W9, Salinity

Question 1

How would you calculate the concentration of salt (in mg /L) if you had the EC of a sample?

Answer 1

[salt] mg /L = 640*EC (dS /m)

Question 2

What are the methods for measuring soil salinity in a lab?

Answer 2

1:5 extract (EC1:5)

→ 1 part soil, 5 parts water

Saturated paste extract, ECe

→ Add only enough water to a dry soil to form a “glistening” paste

→ Much closer representation of what a plant will experience

• EC1:5 < ECe because EC1:5 uses more water
• Use conversion factor to switch between the two (depends on soil texture)

Question 3

Give 2 important reasons for why we must consider geologic history when studying salinity in Australia.

Answer 3

→ Australian soils are old and have persisted through many changes in climate and vegetation cover

• both of these factors influence soil formation
• once was tropical climate and covered in rainforest

→ Parts of Australia’s current land mass was once underwater; i.e. was formed from ocean sediments.

• means that salinity in these areas is natural and not a form of land degradation

Question 4

Provide a definition for soil salinity

Answer 4

A measure of the concentration of total dissolved salts in soil or water.

A saline soil is one where the build up of salts has reached toxic levels for plants

Question 5

At what concentration (ppm) does salt become problematic for most cultivated plants?

Answer 5

3,000 - 6,000 ppm

Question 6

True or False?

Salinity is one of the most devastating forms of land degradation

Answer 6

True.

Hard to remediate.

Question 7

Explain the relationship between salinity and osmosis

Answer 7

• Water moves from regions of high water/osmotic potential to regions of low water/osmotic potential.
• This means that for plant cells in a saline environment, the osmotic potential outside of the cell is greater than inside of the cell, meaning water wants to be drawn from the cell to reach a solute equilibrium, which can cause the cells to plasmolyse.
• It also means that roots have to draw in water against a concentration gradient, which requires energy (CHECK).

Question 8

Explain how the issues related to/caused by salinity are largely indirect

Answer 8

↑ [salt]

= ↓ plant cover = ↑ erosion

= ↓ biodiversity (dominance of salt-resistant species) = alteration of ecosystem structure

Question 9

List some factors that influence/promote salinity in Australia

Answer 9

• Geologic history
• Vegetation change
  
  • Deep-rooted perennials → shallow-rooted annuals
• Irrigation practices
  
  • Changes in hydrologic balance
  • Irrigating with low-quality water)
• Climate change
  
  • Increasing sea level
  • Changes in frequency and intensity of rainfall
  • Changes in vegetation

Question 10

What is the difference between solute and matric potential?

Answer 10

Solute (osmotic) potential, Ψ_s:

• The difference in solute (chemical) concentration
• High [solute] = high solute potential
• No solute = Ψ_s = 0
  
  • Add solute, Ψ_s value becomes negative

Matric potential Ψ:

• The difference in water content/availability
• How strongly water is held to surfaces
• Dry soil = high matric suction

THIS ANSWER NEEDS CHECKING

Question 11

According to the FAO, roughly how much land is salt-affected globally?

Answer 11

≈ 8.31 million km² (831 million ha)

Every continent except Antarctica

Question 12

True or False?

Salinity occurs mostly in arid and semi-arid zones

Answer 12

True.

Occurs in most climatic zones though.

Question 13

True or False?

Only certain soil types are affected by salinity

Answer 13

False.

Question 14

Where would you search for the Australian Government’s assessment of dryland salinity, and how do they conduct it?

Answer 14

The National Land and Water Resources Audit

• Defines lands at risk
• What the impacts on those lands is/will be

Based on:

• Known incidences of human-induced salinity
• Groundwater depth
• Soil factors
• Topography

Question 15

What are the current estimates for the area that will be affected by secondary (human-induced) salinity in Australia by 2050?

Answer 15

15-17 million hectares

Question 16

How does salinity affect infrastructure like houses, roads, bridges, machinery, etc. and why is it such a big problem?

Answer 16

• Increases the rate of corrosion.
• Deteriorates brick, mortar, concrete, bitumen, asphalt
  
  • Saline water crystallizes in the material and basically (physically) weathers it (breaks it apart)
• Can cause foundations to shift or sink

Infrastructure (like roads and bridges) can be extremely expensive to repair.

Question 17

What proportion of Australian farms show signs of salinity?

Answer 17

≈ 14%

> 50% in WA

> 20% in SA

(0.4% of total agricultural land)

Question 18

Where does the salt that causes salinity come from?

Answer 18

• Physical and chemical weathering of rocks
• Rainfall (small inputs that accumulate)
• Wind-transported materials (soil, sea spray, etc)
• Poor quality irrigation water
• Groundwater
• Seawater intrusion
  
  • Tsunamis
  • Sea level rise

Question 19

What is the predominant salt in saline soils?

Answer 19

Sodium (Na⁺)

Question 20

Explain the difference between primary and secondary salinity.

