REN R 210 - Module 2 Exam

Soil physics

The study of solid, liquid, and gas phases and how they interact

Soil matric

The framework of a solid phase and open pore space

What are the distance measurements used in Soil physics?

• Meter (m)

• Kilometer (km) - 10^3

• Centimeter (cm) - 10^ -2 m

• Micro (μm) - 10^ -6m

What are the weight measurements in soil physics?

• Grams (g)

• Kilograms (kg) - 10^3 grams

What are the volume measurements used in soil physics?

• Meters cubed (m^3)

• Litres (L)

What is soil made from and how does it form?

Soil is made of aggregates that are formed through biological, chemical, or physical processes

How do soil aggregates form chemically?

Clay has a negative charge which attracts organic matter and cations (which are both positive)

How do soil aggregates form biologically?

Organic matter and clay materials mixed together like glue

What are the four levels that aggregates can be view on?

Macroaggregates

Microaggregates

Submicroaggregates

Primary particles

Magroaggregates

Roots and hyphae

Microaggregates

Root hairs, hyphae, and polysaccharides

Submicroaggregate

Mineral grains encrusted with plant and microbial debris, plant debris coated with clay

Primary particles

Silt, clay, and humus

What is the thin section technique?

A historical technique that cast soil in resin, which filled the air space and making it possible to thinly slice sections to see through

Scanning Electron Micrograph

Modern technique where soil is encrusted in a thin layer of gold, and bombarded with electrons which reflect to produce an image

How do macropores and micropores influence porosity and bulk density?

• Macropores AKA transmission pores: The pore space between coarse mineral particles, such as sand, which is very large

• Micropores: Tiny particles that have very little volume of pore space compared to macropores

What happens to micropores as texture decreases?

The amount of micropores increases

Porosity (%)

The relative proportion of a volume of soil that’s filled with air and water AKA the pore space

What does porosity effect?

Affects water movement, water availability, and root growth

Soil Bulk Density (Db)

Mass of soil per unit volume

Particle Density (Dp)

Density of solid mineral material (aka rocks) which generally maintains the same value of 2.65 g/cm³

Factors that affect Bulk density

• Porosity ↑ Db ↓

• Texture - Clay ↑ then Db ↓

• OM content ↑ and Db ↓

Cohesion

Water molecules stick to each other because of polarity, forming weak bonds

Adhesion

Water molecules stick to other things, with the strength of the bond varying depending on the thing

Hydrogen bonding

The attraction between the positive hydrogen in water molecules with negatively charged oxygen in other water molecules (Cohesion), or with the negatively charged surfaces of soil particles (adhesion)

Hydrophilic and its angle

Water loving, usually what the soil is. On a flat surface the angle between the top and bottom of the water will be very small because the water and the surface can’t get close enough

Hydrophobic and what it’s angle is

Water repelling (eg: wax/fats, OM). The angle between the top and bottom of water on a hydrophobic surface will be very large because the water can’t get far away enough from it

Capillary action

How high water will reach in a hypothetical tube, due to the tube’s hydrophilic nature and adhesive forces with the height the water will reach in a tube depending on the radius of the tube

What is the difference in adhesive forces between larger and smaller pore soils?

• Large pore soils have weaker adhesive forces because the water is farther away from the soil particles, so it doesn’t rise as high

• Small pore soils have stronger adhesive forces because the water is closer to the soil particles, so it will rise higher

Soil water content

The amount of water in a given amount of soil, which will depend on different factors

Soil water potential

The forces holding water onto the soil matrix (AKA the pore spaces and solids)

Gravimetric water content

the amount of water in a soil based on mass (measured in grams)

Volumetric water content

The amount of water in a soil based on volume, with bulk density necessary to calculate it

Mass

The amount of matter in an object

What is soil water potential determined in reference to?

The reference reservoir

Reference reservoir

water at atmospheric pressure and with an elevation of zero

Work

How much force it takes to move an object a distance, which can also be defined as distance

What question are we trying to answer when talking about soil water potential?

“How much work/energy does it take to transport water in the reference reservoir to a point in the soil?”

Soil water potential

The amount of work that has to be done to transport pure water from one place to another

What are the two types of work in soil water potential

• Negative work: The soil is absorbing the water on its own without any external force

• Positive work: Extracting the water from the soil by exerting a force

Gravitational potential (Ψ_g)

The force of gravity, which is highest when a soil is saturated

Matric potential(Ψ_m)

The soil particle forces, such as forces of adhesion and capillary rise

Osmotic potential (Ψ_o)

The attraction between water and solutes, which is generally negligible and ignored

Water storage

The amount of water stored in soil at a specific time

Name	Main influence	Plant availability	Total potential energy
Saturated/Free water
Field Capacity
Wilting point

Name	Main influence	Plant availability	Total potential energy
Saturated/Free water	Gravitational force	Yes	0.0 kPa
Field Capacity	Matric potential	Yes	-33 kPa
Wilting point	Matric potential	No	-1500 kPa

Saturation

The maximum amount of water a soil can hold, with all pore spaces filled

Provide an example of how water content is a function of texture

Clay holds higher amounts of water than sand because the higher surface area increases the overall matric potential

Wilting point

Water is super close to the soil and it’s too hard for plants to access it

What is water above Field Capacity unavailable to plants?

