W5, Sodicity

Question 1

Why is sodicity an issue?

Answer 1

Decr. Strength of attractive forces between clay particles
= degradation of soil structure
= ↓ Permeability
= ↓ water holding capacity
= ↑ soil strength (penetration resistance)
= crusting
= ↓ seedling establishment
= ↓ Infiltration
= ↓ plant available water
= ↑ runoff
= ↑ erosion
= contamination of waterways (turbidity, nutrients)
= hard-setting
= restricted root growth
= waterlogging
= ↓ plant cover

Question 2

True or False?

Topsoils are generally more sodic than subsoils.

Answer 2

False.

Question 3

How does tunnel erosion occur?

Answer 3

Tunnel erosion occurs when water movement along the A:B interface encourages dispersive, sodic subsoils to erode faster than the overlying topsoil.

Question 4

Roughly how much of Australia’s agricultural land is sodic, and how much does it cost in lost production?

Answer 4

≈30%
$2 billion /year in lost production

Question 5

Explain the cause of dispersion (diffuse double layer forces) in sodic soils

Answer 5

Put answer here

Question 6

Define sodicity

Answer 6

The extent to which the cation exchange capacity (CEC) of the soil is occupied by sodium. More precisely, a sodic soil is one with an exchangeable sodium percentage (ESP) > 6 (> 15 = strongly sodic).

Question 7

How is the ESP calculated?

Answer 7

ESP = 100*[exchangeable Na+]/CEC

Question 8

How is the exchangeable sodium ratio calculated?

Answer 8

ESR = [exchangeable Na+]/(CEC-[exchangeable Na+]) ESR = ESP/(100-ESP) e.g. if ESP = 6, ESR = 6/(100-6)= 0.064

Question 9

T/F: Soil structure can be affected at ESP < 6 in dry conditions

Answer 9

True

Question 10

At what ESP does the United States Salinity Laboratory consider a soil to be sodic?

Answer 10

ESP => 15

Question 11

What is the goal when ameliorating sodic soils?

Answer 11

Increasing the electrolyte concentration in solution, which reduces the diffuse layer size (decreases repulsion between opposing surfaces).

Divalent cations decrease the thickness of the diffuse layer more than monovalent cations, and trivalent cations reduce it more than divalent cations.

Question 12

Explain the diffuse double layer forces that result in dispersion.

Answer 12

There are two main forces acting on cations within soil:
→ electrostatic attraction to negatively charged particles
→ concentration gradients

In other words, there is a tendency for the cations to adsorb to the negatively charged surface, but a concentration gradient exists between the highly-charged surface and the weakly-charged solution, so there’s also a tendency for the cations to diffuse away from the surface, down the concentration gradient, which reduces the van der Waals forces between particles, preventing them from flocculating.

The tendency of the particles to move away from eachother is a result of the system reducing the high concentration of cations between particles by moving them further apart.

Question 13

How is sodicity measured?

Answer 13

→ Exchangeable Sodium Percentage (ESP)

• The extent to which the CEC is occupied by exchangeable sodium.
  = ESP = [Na⁺]ex / CEC * 100
  = ESP = 100 ESR / (1 + ESR)

* ESP > 6 = sodic
* ESP > 15 = strongly sodic

→ Exchangeable Sodium Ratio (ESR)

= ESR = [Na⁺]ex / (CEC - [Na⁺]ex)
= ESR = ESP / (100 - ESP)
= ESR = kG * SAR

* ESR 0.064 = sodic
* ESR 85 = strongly sodic

CEC ≈ [Na⁺]ex + [Ca²⁺]ex + [Mg²⁺]ex + [K⁺]ex (mmol(+) / kg soil)

Question 14

True or False?

T/F: Soils with a pH > 8.5 tend to be sodic.

Answer 14

True.

(The cations present at high pH are generally only sodium and a bit of magnesium.)

Question 15

Order from most soluble to least soluble: MgCO₃, NaCO₃, CaCO₃

Answer 15

CaCO₃ → MgCO₃ → NaCO₃

Question 16

Why does a high concentration of Ca²⁺ not cause dispersion?

Answer 16

Because Ca²⁺ is adsorbed much more intimately, the concentration of exchangeable cations between particles is much less severe, so the system doesn’t need to separate the particles to dilute the cations between them as much.

Question 17

True or False?

You don’t see sodicity in saline soil.

Answer 17

True.

(The tendency to disperse decreases with increasing salinity. High electrolyte concentration in the soil solution negates the need for particles to separate to dilute the high concentration of cations between them.)

Question 18

What does the Gapon equation tell us?

