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Seed in the Topsoil

Why is returning fresh topsoil important?

 
Applied seed can be used successfully to establish plants in rehabilitated areas. However, there are many species where it is impractical or even impossible to collect sufficient quantities of seed to acheive this on a large scale. For many of these species, the natural seed contained in the returned topsoil (soil seedbank) is the best way to re-establish the species in rehabilitated areas.
 
In the jarrah forest of Western Australia the natural soil seed bank ranges from several hundred up to about 1500 native seeds per square metre.  For bauxite mine rehabilitation the soil seedbank can contribute more than 70% of the plant species found in a rehabilitated pit and hence is an important resource for the restoration process.  We know there are large changes in the seedbank over the seasons with summer having the most seed.  Also seeds need to be close to the soil surface to successfully germinate.  Various studies have shown that the seedbank can be lost if the topsoil is not correctly handled.  This knowledge and experience has led to the following recommendations to ensure the best use of the topsoil seedbank for mine rehabilitation;
 
  • Strip the topsoil from new mining areas and return the soil to rehabilitated areas in the dry season (summer to autumn in this climate type)
  • Return the topsoil from new mining areas to nearby rehabilitated areas directly with no storage period (avoid stockpiling the soil).  This is called direct return of topsoil.
  • Carry out the final soil tillage (in our case, ripping) in the dry season (summer-autumn)
  • So that the seeds are not buried too deep to emerge, ensure the top layer of soil (approximately 15 cm) is returned on the top in rehabilitated areas.  This means we take the soil off in two layers and return them in the same two layers when rehabilitating an area.  This is called double stripping of soil.
 
A procedure to concentrate the seed
 
Sometimes fresh topsoil is not readily available, such as when we are decommissioning a crusher site and mining operations have already moved away so “direct return” is not possible.  To overcome this, we developed a method to concentrate the seed in the fresh topsoil, so that pits which receive stockpiled topsoil can also receive some of the seed-rich fresh topsoil.  Our research has found that concentrated topsoil from one hectare of forest can be used to significantly increase the number of plants and plant species on a much larger area of rehabilitation, up to seven hectares.  This is because many seeds can only germinate from less than 10mm deep in the soil profile, so applying topsoil as a thicker layer effectively wastes the seed in the lower part of the layer. 
 

The procedure to concentrate the seeds involves collecting the topsoil in summer when the seed density is the highest.  Because the forest topsoil is very gravely (60-80% gravel in many cases) the topsoil can be screened to remove gravel greater than five millimetres in diameter. The fine material is then transported to a recently rehabilitated mine pit that has received stockpiled topsoil, where it is applied as a ‘top dressing’. This increases the species richness of the old topsoil two to four fold.  Transporting the screened soil is cost effective because the seed has been concentrated in a small volume of soil. This may be the first case of seed concentrating technology being used in land rehabilitation anywhere in the world.
 

soil pouring from soil sieve machine

Click image to enlarge.


Topsoil is screened to gather a seed-rich mix used to maximise growth in the rehabilitated areas.