Sodicity – a dirty word in Australia

Activity 1

A field test for sodicity

Sodicity is identified in the laboratory by measuring sodium levels in the soil. In the field we measure the problem caused by the sodium, that is the cloudiness or turbidity caused by soil dispersion.

Test the surface and the subsoil separately to best understand the problem.

  • Take two clean buckets into the field and collect samples from both the surface and the subsoil, according to standard procedures (a 5 centimetre soil auger would be ideal).

  • Collect samples randomly from a minimum of 5 locations over a uniform 1-2 hectare representative area of the paddock, placing all the surface samples in one bucket and the subsoil samples in the other.

  • If it isn't clear where the subsoil begins, take a sample from the top 10 centimetres of the soil profile. Then take a second 10 centimetre sample from somewhere deeper in the profile, within the range of 20-60 centimetres below the surface.

  • Spread the soil from each bucket into a thin layer on a clean plastic sheet. Place in a well-ventilated location to get it air-dry, which may take several days.

  • Then if necessary break the air-dried soil down into pieces of 1centimetre diameter, and mix the soil in each bucket thoroughly.

Measure the problem using the turbidity test for sodicity

  • From each surface and subsoil sample weigh 100 grams of soil into a clean 600 millilitre glass jar with lid.

  • Measure out 500 millilitres rainwater or distilled water to give a 1:5 ratio of soil to water.

    • act1.jpg – 8798 Bytes
    Making up a ratio of 1:5 soil to water (100 grams of soil and 500 millilitres water).
  • Gently pour this water down the side, without disturbing the soil at the bottom.

  • Invert the jar slowly and gently once and then return to its original position (avoid any shaking). Then let stand for 4 hours, with no vibrations or bumping.

  • Check the suspension above the sediment at the bottom of the jar and score the amount of cloudiness using the photographs for comparison. Make up another soil suspension and repeat the process if unsure.

    • Estimating turbidity (soil sodicity) in a 1:5 soil/water suspension.
        1. Clear or almost clear – not sodic
         2. Partly cloudy – medium sodicity
         3. Very cloudy – high sodicity
  • A white plastic spoon or spatula, that reflects light, when placed in the centre of the suspension can help identify the level of turbidity.

    • Estimating turbidity using spatula visibility.
         1. Plastic spatula visible – not sodic
         2. Plastic spatula partly visible – medium sodicity
         3. Plastic spatula not visible – high sodicity
  • When land managers determine remediation procedures for sodicity, they should take into account other degradation issues such as acidity and salinity – a multi-pronged approach may be needed.

This activity has been provided by the Cooperative Research Centre for Soil and Land Management. The CRC has developed a sodicity-measuring kit which can be purchased from the following sources:

  • DPI Publications, GPO Box 46, Brisbane 4001; phone 07 3239 3773; fax 07 3239 6509

  • NSW Agriculture Book Shop, 161 Kite Street, Orange NSW 2800; phone 02 6391 3433; fax 02 6391 3527

  • NRE Information Centre, 8 Nicholson Street, East Melbourne VIC 3002; phone 03 9637 8080

  • Information Centre, Mobilong House, Seventh Street, Murray Bridge SA 5253; phone 1800 356 446; fax 08 8535 6427
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Posted June 1999.