The Biosphere- soils

Question 1

What is soil?

Answer 1

Soil is a major component of the Earth’s ecosystems at the interface between the atmosphere, lithosphere,
biosphere and hydrosphere.

Question 2

Performs a wide range of essential functions…

Answer 2

– Medium for plant growth (also animals)
– Water reservoir
– Recycles plant and animal material

Question 3

Components of soil

Answer 3

Mineral 45%
Air 25%
Water 25%
Organic 5%

Question 4

Components of soil

1. Mineral particles

Answer 4

– Weathering of parent material

– Primary/secondary minerals

Question 5

Components of soil

2. Soil organic matter

Answer 5

– Litter, humus and biomass
– Breakdown of nutrients by
microorganisms – mineralization
– Peat forms where decomposition is
slowed
– Important – binding, water holding
capacity, porosity, nutrients

Question 6

Components of soil

3. Soil water

Answer 6

– Ecological function
– Different types of soil water – gravitational water, capillary water,
hygroscopic water, plant available water
– Different soil states – saturation, field capacity, wilting point
– Amount of water held in each state depends on soil texture,
structure and organic matter

Question 7

Components of soil

4. Soil air

Answer 7

– Occupies pores not filled with water
– Aeration = ventilation of soil air
– High CO2 content
– Composition varies from place to place
– Composition varies diurnally and seasonally

Question 8

Soil profile

Answer 8

Vertical section through soil from ground surface – parent material

Question 9

Soil horizons

Answer 9

Distinct horizontal layers in the profile

Some profiles have distinct boundaries whereas others
are gradual

Question 10

Translocation of materials by movement of water through

the soil

Answer 10

– eluviation

– illuviation

Question 11

Horizon classification

Answer 11

Lettered according to genesis and relative position in the profile
• Solum = A, E, and B horizons

Question 12

Pedogenesis

Answer 12

Process of soil formation – takes hundreds and thousands of years

Question 13

Pedogenesis

Main inputs

Answer 13

Parent material weathering, surface accumulation, solutes

and particles from precipitation and wind, energy from the sun and gases from the atmosphere

Question 14

Pedogenesis

Main losses

Answer 14

Through wind and water erosion, and leaching

Question 15

Pedogenesis

Mixing

Answer 15

Soil animals, microbes and plant roots, freeze–thaw cycles, shrinking–swelling, ploughing, chemical processes
Translocations and transformations

Question 16

Luvisols/brown soils

Answer 16

Where rainfall > evapotranspiration

– Clay eluviated and redeposited in argillic horizons

Question 17

Podzolization

Answer 17

– Where there is intense leaching and translocation (acidic)
– Organic acids compound with iron and aluminium
– Freely drained sites under forests and heath plants

Question 18

Gleying

Answer 18

– Waterlogging leads to reduction, mobilization and removal or
redeposition of iron compounds
– Alternate phases can result in mottling

Question 19

Laterization (ferralitization)

Answer 19

– Tropical and subtropical oxisols

– Heavy leaching – depletion of base cations

Question 20

Factors affecting soil formation

Answer 20

• Climate (cl) – most influential
– Determines moisture and temperature regime under the soil

• Organisms (o) – plants, animals, microorganisms and
humans

• Relief (r) – altitude, slope and aspect of the landscape
– Can hasten or delay the influences of climatic factors
– Soil relationship with slop is a catena

• Parent material (p) – influence of the weathered material
on the soil
– Some minerals are more susceptible to weathering than others

• Time (t) – soil is continually forming
– Soil rarely reaches steady state because of changes in
environmental factors, e.g. Quaternary glacial cycles

Question 21

Physical properties of soil

• Soil colour

Answer 21

– Use to determine the nature of soil properties and horizons (e.g. OM content, oxidation–reduction)
– Munsell colour chart – hue, value and chroma

Question 22

Physical properties of soil

• Soil texture

Answer 22

– Proportion of sand, silt and clay-sized fractions
– As particles become smaller they have different properties (e.g.
influence of surface areas on water-holding capacity, cation
exchange capacity and rate of weathering)

Question 23

Physical properties of soil

• Soil structure

Answer 23

– Shape (or type), size and distinctiveness of soil aggregates (peds)
– Blocky, spheroidal, platy and prismatic
– The finer textured the stronger the structure
– Determines soil porosity – affects water and air movement

Question 24

pH

Answer 24

A logarithmic method of expressing the concentration of hydrogen cations (H+) i.e. acidity

Most soils – pH 3.5 – 9