Slaking and dispersion
Dispersion is caused largely by high levels of exchangeable sodium in eh soil and excessive mechanical disturbance, especially if the soil is wet. Slaking is caused by soil sodicity or by lack of organic mater inthe soil. It results in the loss of large pores (macropores) and the development of crusting and/or hardsetting characteristics. The micro-instability of some soils may only become evident after some extra energy has been put into the system by re-moulding the soil.
Sodic Soils
Exchangeable sodium percentage (ESP) measures the sodicity of the soil. Non-sodic soils have ESP <6%, sodic soils have an ESP between 6 and 15% while strongly sodic soils have an ESP greater than 15%. Sodic soils occupy almost 30% of the australan landmass and 80% of irrigated agricultural area in Astralia.
Adverse Properties of Dispersive Soils.
Ameliorating Sodic Soils
Amelioration is the attraction of cations to a colloid surface. Many of the poor physical chracteristics of dispersed sodic soils can be ameliorated by the addition of calcium. Gypsum, is a cost-effective and more readily soluble source of calcium ions.
Gypum applications provide a source of calcium ions. THe calcium ions displace sodium ions from the surfaces of colloids of sodic soils. The calcium-dominated colloids form aggregates and open up the soil. THe concenration of unwanted sodium ion is dilutedby excess water passing through the soilin drainage waters.
Gypsum treatments will improve the structure of hardsetting or rusted sodic topsoils and almost impervious sodic subsoils. The practical outcomes are improved soil tilth, reduced crust strength, increased water infiltration and stoage, increased drainage and leaching, reduced runofff, better plant growth and reduced soil erosion.
A program called Gypsy can calculate the amount of Gypsum required based on CEC, ESP, Electrical conductivity and chloridecontent of the topsoil and subsoil.
Gypsum also improves soil permeability. THe flow of water through columsns of a sodic soilis much less than that of the same soil treated with gypsum. THe calcium from gypsum has induced aggregation and has opened up water movement pathways in the soil.
Soil responsivenes to Gypsum can be tested by placing the soil in dionised water then forming it into a cube and dropping it into another cup of water. No change indicats it is not responsive to chypsum, slaking indicates a marginal benefit while dispersion indicates a high responsiveness to gypsum.
Interpreting Emerson Aggregate Test Results
Learn More
Nova: The Dirt on Our Soils
Sodic Soils
Questions
Dispersion is caused largely by high levels of exchangeable sodium in eh soil and excessive mechanical disturbance, especially if the soil is wet. Slaking is caused by soil sodicity or by lack of organic mater inthe soil. It results in the loss of large pores (macropores) and the development of crusting and/or hardsetting characteristics. The micro-instability of some soils may only become evident after some extra energy has been put into the system by re-moulding the soil.
Sodic Soils
Exchangeable sodium percentage (ESP) measures the sodicity of the soil. Non-sodic soils have ESP <6%, sodic soils have an ESP between 6 and 15% while strongly sodic soils have an ESP greater than 15%. Sodic soils occupy almost 30% of the australan landmass and 80% of irrigated agricultural area in Astralia.
Adverse Properties of Dispersive Soils.
- infiltrating rainwater cannot easily infiltrate into sodic subsoils. The water accumulates in teh topsoil so iron and organic mater are reduced chemical and bleached A2 horizons develop. The saturated topsoil loses its strength and becomes spewy. HUman, animal and vehicle movement over such soils increases erosions problems.
- the surfaces of dispersive soils lack strongly connnected macropores through which water can infiltrate and drain away. Such soils, particularly those with heavy (clayey) extures, become sticky and bogy after only light rains and they usually dry out to form a surface seal or crust.
- surface soil crusts inhibit water infiltration, reduce seedling emergence, prevent revegetation, increase runoff and enhance soil erosion. Although thin they may also have a major influence on the emergence of seedlings and thestablishment of an effective plant cover.
- Surface seals form where fine colloidal particles, dispersed by rainfall impacts, clog the pores of the surface soil and form a thin crust of fine, structureless material. Even if the crust is disrupted by cultigvation, without chemical amelioration, it will re-form under subsequent rainfall or irrigation.
- rill and gully erosion tha give the oil a finely fretted appearance indicate a very hig soil pH (greater than 8.5) and dispersive clods usually indicate the occurrence of sodic soils.
- Runoff water will find a pathway along even tiny cracks or root holes in a dispersive soil. The water will wet and disperse the soil, excavating an underground pipe along that zone of weakness. The the pasage of time, the pipe will enlarge into a tunnel whose roof may collapse, producing a gully. The tunnels will often follow rot holes and may extend for a considerable distance from gullies in dispersive soils. Collapse of the soil bridges over the tunnels results in extensive gully erosion. It is not unusualy for tens of meters of gully to appear after an overnight storm.
- Dispersive soils erode where water depths or velocities are increased at the edges of hard structures. Water flows undermine hard structures which eventually collapse.
Ameliorating Sodic Soils
Amelioration is the attraction of cations to a colloid surface. Many of the poor physical chracteristics of dispersed sodic soils can be ameliorated by the addition of calcium. Gypsum, is a cost-effective and more readily soluble source of calcium ions.
Gypum applications provide a source of calcium ions. THe calcium ions displace sodium ions from the surfaces of colloids of sodic soils. The calcium-dominated colloids form aggregates and open up the soil. THe concenration of unwanted sodium ion is dilutedby excess water passing through the soilin drainage waters.
Gypsum treatments will improve the structure of hardsetting or rusted sodic topsoils and almost impervious sodic subsoils. The practical outcomes are improved soil tilth, reduced crust strength, increased water infiltration and stoage, increased drainage and leaching, reduced runofff, better plant growth and reduced soil erosion.
A program called Gypsy can calculate the amount of Gypsum required based on CEC, ESP, Electrical conductivity and chloridecontent of the topsoil and subsoil.
Gypsum also improves soil permeability. THe flow of water through columsns of a sodic soilis much less than that of the same soil treated with gypsum. THe calcium from gypsum has induced aggregation and has opened up water movement pathways in the soil.
Soil responsivenes to Gypsum can be tested by placing the soil in dionised water then forming it into a cube and dropping it into another cup of water. No change indicats it is not responsive to chypsum, slaking indicates a marginal benefit while dispersion indicates a high responsiveness to gypsum.
Interpreting Emerson Aggregate Test Results
- class 1 aggregates are soilswith high tunnel erosion susceptbilty and detection may be difficult in acidic soils
- calss 2 aggregates: some aggregate instability; soil with some tunnel erosion usceptibility; desirable materials for sealing waterstorage structures
- class 3 aggregates: genral stable materials; soils desirable for soil conservation earthworks; leaks can be sealed when wet by roller or stock trampling;
- classes 4-6 aggregates: soil not susceptible to tunnel erosion.
Learn More
Nova: The Dirt on Our Soils
Sodic Soils
Questions
- what is slaking and dispersion?
- list and describe the different classes of aggregates
- list and describe 4 issues with sodic soils
- what can be used to treat sodic soils? How does it work?
- how can soil response to gypsum be tested?