Answer 18

The relative affinity of exchange sites for sodium and calcium under different concentrations of each element

kG = Gapon constant (mmol /L)
= [NaX][Ca²⁺]⁰˙⁵ / [Na⁺][Ca₀.₅X]

Question 19

How is the sodium adsorption ratio (SAR) calculated?

Answer 19

SAR = [Na⁺]/([Ca²⁺][Mg²⁺])⁰˙⁵ mmol /L

Question 20

True or False?

The affinity of exchange sites for sodium and calcium varies depending on soil moisture and the concentrations of Na and Ca in solution.

Answer 20

True.

e.g.
[Na⁺] = 10 mmol /L
[Ca²⁺] = 1 mmol /L
SAR = [Na⁺]/[Ca²]⁰˙⁵ = [Na⁺]/√[Ca²] = 10 / √1 = 10

If a 10:1 DILUTION (e.g. rain) occurs:

SAR = [Na⁺]/√[Ca²] = 1 / √0.1 = 3.2
= reduced affinity for sodium
= increased affinity for calcium
= ↓ sodicity (more Ca on exchange sites)

If a 10:1 CONCENTRATION (e.g. evaporation) occurs:

SAR = [Na⁺]/√[Ca²] = 100 / √10 = 32
= increased affinity for sodium
= decreased affinity for calcium
= ↑ sodicity (more Na on exchange sites)

Question 21

True or False?

The relationship between ESR and SAR is positively linear

Answer 21

True, but the relationship (slope of the line) varies with soil type.

↑ ESR = ↑ SAR

Question 22

True or False?

Saturated or near-saturated soils generally have a 100 : 1 greater affinity for Ca compared to Na.

Answer 22

True.

Depends on soil moisture.
> 100 : 1 under dilute conditions (↑ affinity for Ca)
< 100 : 1 under concentrated conditions (↑ affinity for Na)

Question 23

True or False?

↑ [Na⁺] = ↑ dispersion

Answer 23

True.

↑ [Na⁺] = ↓ energy required to cause dispersion

Question 24

What factors influence dispersion

Answer 24

→ ESP(ESR) / SAR

→ Electrolyte concentration

Question 25

True or False?

Mg is better than Ca at preventing dispersion

Answer 25

False; Ca is far better.

A soil with an ESP of 6 and Ca being the exchangeable divalent cation will disperse a similar amount to a soil with an ESP of 3.5 where Mg is the exchangeable divalent cation.

Question 26

What is the effect of having a sodic soil and irrigating with saline vs non-saline water?

Answer 26

As the ESP (sodicity) of the soil increases, so does the electrolyte concentration (salinity) of the irrigation water need to increase to prevent structural problems (dispersion).

i.e. ↑ soil ESP = ↑ irrigation water salinity required to prevent structural issues

Question 27

How can sodicity be managed?

Answer 27

→ Ca inputs

▪︎ Gypsum (CaSO₄⋅2H₂O)

= ↑ electrolyte concentration
= displace Na⁺ (try to leach) with Ca²⁺
▫︎ Rate: 3-5 t /ha common
▫︎ Application:
- Surface
- Irrigation water
- Subsurface (deep rip - can be expensive)
▫︎ Moderate solubility (16 mmol /L)

▪︎ Lime (CaCO₃)

▫︎ Far less soluble than gypsum
▫︎ Works best in acid soils (dissolves lime quicker)

→ Decrease disturbance

▪︎ ↓ tillage
▪︎ ↑ soil coverage
▪︎ ↓ impact of grazing (especially when wet or lacking cover)

→ Increase SOM

= ↑ aggregate stability
= ↑ general fertility
= ↑ soil coverage

Question 28

Explain the link between alkalinity and sodicity.

Answer 28

Has to do with the solubility (constants) of carbonate complexes (CaCO₃, MgCO₃, NaCO₃).

Sodium carbonate very soluble, while calcium carbonate is quite insoluble (MgCO₃ is in between).

Alkaline soils generally have more NaCO₃ present.

Calcareous soils are less sodic than non-calcareous soils because of the insolubility of CaCO₃.

Question 29

How can going from irrigating with a high quality water source to a poor quality water source influence soil structure?

Answer 29

Initially, there will be no problem. In fact, on a soil with a low ESP, irrigating with poor-quality (high SAR) water will increase the structural integrity of the soil by decreasing the size of the diffuse layer. However, using water with a high SAR will, slowly over time, increase the ESP of the soil, which will cause a problem when water with a low SAR (e.g. rain) is applied.

Question 30

True or False?

Decreasing ESP (exchangeable sodium percentage) results in decreased PAW (plant available water)

Answer 30

False.

↓ ESP = ↓ sodicity = ↑ PAW