Above field capacity there’s no negative pressure to help plants move water

Available water

The difference between field capacity, and wilting point, which accounts for all the water that is accessible to plants

Soil Type	Porosity	Pore size	Field Capacity	Wilting point	Available water
Sandy soil
Clay soil
Loam soil

Soil Type	Porosity	Pore size	Field Capacity	Wilting point	Available water
Sandy soil	Low	Large	Low	Very low	Low
Clay soil	High	Fine	High	High	Low to medium
Loam soil	Varies	Varies	Low to Medium	Low to medium	High

Which soil type is the best for plants and why?

Silt Loam has the most available water, making loams the best soils for plants

As Organic matter increases, Field capacity and wilting capacity ________ at _________ rates meaning that Available water __________

increases, different, increases

What changes porosity?

Aggregation

Why doesn’t particle size change porosity?

Because of the unique rules of spheres that says that in the same overall area (eg same size box) the radius of the spheres within doesn’t matter, and the total volume of spheres in the area will be the same regardless of radius

Macro/Transmission pores

Largest pore size, which drain the fastest at Field Capacity

Meso/Storage pores

Have medium sized pores at hold the most amount of Available water

Micro/residual pores

The smallest pore size and largely contain unavailable water

Infiltration rate

The speed at which water enters soil in a given time period

Which soils have the fastest infiltration rate? Which have the slowest?

Coarse soils, fine soils

Granular infiltration rate

Rapid flow due to small peds

Blocky infiltration rate

Moderate flow due to angular peds

Prismatic/Columnar infiltration rate

Moderate flow due to the salt content slowing water down due to the osmotic potential (because water and salt like each other)

Platy infiltration rate

Slow flow because the water has to find its way around the horizontal peds

Saturated flow

The speed at which water moves through a soil when soil pores are already completely filled with water, making soil matric potential 0 and is calculated using Darcy’s law

Darcy’s law

Describes the movement of water through a medium

K

The hydraulic conductivity that dictates the saturated flow/ movement. It measures the speed of the wetting front and assumes saturated flow moves vertically

As OM increases, Ksat ______ because _________

decreases, OM holds onto water

As clay increases, Ksat ______ because _______

decreases, the positive charge of water clings to clay

Preferential flow

• The fact that water always travels on the path of least resistance, and will favour going through roots and large pore spaces because it’s easier

  • Results in non-uniform and rapid water movement around roots

Ground water

The saturated zone of soil

Water table

The upper limit of the ground water, and the dividing line between ground water and the other levels

Vadose zone

The zone of unsaturated material above the water table (or the zone that spends the majority of the year unsaturated)

Capillary fringe

The zone of capillary rise that is most affected by ground water

Colloid

Homogenous mixture made ultramicroscopic particles of one substance that’s dispersed through a second substance

They don’t settle, and can’t be separated through filtration

Rock types

 Igneous, Metamorphic, Sedimentary

Weathering

Weathering happens in multiple different ways and to every rock that soil is developing on

Types of weathering (describe them)

• Physical: Disintegration, Free/Thaw cycles

• Chemical: Decomposition, molecular changes, water causing hydrolysis

• Biological: Decomposition caused by organisms

Primary minerals

• Inherent properties of Rocks

  • Eg: Sand and Silt

Secondary minerals

• Derived from primary minerals via weathering

  • Eg: Clay

Primary minerals include

Quartz

Muscovite (mica)

Biotite (mica)

Feldspar

Albite

Quartz

The hardest primary minerals

Muscovite (mica) and Biotite (mica)

Softer rocks

Feldspar

Soft?

Albite

Feldspar that is rich in sodium

Quartz weathering rate and what it turns into

Weathering turns it into sand and SiO₂ (silicate), and it is the hardest to weather

Feldspar weathering outcome

Weathering produces base cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and a more chemically complex result than Quart

Micas weathering product

Weathering produces fine textured soils

“Clay” can mean a variety of things including

• Naming a type of soil separate

• Naming a particle size class

• Referring to a textural class

• Referring to secondary minerals

• Referring to the components of a colloidal fraction in soil

Soil Colloid criteria

• Particle size is less than 1-2 μm

• There is a high surface area

• There is electronegative and electropositive charges on the internal and external surfaces

• It absorbs lots of cations due to its negative charge

• Cation exchange occurs when an ion breaks away before its replaced with another ion

• Ionic double layer (The negatively charged colloid is surrounded by a swarm of cations)

Components of the colloidal fraction include

• Crystalline silicate clays AKA phyllosilicates (dominate in AB soils)

• Amorphous clays (aka non crystalline silicate clays) - generally present in volcanic soils

• Fe and Al oxides

• Organic matter in a stable form

Fe and Al oxides

Fe and Al atoms are connected to oxygen atoms and hydroxyl groups, generally present in more weather soils

What are the two building blocks of silicate clay minerals

Silicon tetrahedron

Aluminium Octahedron

Silicon tetrahedron

4-sided with 4 oxygen atoms and a central silicon cation that’s held in place by covalent bonding

Aluminium Octahedron

8-sided, with 6 hydroxyl (OH^-) atoms and aluminum as the central cation

Silicate clays

 Made from connecting sheets of Tetrahedral and Octahedral planes, which may have an interlayer between them where cations and water can enter causing the shrink swell cycle

Covalent bonding:

The sharing of ions, which forms a weak association

What can cause a central cation in a sheet to be replaced? And what must it be replaced by?

Weathering, a lower valence electron

What can replace the central cation in Octahedral and Tetrahedral sheets?

• Si⁴⁺ can be replaced by Al³⁺

• Al³⁺ can be replaced by Mg²⁺

1:1 type mineral structure

One tetrahedral sheet is bonded to one octahedral sheet