Lithosphere IV

Question 1

What is soil texture?

Answer 1

The proportion of particles of salt, silt, and clay in soil, expressed as a percentage based on dry weight of soil.

Question 2

What particles are and are not included in soil texture? Why?

Answer 2

Particles larger than 2 mm are not included because their influence on soil properties is considered to be minimal. OM is also not included.
Particles smaller than 2 mm, including sand, silt, and clay as well as secondary minerals are included.

Question 3

What are the sizes of sand, silt, and clay?

Answer 3

Sand: 0.05-2 mm
Silt: 0.002-0.05 mm
Clay: <0.002 mm

Question 4

Describe sand particles in terms of:
a) Appearance and feeling
b) Composition
c) Charge
d) Nutrients
e) Drainage

Answer 4

a) Visible with the naked eye, feels gritty
b) Consists of quartz and other primary minerals
c) Low surface charge
d) Low in nutrients
e) Rapid water drainage

Question 5

Explain why sand doesn’t have a high charge and the impact this has on its properties.

Answer 5

Sand hasn’t undergone a high degree of weathering, and this makes these particles relatively inert and now have a high charge. Because of this, they don’t bind nutrients or absorb water well.

Question 6

Describe silt in terms of:
a) Appearance and feeling
b) Composition
c) Surface charge
d) Nutrients
e) Drainage

Answer 6

a) Not always visible to the naked eye, feels smooth and silky (not sticky) when wet and powdery when dry
b) More weathered micro-sand particles made of quartz and other primary minerals
c) Some surface charge and reactivity
d) More nutrients than sand
e) Slower drainage and greater water retention

Question 7

Describe clay in terms of:
a) Appearance and feeling
b) Composition
c) Surface charge
e) Drainage

Answer 7

a) Particles make up of many tiny flakes or platelets (high surface area)
b) Composed of secondary minerals from the weathering of primary materials
c) High negative surface charge
e) Very slow air and water movement

Question 8

Can soil texture be altered? Explain.

Answer 8

No, management practices cannot alter soil texture. Instead, one must find a naturally occurring soil that suits one’s needs.

Question 9

What is soil structure?

Answer 9

How the sand, silt, and clay are spatially arranged into stable structural units known as aggregates or peds.

Question 10

Can soil structure be altered? Explain.

Answer 10

It can be impacted by land management and biological activity as well as climate and texture.

Question 11

How can soil structure be improved?

Answer 11

By having a high abundance of aggregates and organic matter.

Question 12

What is an aggregate?

Answer 12

A conglomeration of soil minerals, organic matter, plant roots, and soil organisms. It is also called a ped.

Question 13

Describe the major steps in the life cycle of soil aggregates.

Answer 13

Formation, stabilization, and disintegration.

Question 14

Describe the 5 steps through which clay aggregates form. Be sure to mention when microaggregates vs macroaggregates are formed.

Answer 14

1. Flocculation of clay colloids through electrostatic bonding (bridging)
2. Clay organic-matter complexes continue to flocculate.
3. Microaggregates are formed with the addition of organic debris and glues from bacteria and fungi.
4. Macroaggregates are formed from many microaggregates bound together by roots and hyphae.
5. Several macroaggregates are held together by the enmeshment of roots and fungal hyphae.

Question 15

Explain how bridging works in the formation of clay aggregates.

Answer 15

The negatively charge clay particles are linked together by strongly positive ions, as normally the clay particles would repel each other.

Question 16

What types of ions work best (and worst) for bridging? Explain why.

Answer 16

Best: divalent and trivalent ions like Ca2+, Fe3+, and Al3+. This is because they are strongly positive and thus can counterbalance the repulsion between negative clay particles.
Worst: monovalent cations like Na+, as they aren’t usually strong enough to overcome natural repulsion of clay particles.

Question 17

What is the impact of a lot of sodium in the soil? Explain why?

Answer 17

Having a lot of sodium promotes the dispersion of aggregates, as Na+ is not strong enough to overcome the repulsion between negative clay particles.

Question 18

Summarize the role of organisms in the formation of soil aggregates.

Answer 18

At the microaggregate scale, bacteria and fungi excrete glues that hold particles together. Fungal hyphae and fine roots also hold microaggregates together. At the macroaggregate scale, fungal hyphae and larger roots hold them together.

Question 19

Name 4 reasons why aggregates are beneficial.

Answer 19

1. They help create pore networks, improving water infiltration and microhabitats.
2. Stabilizes soil against erosion
3. High aggregation is associated with less compaction
4. Holds soil carbon, preventing its breakdown and mineralization to CO2.

Question 20

Name 5 ways to promote the building of aggregates.

Answer 20

1. Soil organic matter (acts as binding agent and attractant for organisms that release glues)
2. Fungi
3. Roots
4. Reduced disturbance
5. High clay content

Question 21

What is the impact of tillage on soil structure?

Answer 21

Tillage causes mechanical disturbance in the soil and promotes the breakdown of aggregates. It can lead to a much crumblier texture than you can get in reduced tillage soils.

Question 22

What happens when an aggregate falls apart?

Answer 22

There is often an immediate release of CO2, mainly because the carbon in the aggregate gets exposed to organisms in the soil that decompose it. There is also a dispersal of materials.

Question 23

A key component of soil structure is […] Why?

Answer 23

Pore space, because this is the space where movement and transport occurs, and it also provides a habitat for organism and root growth.

