Land degredation

Question 1

Define land degradation

Answer 1

Land degradation describes a long-term decline in ecosystem function and productivity.

Land degradation is a bio-physical-chemical process driven by socioeconomic and political causes

Question 2

What are the consequences of land degradation?

Answer 2

• Decreased land productivity
• Poverty, hunger, migration, worsened food insecurity + uncertainty
• Reduced water quality and availability
• Resource loss
• Socioeconomic problems
• Biodiversity loss + damage to ecosystems

Question 3

Consequences of land degradation in terms of welfare

Answer 3

• Clean and adequate water
• Clean food and air
• Encourages migration
• Respiratory conditions by dust from wind erosion + other air pollutants

Question 4

What are the three mechanisms that initiate land degredation?

Answer 4

Land degredation can be physical, chemical, or biological.

Question 5

What is physical land degradation?

Answer 5

Decline in soil structure leading to crusting, compaction, erosion, desertification, anaerobism.

Question 6

What is the result of chemical land degradation?

Answer 6

acidification, leaching, salinization, decrease in cation retention capacity, and fertility depletion, chemical toxicity

Question 7

What is the result of biological land degradation?

Answer 7

Loss of soil carbon, loss of beneficial microbes, loss of microbial diversity and function

Question 8

What are two main types of erosion?

Answer 8

• Wind erosion

- Water erosion

Question 9

Different forms of wind erosion based off particle size?

Answer 9

• 1 mm move by rolling (soil creep).
• 0.1 – 1 mm move by saltation, collision by
entrained particles.
• <0.1 mm detach into suspension.

Question 10

What is saltation?

Answer 10

• preferentially strips out quality aspects: soil carbon.

Saltation is the main process that suspends soil particles in the air - particles between 0.1 and
0.5mm in diameter are lifted by wind then fall back to the ground, so they hop or bounce across
the surface. Leads to breaking the boundary layer protecting lower layers from erosion.

When it doesn’t catch large particles and leads to the formation of a smooth/fine layer over the top: a crust that will break away+ allow beneath contents to erode if another particle lands on it and dislodges it.
=> chain reaction using v. fine clay, silts, and carbon
=> leaves larger, inert materials behind (sand) so reduces quality of the soil: will lack the ability to hold nutrients.

• if one grain hits the surface, 3-4 will be liberated.

Question 11

What is creep?

Answer 11

Particles greater than 0.5mm in diameter are usually too heavy to be lifted by wind. Creep is
when wind rolls these particles or they are moved along the surface by bombardment by other
moving particles.

Question 12

What is suspension?

Answer 12

Small particles less than 0.1mm in diameter that have been ejected into the air by saltation
remain suspended as dust and are carried away from the erosion site by the wind. The majority
of particles greater than 0.02mm will settle back to the ground within 100 kilometres of the
erosion site, but finer particles can be carried long distances, as far as New Zealand.

Question 13

What are the strategies to reduce wind erosion damage?

Answer 13

Cloddy Tillage or stubble retention should create soil roughness by leaving clods in the soil or by making ridges and furrows perpendicular to the prevailing wind direction. ….not effective in sandy soils due to lack of cohesiveness

Clay listing sandy soils is used in duplex because they do not produce durable surface clods. Listing ridges the soil and brings up firmer subsoil clay….perpendicular to the eroding wind, …need heavy duty cultivator.

Surface and crop residues can be retained after harvest, which generally coincides with the dry season, to provide soil cover and reduce wind erosion.

Strip cropping reduces the width of the field area where the soil is exposed to wind erosion. Alternating strips of crops and fallow or strips of clean cultivated crops …. not widely practiced.

Intercropping is widely used in many arid regions, and several systems have been developed. …i.e. pasture cropping.

Wind breaks interrupt wind flow and reduce wind velocity through presence of physical barriers.

Question 14

What is the result of reducing fetch?

Answer 14

Reducing fetch means less distance over which wind can gain speed, so less capacity to strip soil away.

Question 15

What are some wind erosion control options for cropping?

Answer 15

• Strip cropping
• Tree wind breaks/shelter belts: create eddying + pressure changes that reduce wind speed.
• Ridging
• Crop residues: about 50% of ground cover needed to control wind erosion. Ground cover prevents erosion.
• Clay delving
• Crop wind breaks

Question 16

What’s the general pattern of a water erosion sequence?

Answer 16

Splash erosion leads to crust formation and reduced infiltration.

This causes sheet erosion. If conc. of water in depressions + increased flow velocity occurs, this leads to rill erosion. If run-off enters dispersive subsoil, tunneling will occur.

