Soil Flashcards
What is soil?
- Formed by pedogenesis over 1000s of years
- An open system with flows going in and out
- Total soil compartment is not finite
What are the inputs of soil?
- Gas inputs
- Plant and animal decomposition
- Solar energy
- Weathering of bedrock
- Accumulation of particles
- Mineral precipitation
What are the outputs of soil?
- Wind and water erosion
- Leaching
Why are soils important?
Soils are the interface between the lithosphere, biosphere, hydrosphere and atmosphere
What does soil produce?
- Soils produce gases such as methane and carbon dioxide as carbon rich plant material decomposers
- Important for global carbon cycle and nitrogen fixation
How much carbon is stored in earths soil?
The amount of carbo stored in earths soil is estimated at 1500 PgC – 1000 billion grams
How does carbon get into the soil
- Photosynthesis, then plant death and then plant consumption by animals and microbes
How does carbon leave the soil?
- Microbial respiration
- Eating larger carbon containing molecules, like sugars and breaking them into smaller components = carbon dioxide and methane
What ecosystem functions and services does soil provide?
- Provides medium for food and fuel production
- Store water and modify flood run off
- Infiltration and and storage in soil pore space
- Provides habitats and important for biodiversity
- recycles plant and animal waste - cycling carbon and nitrogen
Describe soil composition
- Composition by volume
- Highly variable
- Solid - minerals and organic matter takes up 40-60%
- Pore space - water 20-50% or gas 10-15%
- Organic material - 10%
What are the geological controls on soil chemistry?
- Mineral component of soils is strongly dependent on the underlying parent mineral
What are the impacts of soil composition on human health and food security?
- The goitre belt
- Selenium deficiencies
What is the goitre belt?
- Goitre (hypothyroidism), perinatal mortality – caused by iodine deficiency – needed for the production of thyroxine in the thyroid gland
- Main source is dietary – crops, sea weed, shellfish
- Soil iodine concentrations dependent on coastal proximity and chemical mobility
What is a selenium deficiency?
- Essential trace element: healthy immune systems, sperm motility, production of thyroid hormones
- Deficiency – linked to mood swings, cancers and diabetes
- Dietary intake is dependent upon soil chemistry
- Concentration in soils is directly dependent upon the concentration in underlying geology
Why is soil water important?
- Important for delivering nutrients, removing toxins and controlling aeration of soils
What are the 3 key water states of soil?
1) Saturation - no gas present
2) At field capacity
3) Permanent wilting point - plants cannot access it
What is the relationship between soil water and texture?
- Wilting point increase as particle size decreases
- Finer grain particles, more surface area, more water being held onto particles/sediments
What is soil waters holding capacity?
- Varies globally - depends on physical characteristic of soil
- This has consequences for food security and reliability of crop yields
What is the typical soil profile?
- Distinctive horizontal layers due to vertical translocation of materials by water
- Gravity and water flow moves material up and down
- Generally water moves down
What is eluviation?
The movement of material
What are the different layers of soil?
- O - organic horizon
- A - mineral horizon
- E - mineral horizon
- B - mineral horizon
- C - unconsolidated material
- R - bedrock
What are environmental factors affecting soil formation?
- Climate – temperature, rainfall, vegetation, rates of weathering
- Underlying geology – some minerals more easily weathered
- Relief and topography – hillslopes/floodplains
- Biota – bioturbation, decomposition, trampling and compaction
Outline the physical properties of soils texture
- Particles - clays, silts, sands, gravels - defined by diameter
- Solution - anything passed through sieve
- Texture - distribution fo particle size determines soil texture
Outline the chemical properties of soil
- A major component of soils is colloidal and fine- grained clay minerals
- Phyllosilicates or platy minerals comprising layers of Si and Al bound to oxygen
What is the sheet structure of soil?
- Minerals slip over each other
- Sorption capacity
- Isomorphic substitution – negative charge – chemically reactive
What is cation exchange?
Negative sites of clay minerals are every balanced with positively charged cations
These are absorbed and exchangeable - can be displaced by other cations in solution
What is pH?
A logarithmic method of expressing the concentration of hydrogen
What is the pH of most soils
pH is typically between 3.5 - 9
What processes result in hydrogen becoming a predominant cation in soil?
- Leaching of wet cations
- Carbon dioxide from reparation dissolves into carbonic acids
- Crop harvest removes base cations taken up by plants
