Lecture 8 Soil Properties and Classifications

Importance and Relevance

• soil covers virtually the entire Earth surface

• to understand biogeography

• all terrestrial food depends on soil

• sustainable use of soils is essential if problems of exhaustion, erosion and pollution are to be avoided

• geomorphology: erosion rates, river sediment, deposition

• takes thousands of years to develop

Soil as a Carbon Reservoir

• 2.5 gigatons

Soil Definition

• mixture of organic matter, minerals, gases, liquids, and organisms that together support life

Pedosphere Uses (4)

• as a medium for plant growth

• as a water storage, supply, purification

• as a modifier of Earth’s atmosphere

• as a habitat for organisms

Regolith

• unconsolidated material found overlying bedrock

  • may have been formed in situ or been transported

• soil is part of the regolith, and is usually the top part which contains a high concentration of organic material and is affected by weathering

Pedogenesis and Soil Horizons (6)

• generation of soil

• physical and chemical weathering and organic activity

• soil horizons

  • O (unsaturated organic)

  • H (saturated organic, e.g. peat)

  • A (mineral horizon at or near surface)

  • B (subsoil)

  • substratum

  • R (bedrock)

Mechanisms of Chemical Weathering (4)

• dissolution by acids or bases

• redox reactions (the addition or removal of O2)

• adding of water to mineral structures (hydration)

• interaction with organic compounds

Types of Dissolution Reactions

• 2 types

• congruent reactions: mineral is dissolved entirely into solution

  • e.g. quartz, calcite

• incongruent reactions: produces solutes AND forms new minerals

  • usually clays or oxides

  • all silicate minerals, e.g. anorthite

Rock Composition

• depth and temperature very important

• minerals formed at higher temperatures are less stable at the Earth’s surface

  • olivine 1200C

  • quartz 700C

• nitrogen and carbon in excess on surface

  • come from atmosphere

Soil Formation

• controlled by several factors

• pedological processes

  • climate

  • biological activity (organisms)

  • relief

  • parent material (e.g. rock)

  • time

• S = f(CL, o, r, p, t)

• soil properties are the function of climate, organisms, relief, parent material and time

• soil formation is the conversion of saprolite to soil

• rate of weathering

  • no plants, no soil = low WR

  • small plants, little bit of soil = intermediate WR

  • big plants, intermediate soil = lower WR

  • big plants, abundant soil = low weathering rate

• actual soil erosion rates in Europe much higher than tolerable

Saprolite

• chemically weathered rock

• geomorphologist: soil is moving, saprolite is not

Fastest rate soil can form in mm/yr

• Australia: 0.143

• California: 0.077

• Oregon: 0.268

• Australia: 0.053

Soil Classification

• why?

  • to describe a soil for exchange of knowledge

  • mapping

  • to show relationship between soils

• lots of different classification systems

• now FAO/UNESCO method use

• influence of parent material

  • Inorganic Parent Material

    • Residual

    • Transported

  • Organic Parent Material

    • Peat (partly decomposed organic matter)

    • Muck (very decomposed organic matter)

FAO/UNESCO Soil Classification

• 1:5,000,000 soil map of the world

• only truly international system

• world reference base for soil resources

• soil divided into 10 sets related to formation processes

Scottish Parent Materials (4)

• in situ (residual) shattered and weathered rock

• glacial till: unsorted, weathered mineral material, unweathered rock, water-modified, moved by ice

• fluvioglacial meltwater deposits of sand and gravel and, in highland areas, morainic deposits

• mountain-top detritus: frost-shattered debris with common rock and scree

• recent deposits, incl aeolian sand, alluvium raised beach deposits, peat

Worldwide Parent Materials

• arenosols (sand)

  • quartz rich rocks -> sandy soils, easily eroded and leach nutrients

• andosols (volcanic)

  • parents material of volcanic ash, tuff, pumice, etc

British Parent Materials

• sandstones, limestones -> cambisols (rich brown earths)

• acid rocks, e.g. granite -> podzols (poor acid soils)

• glacial till -> cambisols to gley soils (waterlogged clay-rich soil)

Influence of Climate: Precipitation

• rainfall impact on types of weathering

• lower rainfall -> salt crusts/lime layers, evaporation leads to salt deposition

• higher rainfall -> leaching of soluble salts, more clays, organic matter, cation exchange capacity and nutrients increase

Influence of Climate: Temperature

• high temperatures lead to the rapid breakdown of organic material

• small increase in clay minerals with temperature

• evaporation (impact depends on rainfall)

Humid Tropics

• high temperatures and precipitation

• highly weathered, nutrients supplied by vegetation

• e.g. ferralsols, alisols, acrisols (acidic), lixisols (washed out)

Arid and Semi-Arid Regions

• high temperatures, low precipitation

• evaporation and deposition of salts

• solonetz (salty), gypsisols (gypsum), durisols (hard)

Humid Temperate Regions

• seasonal variations in temperature and precipitation

• rich organic material near surface and horizons (very generalised)

• podzols, liuvisols, umbrisols

Permafrost Regions

• cryosols: soils showing influence of frost action and permafrost

Histosols

• peat, much soils, organic soils - waterlogged organic soils with limited decomposition, low pH

• found in high latitudes, temperate and tropical environments

• can be productive, but peats contain large carbon reservoirs and need to be conserved

• have been extensively drained and lost in many areas

• major carbon stores, important part of climate mitigation strategies

• peatlands cover 20% of Scotland, 80% degraded, 1600 million tons of carbon

Influence of Relief (4)

• Altitude

  • in UK, colder and wetter conditions lead to accumulation of organic material, e.g. peats

• Aspect

  • warmth of soil

• Slope

  • mass movement, drainage, overland flow, throughflow, creep, water accumulation hollows/base of slopes

• Catena

  • a sequence of soils down a slope, created by a balance of processes such as infiltration, precipitation, and runoff

Other Soil Properties

• texture (particle size)

• mineral composition

• organic matter

• water

• structure

• pH

• air

• temperature

• colour

• density

Pyramid Graph

• sand, clay, and silt percentages