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