SOIL DESCRIPTION

SOIL DESCRIPTION

1. dynamic soil layer

   • definition = a natural surface layer containing living matter and supporting or capable of supporting plants
   • pedology = study of the soil itself
     • drives
       • 1)ecology
       • 2)geomorphology
       • 3)Palaeoecology
   • Why dynamic?
     • many complex physical and chemical activities are going on simultaneously within it
     • soil-forming activity are expressed differently from place to place
   • boundary between vegetation (the biosphere) and rocks (the lithosphere)

2. factors of soil formation (5)

   • 1. Climate
     • 1. weathering of plant material
     • 2. transportation of parent material if it has been moved
     • 3. development of features by internal soil processes
     • 4. erosion or physical removal of the soil body
   • 2. Vegetation
     • 1. Quantity and quality of living and decaying plants.
     • 2. Organisms responsible for decay of surface organic matter etc.
     • 3. Lack of organic matter may be responsible for reduction in
       • 1) weathering
       • 2) plant nutrient supply
       • 3) leaching
     • 4. Form and structure determine rate of litter addition.
   • 3. Biotic factors
   • 4. Parent material
     • underlying geological material
       • usually older
   • 5. Relief
     • significant relationship between relief and parent material and soil drainage
     • angle of slope and position of the soils
       • effect
         • ALTITUDE
           • height in relation to sea level
         • ASPECT
           • horizontal direction to which a mountain slope faces
   • HISTORY/DEVELOPMENT
     • 19th century - Dokuchaev
       • s = f {c, o, p} to
         • s = soil
         • c = climate
         • p = parent material
         • t = time
           • = independent variable
     • 1941 - Jenny
       • s = f {cl, o, r, p, t}
         • t = time = NO LONGER INDEPENDENT VARIABLE
           • Time play key role
     • Jenny - 1961
       • l, s, v, a = f {Lo , px , t}
         • l = any property of the ecosystem in its totality
         • s = soil properties
         • v = vegetation
         • a = animal properties
         • px = the flux of the system
         • Lo = represents the assemblage of properties at time zero
         • t = the age of the system
       • modified equation to make it more applicable to modern thought concerning ecosystems
     • Importance of time as a factor in the formation of soils

3. soil processes

   • 1. weathering
     • physical weathering
       • Physical weathering = smaller particle size = larger surface area = greater physical weathering.
         • as surface area increases = more sites of chemical weathering also
       • Large temperature range over a short time period.
       • Depends less on moisture than does chemical weathering.
     • Chemical weathering
       • Hydrolysis = H2O -> H+ + OH-
         • IMPORTANT - in breakdown of FELDSPAR minerals
         • Break down of water into Hydrogen + hydroxyl
       • OXIDATION = Addition of oxygen
       • REDUCTION = removal of oxygen
         • saturated soils
       • HYDRATION = addition of H20 (Water molecules)
       • SOLUTION = whole minerals dissolved in water
       • Carbonation = addition of carbonates (CO3) (in form of carbonic acid -H2CO3 )
         • EQUATION = CaCO3+CO2+H20->Ca2+ + 2H203-
           • calcite + carbon dioxide + water -> calcium + bicarbonate
       • efficiency dependent on
         • comp + concentration of solution
         • temperature
           • higher temp = more vigorous reaction
           • 10 degrees = necessary for activation
         • bacterial presence
         • substances taken into the solution from minerals decomposing
     • Biological weathering
   • ALL 3 PROCESSES ACT TOGETHER
   • 2. decomposition of organic matter
     • Biological weathering
     • production of carbon dioxide + organic acids + ammonia + nitric acid - through the break down of dead organisms
   • 3. re-organisation processes and translocation

4. pedogenic processes

   • 1) Eluviation/lessivage = physical transportation of soil particles
   • 2) Leaching = removal of dissolved chemical compounds.
   • 3)Salinisation = upward movement of water and salts in arid climates.
   • Chelation = the picking up of acidic organic extracts capable of combining with iron or aluminium in soils or rocks and removing these elements in solution.
   • Decay by soil organisms
     • depends on
       • temperature
       • moisture of the soil.

5. soil profiles/stratigraphy + properties

   • Soil horizons
     • Distinctive horizontal layers set apart from other soil zones or layers by:
       • 1) physical and chemical composition
       • 2) organic matter content
       • 3) structure
       • 4)combination of those properties
     • Developed by interaction through time of climate, living organisms and relief.
     • Soil horizons occur parallel to the surface of the profile
       • differentiated by soil-forming processes
     • Horizons are not always distinctive
       • can be = gradual and transitional.
   • profile
     • basic unit in pedology.
     • soils represented in 2d profile
     • contains = three or four different horizons
       • each horizon is allocated a letters of notation
         • = defines certain properties and certain processes
   • Soil stratigraphy
     • (4) principles
       • Principle of Lateral Continuity
         • 1) Must be able to trace the soil laterally
         • 2) soil must vary with the topography
       • Principle of Pedogenic Persistence
         • 1) Must be able to recognise features preserved without alteration
         • 2) must be able to distinguish these from altered features
       • Law of superposition
         • In an undisturbed sequence = the youngest stratum is always at the top
       • Law of Separate Identity
         • Soil must be identified as a distinct entity
   • structural groups
     • Crumb
     • Blocky
     • Columnar
     • Prismatic
     • Platy
   • Soil texture
     • most important single property of soil
     • proportion of sand, silt and clay less than 2 mm in the mineral fraction
     • Different ratios of sand, silt and clay give rise to textural classes.
       • Sand 2000µ – 60µ (2 – 0.06 mm)
         • location of formation
           • Quaternary sedimentation has occurred
           • places with sedimentary bedrock
         • Sandy soils composed of quartz (SiO2 ) particles = resist leaching whereas other minerals get washed out.
           • Quartz - 90%, Feldspars (CA, Al, K, Na), Micas.
       • Silt 60µ – 2µ (0.06 – 0.002 mm)
         • location of formation
           • large rivers deposit alluvium on an annual basis.
         • as particles are small = become waterlogged quite easily
         • Very susceptible to wind erosion
           • loess
       • Clay <2µ (< 0.002 mm)
         • synonomous with the term ‘clay minerals’
           • BUT
             • clay minerals have a platy structure
             • clay minerals can be > 2µ.
             • Clay mineral tend to ionise = gain or lose electrons
               • BUT ALSO exist in a neutral condition
               • 1. Cation Exchange Capacity - ability to adsorb cations
               • 2. capacity to absorb water
               • 3. ionic double layer
               • 4. variations in structure mean they do not have a fixed CEC
         • = mineral colloids
         • = increased surface area per unit volume
           • SO = more susceptible to chemical weathering
     • Coarser particles
       • 1) Associated with areas of Quaternary deposits
       • 2) derive from bedrock shattering
       • sand,silt, clay
   • soil colour
     • Ferric iron = yellowish brown
     • Ferrous iron = greys and blues
       • associated with waterlogged soils
     • Bh horizons = brown or black in the subsoil.
       • contain less humus
     • CaCO3 = grey or whitish
     • Aluminium = yellowish
     • colour exerts the least influence on other properties
     • influenced by
       • Organic matter
       • mineral colouring agents
       • Soil texture
       • soil structure

6. basic analytical techniques

   • colour
   • stratigraphy annotations
   • texture
     • coarseness