A Vertosol soil formed of volcanic parent material. Volcanic parent material gives rise to clay soils and they tend to have strong aggregate development. This means that you can see distinct clods or clumps throughout the soil. Clay soils hold moisture strongly and release it slowly for plant uptake.

This uniform fine-textured soil has developed from the slow settling of suspended clays. These are known as black Vertosols or cracking clay soils.

Features of black Vertosols on Tocal are:

  • formed from basic alluviul on backplains and in valley bottoms
  • imperfectly to poorly drained
  • uniform or gradational profiles
  • moderately to strongly developed structure
  • self-mulching surface soil and seasonal cracking when dry/swelling when wet due to presence of montmorillonite clay
  • moderate to high water-holding capacity
  • neutral to alkaline soils reaction trend; fertile; available potassium in calcium-bound form

This profile is called Racecourse Gilgai after the undulating surface of the soil in this area. Gilgai are formed by the shrinking and swelling of clay horizons with alternate dry and wet cycles. This forces 'blocks' of subsoil material gradually upwards to form mounds.

In areas with gilgai pasture growth is likely to be uneven across a paddock due to soil variations and poor drainage conditions in the depressions.

The combination of the heavy soil which is difficult to work and the presence of gilgai mean that this area is better suited to grazing than cropping. Fertiliser and grazing mangement is the key to retaining productive perennial grasses on these soils. If this isn't done, the soils will be dominated by pin-rush - of little value for grazing.


Profile description

  • 0cm: IA horizon.Dark grey-brown light medium clay. Sandy coatings on the lower part. Abundant plant roots. Drainage is fair. Fine crumb structure. pH 5.6
  • 32cm: IIA horizon.Very dark grey-brown sticky light clay. Plant roots common. Drainage is fair. Fine crumb structure. pH 6.0
  • 80cm: IIC horizon.Very dark grey-brown light clay with weak dark yellow brown mottles. Plant roots common to base of the profile. Drainage is fair to poor. Coarse angular blocky structure. White coatings on the soil surface at 75-80cm may be salt. pH 7.0.

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