Lecture 8 - soil physical properties
What is the gravimetric soil moisture equation
(Mass water/ mass oven dry soil) x 100
What other properties can soil colour tell you about
- Drainage - Minerals/ parent material - Organic matter content - Soil moisture content
What is soil moisture important for
- Erosion - Agriculture - Flood risk assessment - Engineering - Nutrient cycling
What does a rise in surface area of soil lead to
- Increased absorbance ability - Increased plasticity - Increased swelling capacity
What does soil moisture influence
- Infiltration - Overland flow generation - Erosion - Plant growth - Microbial habitats
What is the importance of soil texture
- Influence other physical properties - porosity and infiltration - As well as key soil functions - water retention/loss, same for nutrients, erodibility
What are the best ways to manage soil structure
- Minimise tillage - Keep soil surface covered - plant vegetation = less erosion - Incorporate organic matter - fertilisers - Till at the right moisture content - waterlogged soil tillage destroys structure
Due to soil structure providing much of the soils porosity what does it control
- Permeability - Aeration - Root movement and dev - Microbial habitats - Nutrient movement
What is soil structure characterised by
- Shape or type - Size - Grade (distinctness) They determine properties
What does soil moisture depend on
- Texture - Structure - Organic matter content - Antecedent rainfall
How big is silt
0.05-0.002mm
How big is sand
2-0.05mm
How much of a standard top soil does air account for
25%
How much of a standard top soil does water account for
25%
How much of a standard top soil does mineral matter and nutrients account for
45%
How much of a standard top soil does organic matter account for
5%
How big is Clay
< 0.002mm
What is the soil texture triangle
A way of determining the type of soil based off the relative percentages of sand, silt and clay contents
At what scale does the soil colour change
Across the soil profile
What type of soil structure has a moderate permeability
Blocky and columnar/ prismatic
What 2 groups can mineral particles be separated by sizes into
Coarse fragments > 2mm Fine earth fragment < 2mm
How can soil structure development be described
Dynamic- can be changed over time by physical and chemical processes
What happens to the components for different types of soil
Give an example They change - e.g. Peat would have much more organic matter.
What type of soil structure has a high permeability
Granular and aggregated
What soil colour does a waterlogged soil have
Grey and mottling
What influence on the ability to hold nutrients does clay have
High
What influence on the aeration does sand have
High
What influence on the compaction does clay have
High
What influence on the drainage rate does sand have
High
What influence on the leaching of pollutants does sand have
High
What influence on the organic matter decomposition does sand have
High
What influence on the water holding capacity does clay have
High
When is there a strong stable structure
If there is lots of organic matter
How can we estimate particle density
If we know the texture and parent material
Where is particle density hard to test
In the lab
What type of bulk density does a compressed soil have
Larger
What influence on the ability to hold nutrients does sand have
Low
What influence on the aeration does clay have
Low
What influence on the compaction does sand have
Low
What influence on the drainage rate does clay have
Low
What influence on the leaching of pollutants does clay have
Low
What influence on the organic matter decomposition does clay have
Low
What influence on the water holding capacity does sand have
Low
How is soil bulk density influenced by human activity
Machinery and livestock - these compact the soil Consequences include reduction in porosity and loss of structure Which negatively impacts flood responses and agricultural productivity.
