In Nillumbik we are faced with predominantly clay soil. The most common soil type in Nillumbik Shire is a dark grey-brown loam surface layer with a lighter grey subsurface layer over yellow-brown mottled clay subsoil. Soils are often shallow and gravelly on steep slopes.
There are pockets of other soils types, particularly near Kangaroo Ground where there are areas of two other types, a dark grey to black heavy clay and dark grey sand over mottled clay. To the north-east of the shire there are areas of dark, slightly acidic clay loams over dark grey silty or medium clay subsoil.

Difficulties with clay soil :

  • Can become compacted and hard
  • Drainage and aeration may be poor
  • Drop in bioavailability of nutrients with falling temperature can be greater than other soil type

Solutions and recommendations: 

Compost

  • Can help break up clay and add organic matter
  • Will require working through the soil which can be difficult with hard lumps of clay

Soil Conditioners

  • Usually a mix of composted material like bark, gypsum, and soil wetter.
  • Effective, but can be slightly more expensive.

Mulching

  • Regular mulching in spring doesn’t just help save water, it rots down into the soil through the year, gradually adding organic matter.
  • You need to do other soil amendments as well, mulching alone isn’t enough to break a heavy clay soil.

Gypsum

  • 1Kg per sq m.
  • Can break up clay soils, but only improves dispersive clay. 

How to test if your soil disperses:

  1. Take some air-dry soil peds (aggregates) about 5mm in diameter
  2. Put a few centimetres of distilled or rainwater into a clear jar, or dish. Do not use chlorinated tap water as this can interfere with the test
  3. Place the peds gently in the water.
  4. Check the peds after 30 minutes, 2 hours, and 24 hours.

What you looking for?

If the topsoil is dispersive the water around the peds becomes cloudy or milky from clay in the water. The more cloudy the water, the more dispersive the soil is. See Agriculture Victoria’s time-lapse videos of dispersion. Gypsum makes these soils more stable and less dispersive.

If the ped has crumbled but the water is still clear, the soil is not dispersive. Adding gypsum to non-dispersive soils will not improve soil structure. If you don’t need to add Ca and/or S for nutrition, you don’t need gypsum.

http://agriculture.vic.gov.au/agriculture/farm-management/soil-and-water/soils/melbourne-soils

The positive side of clay Clay soils may be hard work but they also have benefits too

  • Clay particles are negatively charged which means they attract and hold negatively charge particles such as potassium, calcium and magnesium, making a more nutrient rich soil.
  • Clay soils also have a good water holding capacity.