts, slower weathering
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Organic Fraction
Residues of Plants and Animals:
- prairies more organic matter than forests- type of vegetation affects amounts- temperature and moisture:
warm & moist - no gaincold & moist - accumulationtillage - loss of organic matter
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Topography
Influences drainage and runoff:
- erosion
- less percolation
- leaching (internal drainage dependent)
- gentle slopes in heavy vegetation form well-defined profiles
- topography affects climate affects vegetation
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Time
Parent Material affects rate of decomposition
- harder rocks (granite) take longer to form soil
- softer rocks (limestone) less time
biological and chemical reactions form profile:
differentiates into Horizons over time
more prominent older soils
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Soil Profile
The vertical section of a soil through all its horizons (layers), ending in the parent material
Each horizon differs chemically/physically
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A Horizon
Often called topsoil . . .
- Higher in organic matter (darker) - Zone of leaching (nutrients, clays)
- Zone of biological activity- Highest level of nutrients
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B Horizon
Referred to as subsoil . . .
- Less organic matter - Smaller particles
- Lighter in color ? Depends on materials leached- Zone of accumulation (soluble nutrients, clays)
A and B together: Rooting Zone . . . Solum
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C Horizon
Sometimes called substratum . . .
- Extends from B horizon to bedrock
- Zone of least weathering (parent material)
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OLDER SOILS
More clearly defined horizons, thus . . .
MORE COMPLETE DESCRIPTION OF THE PROFILE
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Typical profile:
1 m for a temperate-zone soil
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SOIL PHYSICAL FACTORS
Textureindicates the percentage of:
Sand
Silt
Clay
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Percentages are applied to . . .
Textural Triangle:
- Percent Sand, Silt, Clay
- 12 Textural Classes (fig 8-6, p. 146)
- Texture by “Feel” with experience
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Textural Triangle
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Structure(p.147)
Indicates how individual soil particles groupe.g. Clay - develops blocky, massive structureSand – no structure (each particle independent)
Soils with some smaller particles and O.M. demonstrate . . .
AGGREGATION – very important to soil quality
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Aggregation improves:
Aeration
Percolation
Root penetration
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SOIL MOISTURESoil is the plants water reservoir…
Water uptake is by:
Diffusion
and
Osmosis
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Water POTENTIAL
Refers to the ability of water to move in soil
More water in soil = More water potentialAt saturation, potential is near 0 (zero)As soil dries, values become more negativeWater is held more tightly by soil
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FOUR CATEGORIES OF SOIL MOISTURE
Chemically combined . . . unavailable
Hygroscopic . . . unavailable
Gravitational . . . moves downward by gravity
Capillary . . . taken up by plants
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SATURATED SOILS
Sandy soil:gravitational water moves rapidly downward
Clay loam:gravitational water retained 2-3 days afterward
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Once soils lose gravitational water (drain) movement is by . . .
Capillarity – movement due to attraction between water molecules and soil particles
Rapid in sandy soils but limited in distanceSlow in clay soils but may move great distances