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 % land

70 % water 70%

30%

LandWater

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 gaincold & moist - accumulationtillage - loss of organic matter

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

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

Textureindicates 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 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

http://ltpwww.gsfc.nasa.gov/globe/pvg/prop1.htm

Aggregation improves:

Aeration

Percolation

Root penetration

SOIL MOISTURESoil 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 potentialAt saturation, potential is near 0 (zero)As soil dries, values become more negativeWater 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 distanceSlow in clay soils but may move great distances