Manufacturing topsoil to reduce costs for the industry

By Tania Costantini


31st Mar 2015

Ysgol Ffwrnes offered an excellent case study for the use of certified compost for large-scale urban landscaping in the construction industry.

Certified compost is already being used successfully in topsoil manufacturing on development projects. Topsoil is produced by mixing compost with recycled inert materials already present on site, such as surplus subsoil, stone fines and other mineral soil-forming materials. The particular blend of certified compost and soil-forming material will vary according to the landscaping needs to each individual site, but all manufactured topsoil should conform to the requirements of BS3882:2007.

A trial was undertaken comparing the performance of amenity and wildflower grasses established on manufactured and imported topsoil. One test area used standard imported soil. The second test area used topsoil that was manufactured onsite by incorporating PAS100 certified (green) compost at the rate of 10% by volume within the top 20 to 40 cm of prepared subsoil. This was achieved by spreading the compost to a depth of 4 cm across the subsoil surface and rotavating it in.

The plots were monitored visually, and grass production was measured over a period between July and October to determine the effectiveness of manufactured soil compared with imported soil.

Laboratory results show the compost had twice the organic matter content of topsoil, nearly twice the total nitrogen content, five times the readily-available nitrogen, and twice the available potassium and phosphorus. Consequently, using compost allowed for the importation of a concentrated form of plant nutrients onto the site without bringing in the weight of the mineral matter in topsoil. The grass growth trials have shown that over a single season between June and October 2014, the topsoil manufactured with certified compost outperformed the imported soils by a factor of two. This means it had approximately 100% more growth than the imported soils.

WRAP Testing Comparison

Watch this video to see the results of the trial and get some good practice guidance.

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