Question 24

Bulk density provides information about […]

Answer 24

Soil particles and the amount of pore space.

Question 25

What is the formula for bulk density?

Answer 25

Bulk density (Db) = soil mass (g)/total soil volume (cm3)
Soil mass includes minerals and OM
Soil volume includes all particles, OM, and pore space

Question 26

Explain what a lower bulk density reveals about a soil vs a higher one.

Answer 26

A low bulk density = lower weight and more pore space.
A high bulk density = higher weight and less pore space.

Question 27

If you have 2 soils of the same volume but with a different bulk density, how are they different?

Answer 27

The one with the lower bulk density has more pore space and is less compacted, while the one with higher bulk density has less pore space and is more compacted.

Question 28

If you have 2 soils of the same dry mass but with a different bulk density, how are they different?

Answer 28

The one with the lower bulk density has a greater volume and is less compacted, while the one with the higher bulk density is more compacted and has less pore space.

Question 29

Explain the relationship between bulk density and infiltration.

Answer 29

Higher bulk density = more runoff than infiltration, as there is less pore space for the water to be absorbed. Lower bulk density = more infiltration than runoff, as there is more pore space for the water to be absorbed.

Question 30

Explain the impact of harvesting on bulk density.

Answer 30

The bulk density of the soil being harvested will be higher due to the weight of the equipment and the mechanical disturbance.

Question 31

Explain the impact of grazing on bulk density.

Answer 31

Increased grazing increases bulk density, as the hoof action on moist to wet soil can create a compacted soil layer (called a Cow Pan/Traffic Pan). This impedes water and root penetration.

Question 32

What are the major types of pores? What determines the pore types in a soil?

Answer 32

Macropores (> 0.08 mm) and micropores (< 0.08 mm). Soil texture determines the proportion of micropores and macropores.

Question 33

True or false? Bulk density provides insight into pore shape.

Answer 33

False. It only provides insight into total porosity.

Question 34

Describe the major properties of macropores.

Answer 34

They are wide enough that water and air can move easily, and water cannot be held against the force of gravity. They accommodate root growth and tiny creatures.

Question 35

In what type of soil do macropores tend to form?

Answer 35

Between large sand particles and between aggregates.

Question 36

Describe the major properties of micropores.

Answer 36

Water and air movement is slow, and water can be held more tightly against gravity. Water is not always plat available.

Question 37

In what type of soil do micropores typically form?

Answer 37

In fine-textured, clayey soil

Question 38

Describe how total amount of pore space varies between soil textural classes. Explain why this is.

Answer 38

As you move towards finer textured soils (sand -> clay), there will be an overall increase in pore space. This is despite there being fewer macropores in finer soils. This is because in sand, there are a lot of macropores but few micropores, since the grains are so large. In clay, although there are fewer macropores, there are many micropores in the aggregates, which overall make up more total pore space.

Question 39

Describe how water movement in soil is affected by soil texture.

Answer 39

In sandy soils, gravitational pull is the dominant force, so water will move more quickly downwards and less so sideways. In clayey soils, capillary action is more dominant due to smaller pores, so water will move sideways more quickly and make its way down more slowly.

Question 40

What is the role of earthworms in soil structure?

Answer 40

They move through the soil and ingest part of it. They excrete it as aggregates that are rich in nutrients. They also create biopores via burrowing activity. They overall improve the structure of the soil.

Question 41

Explain the relationship between soil texture and surface area.

Answer 41

Sand has a lower surface area than clay, as clay particles consist of many tiny plates stacked together, creating many grooves that add to the surface area.

Question 42

Compare macroaggregates and microaggregates in terms of:
a) Pore size
b) Pore connectivity
c) Surface area
d) Decomposition of carbon
e) Gas exchange
f) Water retention

Answer 42

a) Higher in macro than micro
b) Higher in macro than micro
c) Lower in macro than micro
d) C less decomposed in macro
e) Diffusion higher in macro
f) Water retention lower in macro

Question 43

What are the approximate proportions of sand, silt and clay in loam?

Answer 43

25-51% sand, 28-50% silt, and 6-26% clay

Question 44

Explain how wind erosion moves particles differently.

Answer 44

Finer particles (clay and silt) get suspended in wind and can get carried further distances. Small sand particles can get picked up somewhat, and will undergo saltation. Bigger sand particles are too heavy to pick up, so they will creep on the ground. Sand generally does not travel that fair.

Question 45

What was the Western prairie dust bowl?

Answer 45

A massive errosive process by wind.

Question 46

Describe the 5 events that led to the Western Prairie Dust Bowl.

Answer 46

1. Increased need for cultivating land
2. Farm mechanisation led to more prairie lands cultivated for agriculture.
3. A series of droughts dried out the soil in the 30s, making it more susceptible to wind erosion.
4. Extreme, extended drought further decreased plant cover and increased plant mortality.
5. Poverty increased and many families lost their farms, leading to migrations further west.

Question 47

What was done after the western Dust Bowl?

Answer 47

Trees were planted to create windbreaks.

Question 48

Explain the trends in no-till and summer-fallow and the impact they have on soil cover.

Answer 48

In the prairies there has been an increase in no-till practices and a decrease in summer allow practices, which is typically done to allow the soil to accumulate more water for the next crop. This has increased soil cover there. This is because no-till accomplishes the same goal as summer-fallow, as the lack of mechanical tillage prevents compaction and allows water to stay in the soil better, eliminating the need to summer-fallow.