Environmental
Factors
Soils
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
Earth’s Surface
30%
30 % land
70 % water
70%
Land
Water
SOIL PROVIDES
Physical support
Water
Nutrients
5 Components of Soil
Mineral Particles
Organic Matter
Water
Atmosphere
Organisms/Microorganisms
5 Soil Forming Factors
Parent Material
Climate
Organic Material/Organisms
Topography
Time
5 Soil Forming Factors
Parent Material
unconsolidated accumulation forming from the
breakdown of parent rocks by chemical and
physical weathering
Parent Material Chemical Weathering
Four processes:
Carbonation – reactions with carbonic acid
Hydration – adds molecular water
Hydrolysis – reactions with water
Oxidation – reactions with oxygen
Parent Material Physical Weathering
Examples:
Exfoliation by temperature changes
Varying coefficients of expansion
Expansion of freezing water
Glacial grinding
Moving water
Wind “sandblasting”
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
Organic Fraction
Residues of Plants and Animals:
- prairies more organic matter than forests
- type of vegetation affects amounts
- temperature and moisture:
warm & moist - no gain
cold & moist - accumulation
tillage - loss of organic matter
Topography
Influences drainage and runoff:
- erosion
- less percolation
- leaching (internal drainage dependent)
- gentle slopes in heavy vegetation form welldefined profiles
- topography affects climate affects vegetation
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 in older soils
Soil Profile
The vertical section of a soil through all its
horizons (layers), ending in the parent material
Each horizon differs chemically/physically
A Horizon
Often called topsoil . . .
- Higher in organic matter (darker)
- Zone of leaching (nutrients, clays)
- Zone of biological activity
- Highest level of nutrients
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
C Horizon
Sometimes called substratum . . .
- Extends from B horizon to bedrock
- Zone of least weathering (parent material)
OLDER SOILS
More clearly defined horizons, thus . . .
MORE COMPLETE DESCRIPTION OF THE
PROFILE
Typical profile:
1 m for a temperate-zone soil
SOIL PHYSICAL FACTORS
Texture
indicates the percentage of:
Sand
Silt
Clay
Percentages are applied to . . .
Textural Triangle:
- Percent Sand, Silt, Clay
- 12 Textural Classes (fig 8-6, p. 146)
- Texture by “Feel” with experience
Textural Triangle
Structure
(p.147)
Indicates how individual soil particles group
e.g. Clay - develops blocky, massive structure
Sand – no structure (each particle independent)
Soils with some smaller particles and O.M.
demonstrate . . .
AGGREGATION – very important to soil
quality
http://ltpwww.gsfc.nasa.gov/globe/pvg/prop1.htm
Aggregation improves:
Aeration
Percolation
Root penetration
SOIL MOISTURE
Soil is the plants water reservoir…
Water uptake is by:
Diffusion
and
Osmosis
Water
POTENTIAL
Refers to the ability of water to
move in soil
More water in soil = More water potential
At saturation, potential is near 0 (zero)
As soil dries, values become more negative
Water is held more tightly by soil
FOUR CATEGORIES OF
SOIL MOISTURE
Chemically combined . . . unavailable
Hygroscopic . . . unavailable
Gravitational . . . moves downward by gravity
Capillary . . . taken up by plants
SATURATED SOILS
Sandy soil:
gravitational water moves rapidly downward
Clay loam:
gravitational water retained 2-3 days afterward
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 distance
Slow in clay soils but may move great distances