Week 11 - Sodicity and Salinity

Question 1

Define saline or salt affected soils.

Answer 1

Soils containing high levels of dissolved ions in the soil solution.

Question 2

What ions are commonly present in soils?

Answer 2

The ions which are commonly present are:
i. Cations: Ca2+, Mg2+, K+, NH4+, Na+
ii. Anions: HCO3-/CO32-, SO42-, NO3-, Cl-

Question 3

What is the most common salt occurring in the soil solution?

Answer 3

Na+ is very weakly held on cation exchange sites, and Cl- is rarely held in soil at all, the most common salt occurring in soil solution is NaCl.

Question 4

What does it mean for a soil to be sodic?

Answer 4

• Where NaCl is the major salt, some Na + from solution will exchange with other cations on the cation exchange sites.
• Thus the soil will contain some exchangeable sodium (Ex. Na+).
• Once the Ex. Na+ concentration becomes sufficiently high, that is, Na+ occupies a high proportion (6-20%) of the cation exchange sites the soils are said to be sodic.

Question 5

Define exchangeable sodium percentage (E.S.P).

Answer 5

The percentage of the cation exchange sites occupied by Na+ is called the exchangeable sodium percentage (E.S.P.).

Question 6

Define sodium adsorption ratio (S.A.R.)

Answer 6

The ratio of the Na+ concentration to the concentration of other cations in the soil solution.

Question 7

When do salt concentrations become a problem for plant growth?

Answer 7

• Once the salt concentrations exceeds 0.1-0.2% of the soil by weight, plant yield is severely reduced on average by 50%.
• Plants are particularly sensitive to salting during germination.

Question 8

What causes the effect of salinity on plant growth?

Answer 8

• Reduced availability of water in soil. Ions in solution, especially Na+, adsorb water on their surface making the water less available for uptake (reduces the water potential). Since salinity commonly occurs in arid environments where water supply is already a problem, salinity makes the water availability problem worse.
• Specific toxicities. Often the salts contain some elements at levels which are toxic, e.g. borate’s, or high Mg2+ (this decreases Ca2+ uptake). These will act to reduce plant growth.

Question 9

What causes the effect of sodicity on plant growth?

Answer 9

Once Na+ has entered the soil system, the Ex. Na+ concentration increases.

Problems occur if the salt concentration is reduced by leaching with relatively salt free water (for example rainwater). The soil has high E.S.P. (sodic) but now the salt concentration in the soil water is low. The effects on the soil are on both the physical and the chemical aspects of production.

High concentrations of Ex. Na+ with low soluble salt concentrations result in collapse of soil structure (dispersion). This results in soils with extremely poor physical condition, i.e. poor aeration, slow infiltration and drainage of water.

Question 10

At what exchangeable sodium values (E.S.P) will structure problems become apparent? In Australia, a soil with what E.S.P value is considered sodic?

Answer 10

The structure problems may become apparent at E.S.P. values as low as 5 to 6.

In Australia, a soil with an E.S.P > 6 is considered sodic.

Question 11

What are the chemical effects of high E.S.P. and low soluble salt concentrations?

Answer 11

That the Ex. Na+ can be hydrolysed from the exchange sites resulting in the production of an alkaline soil solution,

Ex. Na+ + H2O <=====> Ex. H+ + OH- + Na+

Soil pH can exceed 9. Under these conditions the adverse effect of high pH on nutrient availability become apparent. Deficiencies of Fe, Mn, Zn and Cu are some of the common induced nutrient deficiencies at these pH values.

Question 12

What are the origins of soluble salts in soils?

Answer 12

• Weathering of Parent Materials
• Cyclical Salt
• Direct inundation of coastal land by sea water.
• Rising water tables.
• Irrigation
• Fertiliser

Question 13

How does the weathering of parent materials contribute to soluble salts in soils?

Answer 13

• As rocks weather, the minerals dissolve.
• If these are not removed by the leaching process they will accumulate in the soil as salts.
• Low leaching regimes are associated with semi-arid environments.
• Rocks which contain high concentrations of Na minerals such as granites are most likely to produce soils with salinity problems.

Question 14

How does cyclical salt contribute to soluble salts in soils?

Answer 14

These salts are present in the atmosphere and are returned to the soil in rainfall. Cyclical salt returns are high in coastal environments (hundreds of kg ha-1 per year) but decrease rapidly as you move away from the coastal regions

Question 15

How does rising water tables contribute to soluble salts in soils?

Answer 15

When the water table is saline or there are salt deposits in the subsoil which are mobilised by the rising water table.

Question 16

How does irrigation contribute to soluble salts in soils?

Answer 16

• Irrigation with saline or slightly saline/sodic water can result in increasing soil salinity.
• When the water is lost to the atmosphere through plants, the salt is left behind.
• Even with good quality irrigation water (containing a low salt concentration) a potential salt problem exists.

Question 17

How does fertiliser contribute to soluble salts in soils?

Answer 17

Where high rates of soluble fertiliser (fertilisers are chemically, salts) are applied, the salt concentration builds up in the soil solution.

Question 18

What are the basics of managing saline or sodic soils?

Answer 18

1. Define the soil problem –
2. Identify the source of salt/Na and where possible reduce it
3. Where Na+ is present, the soil structure must be maintained (saline soils) or improved (non-saline soils). This is often achieved by applying gypsum (CaSO4) at rates ranging from 1 to 10 t ha-1. The Ca2+ applied is to displace Na+ from the exchange sites by cation exchange and increase soluble salt levels with CaSO4.
4. Leach the excess salt/Na+ out of the soil.
5. Establish plants preferably with some tolerance of salinity or structure problems from sodicity. The objective is to build up organic matter levels and general fertility.
6. Endeavour to keep salt/Na+ levels at a low level during subsequent land use. Rarely do the problems go away so on going management of water and salt is essential

Question 19

Explain the reasons why a high concentration of soluble salts retards plant growth.

Answer 19

• Reduced availability of water in soil. Ions in solution, especially Na+, adsorb water on their surface making the water less available for uptake (reduces the water potential).
• Specific toxicities. Often the salts contain some elements at levels which are toxic, e.g. borate’s, or high Mg2+ (this decreases Ca2+ uptake). These will act to reduce plant growth.

Question 20

Explain in detail why gypsum is the favoured amendment to overcome sodicity.

In your answer provide headings followed by description.

Question 21

Explain why lime is not commonly used to treat sodic soils.

Question 22

Explain why any estimate of sodicity is only a guide to the likelihood of dispersion.

Answer 22

• High concentrations of Ex. Na+ with low soluble salt concentrations result in collapse of soil structure (dispersion).
• The extent of the dispersive effect of sodicity decreases as the salt or electrolyte concentration increases.
• At an ESP of 20, soil structure will be dispersed at low electrolyte concentrations but remain stable if the electrolyte concentration is high.

Question 23

Define two properties that confirm that a soil is sodic.

Answer 23

i. Has a high concentration of the cation sodium Na+ relative to other cations.

ii.

Question 24

Describe in detail how low concentrations of sodium can lead to deflocculation of clays.

Question 25

Describe the sequence of structural changes that occur in a soil profile following large additions of NaCl and the subsequent leaching of Na. with Na free rain water.