Measuring Soil Texture in the Laboratory – New South Wales

Key points

• Particle size analysis breaks a soil into texture classes – sand, silt or clay.


• Soil texture influences nutrient retention, water storage and drainage.


• Particles greater than 2 mm are removed before analysis.


• The soil textural triangle is used to determine soil type based on sand, silt and clay percentages.

Background

Particle size analysis (PSA) determines the relative amounts of sand, silt and clay in a soil. These size fractions are the mineral component of a soil and together determine soil texture. PSA is a laboratory alternative to field texturing (see Measuring Soil Texture in the Field fact sheet) and offers a more reliable determination of particle size distribution. There is only an approximate correlation between hand texturing and PSA (McDonald et al., 1998), because hand texturing relies on qualitative interpretation of texture while PSA measures exact amounts of individual particle sizes.

Soil texture is an inherent soil quality property that has a major influence on several other properties that influence agricultural potential (White, 1997). In particular soil texture influences nutrient retention, water storage and drainage. Soils with a higher proportion of sand retain less nutrients and water compared to clay soils.

Mineral components of soil

Coarse fragments – particles greater than 2 mm and include coarse quartz, rock fragments and cemented material. This is commonly called the “gravel fraction”.

Sand – comprised of quartz and resistant primary minerals such as mica. Sand particles are between 2 mm and 20 microns in size (Note: there are 1000 microns in 1 mm).

Silt – typically composed of quartz and small mineral particles such as feldspars and mica, and are between 2 and 20 microns in diameter.

Clay – made up of secondary clay minerals and oxides/oxyhydroxides of iron and aluminium, and are less than 2 microns in diameter.

New South Wales

Soil information collated from many projects is maintained by the SALIS team at NSW Office of Environment and Heritage (OEH), and was used to produce the map (figure 1) indicating the distribution of different soil texture groups in NSW.

Figure 1: Map of typical soil profile textures across NSW. (Image: NSW Office of Environment and Heritage).

Particle Size Analysis

PSA is a reliable, reproducible technique that eliminates factors that may affect field texture such as organic matter content, clay mineralogy, cation composition and the presence of cementing agents (Bowman and Hutka, 2002). The method comprises two parts, dispersion of the soil and separation of the particles into size groups.

Dispersion and pre-treatment

Pre-treatment of the soil may be needed to remove organic matter and salts such as gypsum. Iron oxides, calcium carbonate and magnesium carbonate should also be removed as they are common cementing agents in Australian soils (Bowman and Hutka, 2002). Pre-treatment of the soil will allow it to disperse completely.

Fractionation

Fractionation involves removing each particle size group (sand, silt and clay) from a pretreated soil and water mixture settled in a cylinder (figure 2). This is achieved by allowing the soil particles of different size to settle out of solution at different times (small clay particles take the longest). The fractions are subsequently dried and weighed and the sand, silt and clay must add up to 100%. Some calculations are needed for this method including the use of a scaling factor for the pipette analysis and a calculation for the sieve analysis. For a complete method, refer to Bowman and Hutka (2002).

Figure 2: Cylinder used for fractionation of sand, silt and clay particles. The sand fraction settles first followed by the silt then the clay fractions.

Using the soil texture triangle

The soil texture triangle (figure 3) is used to convert particle size distribution into a recognised texture class based on the relative amounts of sand, silt and clay as a percentage, for example:

Figure 3: Once soil fractionation has occurred a soil textural triangle is used to determine soil texture. Image adapted from Hunt and Gilkes (1992).

Further reading and references

Bowman GM, Hutka J (2002) Particle Size Analysis. In Soil Physical Measurement and Interpretation For Land Evaluation. (Eds. NJ McKenzie, HP Cresswell, KJ Coughlan) pp 224-239. (CSIRO Publishing: Collingwood, Victoria).

Hunt N and Gilkes R (1992) Farm Monitoring Handbook. The University of Western Australia: Nedlands, WA.

McDonald RC, Isbell RF, Speight JG, Walker J & Hopkins MS (1998) Australian soil and land survey field handbook, Australian Collaborative Land Evaluation Program, Canberra.

McKenzie NJ, Jacquier DJ, Isbell RF, Brown KL (2004) Australian Soils and Landscapes An Illustrated Compendium. CSIRO Publishing: Collingwood, Victoria.

The NSW Department of Primary Industries maintains soil information resources on the website.

Authors: Georgina Holbeche (The University of Western Australia), reviewed for New South Wales by Stephanie Alt (NSW Department of Primary Industries).

 

The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.

The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.