Upper South East Dryland Salinity and Flood Management Program

This update published 1^st August 2007
First published 24^th January 2005

Changes to Saline Soils When Drained

Extracted from CSIRO Land and Water Technical Report 02/03.

Effects from water erosion:

loss of topsoil, carbon and fertility,
drainage of perched wetlands,
erosion and gully formation with deepening and increasing local drainage,
movement of dispersed clay,
deposition of soil/sediment on soil surfaces.

Effects from accession of low EC water:

formation and movement of dispersed clay forming throttles in B horizons,
tunnelling and gullying within sodic layers,
transport of salt and nutrients to drainage lines, formation of sulfidic layers in drains.

Residual effects from (salt) chemistry with drainage:

formation of sodic layers,
formation of sulfidic material under reducing conditions in drains (if SO4 = is present),
high sodium carbonate concentrations leading to high soil pH (> 8.5), altered solubility of soil minerals, low availability of some nutrient elements (e.g. Mn, Fe, Zn) and possibly reprecipitation of silica forming pans in E horizons,
deterioration of soil structure,
salt fretting.

Formation of layers with low hydraulic conductivity:

formation of “pavements” or surface hardpans– usually highly sodic, which shed water,
increase sheet erosion are barriers to capillary rise and infiltration. These are sometimes overlain by sandy materials,
sodic B horizons may retard vertical leaching of salts or upward movement of groundwaters,
iron cemented pans at the surface in some discharge areas may form a barrier to capillary water flow.

Effects from loss of vegetation:

increased erosion,
introduction of exotic tolerant plants altering biodiversity.
loss of organic carbon inputs and root channels.

Alteration from redox condition:

drainage increases oxygenation with potential positive effect on depth of root penetration,
formation of iron cemented pans that may be a barrier to water movement and root penetration.

Changes from increases in bulk density:

pugging, an undesirable effect of grazing,
increases in sodicity,
increases lower air and water filled porosity,
associated with siliceous pans in E horizons of sand over clay soils with illuviated clay,
high bulk density prevents root penetration.

Effects of time:

Although the passage of time may result in decreased salinity levels in a drained soil profile, detrimental effects (such as erosion or sodicity) may intensify unless remedial actions are taken.

Risk Management Planning

Risk management plans should address the following issues:

Is it economically/socially/politically acceptable and feasible?
Is it possible to provide appropriate drainage?
Is leaching water available?
Is the soil and water chemistry right?
Are appropriate vegetation, other biota and land management systems available for adoption?
Should appropriate biota be introduced or re-introduced?
Will there be unacceptable off-site effects?