There are a variety of methods for soil rehabilitation depending on machinery access, parent geology, topography, slope, aspect, and availability of soil additives.
Auckland Regional Council Technical Report TR2009/083 Landscape and Ecology Values within Stormwater Management (Lewis et al., 2010) provides further guidance on soil rehabilitation practices, but some of the common approaches are discussed below.
Where larger earthmoving equipment has access, soil rehabilitation methods can involve deep tillage or chisel ploughing to break up deep soil layers to about 900 mm without mixing in surface soil layers. These methods disaggregate and aerate compacted soils. In other areas, such as around existing buildings or on small sites, it may be possible to do shallow soil remediation using tractor mounted or hand operated equipment such as a rotary hoe.
Allophanic and granular soils can be enhanced using a combination of mechanical cultivation and amendments, as they fracture into a fine tilth over a wide range of moisture contents. These topsoils can be deepened to 300 mm if soils are aerated and compaction is limited to provide increased water percolation and root movement into subsoils.
Amendments and planting
Most soils in earthworks will be physically or chemically degraded to some extent, especially if they have been stockpiled. An effective method of reviving degraded soils is to incorporate organic material such as compost to promote plant growth, water uptake and water storage. Mycorrhizal fungi can also be incorporated (through inoculation by spray) into topsoil horizons to accelerate soil biodiversity and productivity.
Grass heights over 100 mm or dense tree planting increase soil macroporosity and permeability through root activity. Plants transpire, increasing water absorption capacity in the soil. This can be encouraged by planting in grids like orchards. Plants also add organic matter, which enhances the diversity of biological interactions in the soil.
It would be necessary to collect and analyse soil samples to ascertain whether any existing soil contamination issues are appropriate for intended land uses. Options to avoid excavation include capping contaminated soils with hydrophobic clays or re-surfacing with topsoil and compost to allow natural rehabilitation of soils.
Where there are low levels of contaminants, relative concentrations can be reduced through their dispersal throughout a site and phytoremediation processes, i.e. utilising plants to metabolise pollutants to innocuous forms or that are readily transported as coppice or leaf litter.