What is soil?

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LECTURE 1

• What is soil?
• What are its functions?
• What does it consist of?

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Definitions

• Nutrient cycle..
• (R. Manchip)
• Solum.
• (A. Graham)

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What is meant by the term soil?

• The collection of natural bodies occupying parts
  of the earths surface that support plants and
  have properties due to the integrated effect of
  climate and living matter acting upon parent
  material, as conditioned by relief over periods
  of time. Brady Weil (1999)

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Why should we study soils?

• We are very dependent on soils
• We will be even more dependent on them in the
  future!
• Nearly all food
• Much of our fiber
• Medicines (incl. anti-cancer drugs)
• Biomass as a source of energy, especially as
  petroleum supplies are

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• So what are the functions of soil in our
  ecosystem?

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Medium for plant growth
Recycling system
SOIL
Habitat
System for water supply purification
Engineering medium
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Medium for plant growth

• Root systems can be even larger than the part of
  the plant that we can see!
• What do plants obtain from soil?
• Physical support
• Ventilation
• Water
• Temperature moderation
• Protection from phytotoxic substances
• Supply of inorganic mineral nutrients in the form
  of dissolved ions

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Regulator of water supply

• Quality and quantity of water in rivers, lakes
  and dams is a concern!
• All water has traveled through or over soil
• Nature and management of soils will have a major
  influence on the purity and amount of water in
  aquatic systems
• Soil absorbs some water, other water flows
  through the soil to reach groundwater
• Water cleansed as it flows through upper soil
  layers
• Rate of water movement in catchments controlled
  (link with Prof. Rowntrees work)

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Recycling system

• Re-use of nutrients very important to ecosystem
  functioning
• Soils play a major role in geochemical cycles
• Soils assimilate organic matter and convert
  mineral nutrients into forms that can be used by
  plants and animals
• Plants and animals return carbon to the
  atmosphere as carbon dioxide that becomes part of
  living things again via photosynthesis
• Accumulation of large quantities of CO2 can
  affect global changes, e.g. greenhouse effect

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Habitat for soil organisms

• A cupful of soil can contain billions of
  organisms belonging to thousands of species
• There is a vast range of niches and habitats even
  in a uniform soil
• Soils harbour much of the earths genetic
  diversity
• Soils are vital components of larger ecosystems

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Engineering medium

• Concept of terra firma
• Most structures rest on soil, most construction
  projects require excavation into the soil
• Some soils not as stable as others
• Reliable construction requires knowledge of the
  diversity of soil properties
• 1 location not necessarily the same as another
• E.g. swelling properties of clays

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• Working with natural soils/soil material is not
  the same as working with manufactured material
• Bearing strength
• Compressibility
• Stability

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How can we think about soil?

• The soil or soil refers to soils
  collectively, like the term vegetation refers
  to plants.
• A soil is an individual, 3-d body that can be
  classified as a particular soil type.

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How does the story of soil start?

• Parent material weathers
• layer of unconsolidated debris (regolith)
  overlying hard, unweathered rock
• living organisms act on regolith physically and
  biochemically
• SOIL FORMATION BEGINS!

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• Soil occurs at the meeting place of
• LITHOSPHERE
• ATMOSPHERE
• HYDROSPHERE
• BIOSPHERE
• Soil is the product of both destructive and
  creative (synthetic) processes

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Soil development processes

• Destructive processes
• Weathering of rock
• Microbial decay of organic residues

• Synthetic processes
• Formation of new minerals and new stable organic
  compounds
• Formation of soil horizons in upper regolithWhat
  does soil consist of?

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What does soil consist of?

• Air
• Minerals
• Water
• Organic matter
• The relative proportions of these constituents
  greatly influence the behaviour and productivity
  of soils

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Mineral constituents

• Various mineral particles (particles of weathered
  parent material)
• Range of sizes
• Sand (2,0 0,05mm)
• Silt (0,05 0.002mm)
• Clay (lt0.002mm)
• Different mixtures of these result in various
  soil textures

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Organic matter

• Wide range of carbonaceous substances including
  living organisms, remains of organisms and
  compounds produced by current and past metabolism
  in the soil
• Organic matter is lost to the soil over time as
  CO2 which is produced by microbial respiration
• Repeated additions of new plant/animal residues
  is necessary to maintain a reasonably constant
  level of soil organic matter
• The influence of organic matter on soil
  properties is far greater than its low by
  weight and volume would suggest

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Soil water

• Essential for the survival and growth of plants
  and other soil organisms
• The movement of water (and the substances
  dissolved in it) through the soil profile is of
  great consequence to the quality and quantity of
  local and regional water resources

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Soil Water (cont.)

• Held within soil pores according to the amount of
  water and the size of the pores within the soil
  matrix
• This restricts the ability of the water to flow
  freely
• Not all of the water in the soil is available to
  plants
• Never pure water but contains hundreds of
  dissolved organic inorganic substances
• More accurate to call it the soil solution
• Contains soluble inorganic compounds essential
  for plant growth
• Acidity / alkalinity of the solution is NB!

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Soil air

• Consists of a changing mixture of gases
• Differs from atmospheric air
• Varies greatly from place to place in the soil
• Local pockets where some gases are consumed by
  plant roots and chemical reactions, whilst others
  are released
• Generally has higher moisture content than the
  atmosphere
• CO2 higher, O2 lower
• Amount and composition of soil air is largely
  determined by the soil water content