Reuse of site soils
Many of SESL Australia’s clients remove large quantities of soil and fill from landscaping sites to replace with commercial soil for the plantings, and then wonder what to do with it. In most cases, in fact, they can reuse the soil and reduce both costs and problems.
More often than not, site soils can and should be reused in the new landscape. Not only should a “green” industry try to recycle its resources for economic and environmental reasons, but also very often the site soils are of better quality than any commercially available landscaping soil.
Only a few materials cannot be reused in landscaping. These include heavy saline, sodic or alkaline clay, demolition fill which contains cement, lime and mortar, and heavy plastic clay. Natural topsoil can nearly always be reused, and many earthy and sandy fill materials, screened of rubbish, can also safely be reused.
Any landscape job of any size should begin with an appraisal of the site’s attributes. One of the most important of these attributes is the soil. To begin the appraisal, soil maps can prove useful. In most populated areas, good soil maps are available. Sydney is covered by some excellent soil maps (e.g. Chapman and Murphy 1989). This resource will allow the planner to pinpoint almost to the street the expected soil type for the location. A full description of the natural vegetation, soil relationships and physical characteristics can be found. A quick examination of the site will reveal whether the soils fit the descriptions. Quite reasonable stripping and reuse plans can be based on these descriptions alone. Additional information can be found in more general texts (e.g. Stace 1972, Northcote et al. 1975, Charman 1978).
If more detail is needed, a trained soil scientist can survey the site and make precise specifications for a reuse plan. A soil scientist is almost essential where the site does not fit the description because of previous disturbance, filling or other alteration of the natural soil conditions. A soil scientist’s input is essential in large projects, particularly those involving urban redevelopment where natural profiles are seldom found. The alternative can be failure of the plantings and the loss of thousands or millions of dollars’ worth of plants.
The soil scientist’s report should concentrate on the features of the soil relevant to landscaping, rather than just a soil classification, and present the results in plain English.
Details important to the landscape plan include a map of how the soils vary across the site, topsoil depths, precise stripping depths and reuse recommendations. The plan should include the following recommendations:
- Topsoil must be separated carefully from subsoil and stockpiled separately. They’re not the same and mustn’t be mixed. Small stockpiles are preferable, but topsoil spread from large piles can be left to aerate for a few days before planting.
- Soil analysis should be used to determine the need for any lime, gypsum or sulphur for pH correction. Chemicals can be added while stockpiles are being made to that the soil can be reused as soon as the site is ready.
- Stockpiling can reduce the viability of many weeds, and pre-emergent herbicides can be incorporated in the stockpile to kill seeds. The inclusion of grass and fine plant material with the topsoil will improve soil organic matter levels as it rots down in the stockpile.
- Soil analysis will also indicate the need for the addition of fertiliser or organic soil improvers as necessary to support the landscape plan.
- This serves an important function beneath the topsoil. Often we see topsoil placed right on top of cut bedrock. This is a waste of topsoil (and can create soil moisture problems), which can be offset with some of the site subsoil, thus mimicking the natural situation.
Site soil conditions will rarely dictate the landscape plan and planting lists, but a site analysis will reveal any serious impediments. Problems can arise from impermeable fill and clay. Even small amounts of clay in sand can reduce permeability, preventing some soils from draining. Marine saline sediments can pose problems, but fortunately Casuarina and Allocasuarina cope well. In other situations, high phosphate levels can totally exclude some species from the landscape unless imported soils are used.
Having seen first-hand the many problems of imported soils, SESL staff would prefer to work with and improve the natural soil, or even fill soil, any day.
Chapman GA, Murphy CL. 1989. Soil Landscapes of the Sydney 1:100,000 sheet. Soil Conservation Service of NSW, Sydney.
Charman PEV (ed.). 1978. Soils of New South Wales, Their Characterisation, Classification, and Conservation. Soil Conservation Service of NSW Technical Handbook No 1.
Northcote KH, Hubble GD et al. 1975. A Description of Australian Soils. CSIRO, Melbourne.
Stace HCT, Hubble GD et al. 1972. A Handbook of Australian Soils. Rellim Technical Publications, Adelaide.