Exchangeable Sodium Percentage (ESP)

The presence of excessive amounts of exchangeable sodium reverses the process of aggregation and causes soil aggregates to disperse into their constituent individual soil particles. This is known as deflocculation and occurs in sodic soil.

Defloculation occurs because unlike the polyvalent cations of calcium and aluminium, sodium is monovalent. When sodium is adsorbed onto the negatively charged particles it can only attach to one and hence unlike with calcium and aluminium it can not form a bridge between the one it is attached and any nearby. This means the two particles will repel each other and create a dispersed soil condition.

The result is the establishment of tight arrangements of dispersed soil particles saturated with sodium. In general, macroporosity in sodic soil is greatly reduced and water infiltration slows to near zero. When wet sodic soil has poor infiltration and drainage and when it dries become quite hard.

The major issues arising from high sodium levels relative to the other exchangeable cations is on the physical properties of soil. In surface soil horizons this imbalance in the ratio of cations results in poor soil structure. This is evidenced by surface soil crusts or the setting of soil into large blocks on drying. As a result seedling germination and plant growth are problematic.

A sodic soil with few stabilizing agents (e.g. humus, clay or sesquioxides) in the topsoil will ultimately be susceptible to erosive soil loss during intense rainfall or irrigation cycles via rill and gully erosion.

This is because water intake is usually so slow, owing to the poor soil structure. This is especially the case in soil high in silt and clay particle size fractions. In the subsoil, soil sodicity leads to decreased permeability to water and air and poor soil drainage over time.

In many cotton growing areas of south-eastern Australia, large amounts of exchangeable sodium dominate the exchange complex.  This is particularly the case at depths of 0.90-1.0 m. The reason for this is that the process of sodium leaching is incomplete because most cotton growing districts are located in arid and semi-arid areas.

A sodic soil, by definition, contains a high level of sodium relative to the other exchangeable cations (i.e. calcium, magnesium and potassium). A soil is considered "sodic" when the Exchangeable Sodium Percentage (ESP) is 6% or greater. The exchangeable sodium percentage (ESP) is calculated as follows:

ESP = Exchangeable {(Na)/(Ca + Mg + K + Na)} x 100

In Australia, soil with an ESP greater than 6 % is considered to be sodic. However, soil dispersion problems may occur at a higher or lower ESP depending upon clay type.

Exchangeable Classification Non-sodic Sodic Moderately
Strongly Sodic Very strongly Sodic
Sodium Percentage <6 6-10 10-15 15-25 25

Further, ESP values as low as 2 can cause soil structure problems if the concentration of salt in the soil solution is very low. This is an issue in irrigated areas when the ratio of soluble sodium to calcium and magnesium ions in water is high. This ratio is expressed as follows and is called the Sodium Adsorption Ratio (i.e. SAR).

SAR = Exchangeable {(Na)/(Ca + Mg) -0.5}

In order to counteract the effect of excessive sodium on the exchange complex and to reinitiate the process of soil aggregation, calcium needs to be reintroduced into soil solution.

This is best achieved by the application of gypsum. This is because the calcium (Ca2+) in gypsum (CaSO4.2H2O) displaces sodium (Na+) on the exchange site. In turn, the sodium reacts with sulfate (SO42-) to form sodium sulfate (Na2SO4), which is a highly water soluble material that is leached from the soil.

The addition of gypsum leads to the removal of sodium and its replacement by calcium on the exchange sites, which reduces deflocculation and allows natural aggregation of particles that eventually, restores good soil structure. Gypsum is very useful when soil structure deteriorates because of high sodium.


Natural resource management for cotton growing regions

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