Answer 20

Primary salinity
— The result of rainfall interacting with geological features over thousands (or millions) of years
— Occurs naturally

Secondary salinity
— The result of human activity
— Either produces more salt (e.g. through irrigation) or causes primary salinity to rise to the land surface (e.g. by removing deep-rooted perennial vegetation)

Question 21

Is having salt-affected land really a problem?

Answer 21

Yes and no.

Primary salinity, i.e. that which is caused naturally, isn’t as much of a problem as secondary salinity, which is caused by our (poor) land management.

Primary salinity can often go unnoticed, or not be a significant issue, because the salt deposits are deep underground, beyond the rhizosphere.

The issue comes about when poor land management results in those salt deposits reaching the surface, or at least the rhizosphere, where plants, animals, and infrastructure can be affected.

Question 22

Explain how secondary salinity occurs

Answer 22

• Deep-rooted perennials, whose roots absorb most of the water entering the soil, are removed or replaced with shallow-rooted annuals that allow for increased deep drainage (groundwater recharge).
• Irrigation has a similar effect by increasing the amount of water that can make it beyond the root zone (deep drainage)
• As groundwater levels rise, they intercept and dissolve historical salt deposits and carry it towards the surface.

Question 23

What is meant by ‘the vadose zone’?

Answer 23

AKA the unsaturated zone

The area between the soil surface and the top of the water table.

Tim referred to it as the area beneath the root zone (and above the water table), but I don’t think this 100% accurate.

Question 24

True or False?

If there isn’t a significant historical salt deposit underground, increasing the water table isn’t necessarily a bad thing unless it creates waterlogging.

Answer 24

False. The groundwater itself might be saline, contaminating the soil it comes into contact with as it rises.

Question 25

What is the expected deep drainage (mm/year) under deep-rooted native perennial vegetation vs shallow-rooted annual crops?

Answer 25

• *Deep-rooted perennials:** ≈ 0.1 mm/yr
• *Shallow-rooted annuals:** ≈ 10 mm/yr

Question 26

What is the main benefit of maintaining deep-rooted perennial vegetation?

Answer 26

Keeps the water table low.

Question 27

What are some of the issues associated with remediating saline land in Australia?

Answer 27

• The (vast) scale of the problem
• The geographical coordination that has to happen to implement changes (what one person does on their land can impact the hydrology on land quite a long way away)
• There can be a significant lag time between implementation of amelioration strategies and resulting environmental change
• There are limited viable options for farmers (especially) to implement recommended land use changes
  
  • Limited economic return from large, native, deep-rooted perennials (e.g. trees)
• Disagreement about who should cover the cost
  
  • Land owner or government
  • This generation or future generations

Question 28

What are some ways that climate, and therefore climate change, impacts on salinity?

Answer 28

Movement of salts in the landscape is driven by water, so a change in the precipitation will have an effect on salinity.

• The amount of rain
• Frequency of rain
  
  • Large rainfall events can leach salts
• Evapotranspiration
  
  • ↑ temp = ↑ evapotranspiration
• Nature of precipitation (rain or snow)
  
  • Snow sits on top of the ground, water infiltrates far better

Climate change impacts are expected to include:

↓ rainfall
↑ temp
= ↑ evapotranspiration
= ↓ water available overall

Arid environments are more conducive to salt accumulation and less conducive to OM accumulation.

Question 29

How is salinity measured?

Answer 29

Electrical Conductivity (EC)

Specifically, EC_1:5

Question 30

Explain how an EC_1:5 measurement would be performed.

Answer 30

Calibrate EC meter by measuring standard solutions.

• Calibrate to 1 standard solution (the one closest to what you’re anticipating the unknown solution to be)
• Test the second standard solution
• If it isn’t accurate, divide the measurement for the second standard by what it was expected to be. This is the calibration factor to use after measuring the unknown solution.

Measure EC_1:5

• Mix 1 part soil with 5 parts water
• Shake
• Measure EC
• Multiply by calibration factor

CHECK SOIL & WATER RESOURCES PRAC MANUAL TO MAKE SURE THERE ISN’T ANYTHING I’VE MISSED

Question 31

Describe some ways to manage salt affected farms and landscapes

Answer 31

* Manage groundwater balance (amount of deep drainage/recharge)

• Improve soil condition to promote leaching of salts (esp. clays)
  
  • ↑ OM = ↑ aggregation = ↑ water movement
  • Apply gypsum if soil is sodic
• Use variable-rate irrigation
  
  • match the amount supplied with the demand
• Use high-quality irrigation water
• Perform a leaching event (large irrigation in a short amount of time to leach salt downward (assuming the issue isn’t a high water table))
• Choose salt-talerant species
  
  • GMO potential
• Plant more deep-rooted perennials
  
  • Pasture-cropping
  • Agroforestry
  • Silvopasture
  • Native fodder crops
• Use a holistic/systematic/multi-faceted approach

Prevention is better than cure