If roof collapse occurs in tunneling, or further conc. of flow and headwall development occurs in rill erosion, a gully will form.

Question 17

What is splash erosion?

Answer 17

Raindrop impact acts as soil bombardment which causes a crust to form.

• First stage in erosion sequence.
• Clogs up pores in soil that normally absorb rain water, forming crusts that are mostly impermeable, so puddles of unabsorbed water begin to form, flowing downhill in a thin sheet.
• Carries away fine particles in run-off.

Question 18

What is rill erosion?

Answer 18

Rill erosion results from the concentration of surface water (sheet flow) into deeper, fasterflowing channels.

Question 19

Gullies vs rills?

Answer 19

Gullies are rills that are larger than 1 foot deep and 1 foot wide.

Question 20

What’s the problem with sodic/dispersive soils?

Answer 20

Erosion
• Gully erosion
• Tunnel erosion
• Dam collapse

Water Quality
• Extreme turbidity (never settles)
• Sediment deposits

Construction / Development
• Collapse of Fill
• Pipe & Cable Collapse
• Breaching Septic Trenches
• Dam Failure

Question 21

what are dispersive/sodic soils?

Answer 21

Dispersive Soils
• Appear to ‘dissolve’ in water.
• Responsible for tunnel erosion.
• Dispersive soils are usually sodic.
• Structurally unstable, require special consideration for development.

Question 22

How does sodicity lead to dispersiveness

Answer 22

Increased ion concentration on particle surface prevents formation of aggregates.

• Normally clay platelets would stack together + clump.
• More than 5% sodium leads to dispersive behaviour.

Question 23

Why is salinity a problem?

Answer 23

salinity is a problem because salt affects water and nutrient uptake by plants.

• salt can be toxic to plants
• remediation of salty land is hard

Question 24

Why is compaction a problem?

Answer 24

Compaction is a problem because it reduces infiltration and draining, making it harder for roots to penetrate.

Subsurface compaction restricts root
development: don’t get large fibrous nets but short bent roots.

Question 25

What are the properties of good soil structure?

Answer 25

• Porous clod faces.
• Many pore spaces and root
holes.
• A good range of aggregate sizes.
• Lots of fine fibrous white roots.
• large network of soil pores (high
porosity) 
• loose friable structure
• high permeability for rainfall
• good plant available moisture
range
• low soil strength
• Minimal surface ponding

Question 26

What are the causes of compaction?

Answer 26

Soil Moisture: The most important
factor ! Soil moisture affects both
the depth and severity of the
compaction.
Vehicle Weight: Increases the
depth of the compaction
Number of Passes: 90% of the
damage is done in the first pass
Tyre Pressure: Ground pressure is usually
1-2 PSI above the tyre pressure regardless of weight.
Tyre Width: Has minimal effect unless inflation pressure is reduced i.e.
Flotation tyres.
Soil Type: Compaction occurs more readily in soils with high Silts, fine
sands content, Low organic matter, high sodium ~ Sodic
soils

Question 27

How to avoid compaction?

Answer 27

HOW TO AVOID
• Avoid compression when susceptible
• Improve soil structure (SOM)
• Controlled traffic/ fixed bed cropping
• Cultivation strategies (eg deep ripping)
• Choice of machinery

Question 28

Why is soil organic matter important?

Answer 28

• Stabilises and holds soil
particles together
– reduced erosion
– improved structure
– Reduces susceptibility
to compaction
• Source of carbon and energy for soil biota
• Stores and supplies nutrients
• Binds pesticides
 SOM reduced by cultivation
• Increased by pasture, green manure crops,
addition of organic matter
• Effects of management
practices vary with
climate, soil type

Question 29

Relationship between soil carbon and nutrient holding capacity?

Answer 29

A 0.5% loss in organic matter may lead to 14% loss in nutrient holding capacity, 12% loss in water holding capacity.

Question 30

What causes acidification?

Answer 30

Acidification results from:
• Ammonium-based nitrogen fertilisers
• Leaching of nitrate nitrogen
• Continual removal of plant and animal produce and waste products
• Leaching of cations ie Calcium, Iron, Magnesium
• Sulphur fertilizers – ie ammonium sulphate, Potassium sulphate

Question 31

What does acidification cause?

Answer 31

Makes some nutrients less available and some toxins more available.
- Ca and P deficiency, Mn and Al toxicity

Question 32

why is degraded land an issue?

Answer 32

Degraded land bad bc
• costly to reclaim
• may no longer provide a range of ecosystem functions and services
• loss of the goods and potential environmental, social, economic and non-material benefits that are critical for society and development.