The exchangeable sodium percentage ESP measures the proportion of cation exchange sites occupied by sodium. The dispersive behaviour of a soil indicates the level of sodicity Figures 1 and 2. The procedure for a soil dispersion test is:. Aggregates will often, but not always, slake crumble soon after being placed in the water; however, this is not dispersion. The water around the edges of the soil aggregate in a dispersive soil will become cloudy and milky looking water looks dirty because of the dispersed clay Figures 1 and 2.
For a highly dispersive soil, dispersion will be obvious after about minutes; for a moderately dispersive soil, it may take 2 hours for dispersion to be obvious Figures 1 and 2. Dispersive soils are susceptible to water erosion. Dispersive soils used for dam walls are very susceptible to tunnel erosion , and these dam walls often fail if there is a sudden inflow of fresh water.
While some dispersive soils can be acidic, the majority of dispersive soils in the Western Australian grainbelt are strongly alkaline with soil pH Ca greater than 8.
These strongly alkaline dispersive soils can also contain high concentrations of salt and boron. High boron levels are toxic to some crops and pastures and can reduce yields. Salt tends to reduce the dispersive behaviour of soil, and dispersion may only be seen in saline soils when the soil is disturbed or large amounts of fresh water dilute the salt. Dispersion may be completely prevented in highly saline soils despite a high percentage of exchangeable sodium.
Identifying dispersive sodic soils. Page last updated: Tuesday, 16 March - pm. Please note: This content may be out of date and is currently under review. By sodium, this means sodium ions; not salt or NaCl. In fact salt helps improve soil structure and aggregation as salts dry out the soil causing clays to cluster together. Note clays are mostly negatively charged — and opposites attract. But worse still, the sodium ions have very large water halos hydration shells around them, making the ion even weaker charged at its edge.
See below a sodic clay aggregate dispersing — this happens when the soil is placed in distilled pure water e. Soils rich in sodium ions are called — Sodic soils or Sodosols. Sodic soil layers occur most typically in the clayey subsoils and lower layers of profiles in semi-arid areas of Tasmania and often are worse near the coast.
These are areas where sodium ions are added to the soil via rainfall but because of high rates of evaporation in these dry areas the sodium ions are not washed all the way out of the soil profile and instead attach to the soil clays lower in the soil profile.
SE Tasmania has many clayey soils with sodic sub-layers. Assessment of dispersion To assess your soil to see if it will disperse, there is a relatively quick and simply test that you can do yourself at home. Practical Note: Aggregate Stability Well aggregated soil is important. It has pores between aggregates and within the aggregate. Large pores allow for the exchange of oxygen and other gases with the atmosphere, while small pores hold plant available water and dissolved nutrients.
Quick Reference Guide: Assessing aggregate stability When a fragment of soil is immersed in fresh water, there are four things that can happen: It can remain unchanged It can swell It can fall apart into smaller fragments it slakes It can disperse into a fine milky suspension Treating dispersive soils If you have a dispersive soil, the first step is to find out if you soil is dispersive throughout the soil profile.
These numbers will provide you with the information to calculate whether treating the soil with gypsum is practicable and cost effective for you.
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Access to higher quality images can also be provided on request. Your gateway to a wide range of natural resources information and associated maps. Copyright Privacy Disclaimer Accessibility. This page was last updated on. No-flocculated soil sample. If the shell is thin, the particles can come close together for attractive Van der Waal's forces to take hold and for the clay particles to flocculate.
Flocculated soil sample.
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