9.4: Caring for the Country

Salinity

Salinity is the saltiness, or dissolved salt content, of water or soil and is a general term used to describe the presence of elevated levels of salts. These salts include:  Chloride

  • Sodium
  • Magnesium
  • Calcium sulphates and bicarbonates

There are three main types of salinity. These are:

  • Irrigation Salinity– This occurs when water is overused for irrigation causing rising water tables.
  • Dryland Salinity– This occurs when salt water seeps scald the surface of land that is not irrigated, which then affects plant growth and severely degrades the soil structure.
  • Saltwater Intrusion – This occurs in coastal aquifer systems when sea water replaces groundwater that has been overused and depleted.

Where and how salinity occurs depends on several factors. The main factors which are the most important to remember are:

  • The climate of the area
  • How humans have affected the area (clearing, housing etc…)
  • The characteristics of the landscape (bush type and density, river systems etc…)

The amount of salt that can accumulate in the soil depends greatly on the properties of the soil and the local rainfall. It is known that clay soils (which occur extensively in inland Queensland) usually have the highest amount.
There are many signs to be aware of where salinity is concerned including:

  • Rising groundwater levels in bore holes
  • Deterioration and crumbling of roads
  • The ground becomes permanently or seasonally damp or waterlogged or continues to remain damp after extended rain
  • The dieing back of vegetation in low lying areas
  • The failure of plants to germinate
  • Areas of bare soil
  • An increase in the growth of salt-tolerant plants
  • A deterioration in the quality of the groundwater 
  • Rising damp in buildings
  • A change in pasture composition and a reduction in the diversity (with couch grass and other salt tolerant plants dominating the pasture)
  • Salt scalds on bare soil

Salinity is one of the most prevalent and significant environmental problems in Australia. Although salt is naturally present in many of our landscapes due to the geology and age of the Australian continent the European farming practices which took place in the early years of our countries history have had a major lasting effect. These practices replaced much of Australia’s native vegetation with exotic and shallow-rooted crops and pastures which have caused the water table in many areas throughout Australia to rise.

There has been salt present under the ground in Australia for millennia but rising groundwater levels have caused these salts to become dissolved and mobile in the environment once again. Because of this, salt is now being transported to the root-zone of crops, pastures, remnant vegetation and directly into our wetlands, streams and river systems. Salinity also can have a damaging effect on buildings, roads, pipe networks and underground cabling as the salt ions (Na+ and OH-) ‘eats’ away at cement joins, increases rust in iron and steel reinforcements and causes instability in many structures.

Figure 1:  The Water Cycle and Dryland Salinity

Outcome 9.4.3 Students learn to examine the possible consequences for soil salinity of land clearing and irrigation and outline precautions that could minimise the problem in each case