What is the relationship between soil texture and pore size for medium pores
Medium particle size/ mixture - loam = medium percolation rate
What is one of the first thing soil scientists do/ test for
Mineral particle size, as it tells us a great deal
What sticks the different materials in soil together
Organic matter
What are the 4 components of standard top soil
Organic matter Mineral matter and nutrients (from parent material) Air Water
What are 2 mechanical properties of soil
Plasticity Swelling capacity
What type of soil structure has a low permeability
Platey and massive
What kind of soil components are air and water
Pore spaces
What soil colour does an iron rich parent material have
Reddish
What are mottles significant in regards to
Soil moisture status
What does soil structure provide much of
Soil porosity
What does soil colour have little effect on
Soil properties - but tells you lots about other properties
What are some controls on bulk density
Soil texture - soils with a greater surface area typically have lower bulk densities Organic matter - bulk density is reduced by a high OM content
What soils have an unstable structure
Soils that would naturally lose all aggregation
What kind of soil components are organic matter and mineral matter& nutrients
Solid materials
What soils naturally have a good structure in the long term
Stable soil structures
What is the main difference between soil structure and soil texture
Texture cannot be altered by human activity - however structure can
What does soil structure have inherent properties of
The parent material - can be altered by physical processes
What is an example of how soil structure can be modified by humans
Tillage - major cause of loss of soil structure
How is soil colour determined
Using the Munsell chart
How do we measure soil moisture content
Using the gravimetric soil moisture equation
What soil colour does an carbonate rich parent material have
White and pale
Understandings of what can be gained from classify soils based on mineral particle sizes
aeration properties or how well water can leach through the soils
Why are subsoil horizons with less organic matter red or yellow if well drained
bc presence of oxidised iron (Fe III)
Why are subsoil horizons with less organic matter blue or grey if poorly drained
bc presence of reduced iron (Fe II)
Why is it important to classify soil based on fragment size
because it tells us how well particles can stick together - about pore space - small particles = stick together tightly = little amount of pore space.
Why is size of finer particles important
because of surface area - indicates properties of soil
What do spatial and temporal changes in soil moisture include
biological processes and soil structure.
How does soil colour give a good indication of soil moisture content
bright and consistent colours = lots of aeration lots of microbial processes occurring. Waterlogged = grey/blue mottled colour
How can we find the soil texture in the laboratory
by looking at settling rates
What soil colour does organic matter typically have
darker colours - due to the carbon which covers the other colours in the soil
What do finer fragments of soils influence
determine soil texture - sand, silt and clay
What is particle density useful for
determining nature and origin of the soil (i.e. Mineral or organic)
What soil biota can change soil structure
earth worms can change structure, as well as roots.
What are examples of how soil structure can be changed over time by physical and chemical processes
flocculation and volume change
What do coarse fragments of soils influence
function of soil but does not determine soil texture
What does bulk density determine
how easily plant roots will penetrate the soil
What can soil structure help do
inform farmers - because if there is stable soil can use large machinery - roots can penetrate into soil.
What is the relationship between soil texture and pore size for macro pores
larger particle sizes - sand = good percolation rate
What does an increased absorbance ability entail
larger potential for nutrients and water to get into soil
What is particle density
mass of soil in a given volume excluding pore spaces
What is bulk density
mass of the soil in a given volume including pore spaces
What is loam
mixture of all 3 (clay, silt, sand)- good distribution between micro and macro pores and lots of moderate pores - can influence infiltration, percolation and leaching
What do soil particles form when they aggregate together
peds
What is flocculation
positively charged calcium can stick to negatively charged clay and form flocs
What is structure stability
resistance of soil structure to external factors
How does size of finer particles indicate properties of soil
same volume of soil can have higher surface area with smaller particles - exponential increase in surface area when decrease soil particle sizes.
What is the relationship between soil texture and pore size for micro pores
smaller particle size - clay (fine texture) = waterlogged conditions
What are some biological processes that affect soil structure over time
soil biota and organic matter.
What is volume change
soils can absorb moisture, when dry out leave large cracks (macropores)
What are mottles
spots of different colours
What is the definition of soil structure
the arrangement of sand, silt, clay and organic matter into groupings (peds)
What things is the Munsell chart used to determine
the dominant colour (hue), the value (lightness) and then the chroma (intensity of the pigment) which will then yield a colour name for the soil.
What does bulk density measure
the impact of agricultural machinery
What is soil texture
the ratio of sand, silt and clay in a mass of soil.
What soil colour does a soil with good aeration have
uniform bright colour
How else is soil structure knowledge helpful
useful to building and construction industry - can use maps on topsoil structure stability