Bite-size paper: Artificial soils – a solution or potential polluter?

Are artificial soils the solution to our over use of landfills and soil degradation problems, or do they pose a risk to aquatic environments?
Fabricated soil. Image credit: Plymouth Electron Microscopy Centre

Can artificially made soils can be used as a sustainable solution against soil erosion?

Researchers from the University of Plymouth, alongside colleagues from the Universities of Exeter and Worcester and industry partners at the Eden Project, are investigating whether artificially made soils can be used as a sustainable solution against soil degradation, whilst reducing the quantity of waste sent to landfill.

The year-long experiment, led by Professor Mark Fitzsimons, measured the amount of nutrients the soil lost due to the movement of water through it. ‘Leachate’ is the term used for the nutrient-bearing water, after it has passed through the soil. The researchers tested the chemical content of the leachate, as a measure of how well the artificial soil could retain nutrients.

Specifically, the focus of the study was on the ability of the soil to retain nitrogen, especially once carbon quantity in the soil becomes low (limited). This is important as British soil is losing 0.6 per cent of its carbon annually due to climate change related processes. Whether the soil is appropriate for use in projects, ranging from urban landscaping to farming depends heavily on how well it retains nitrogen.

The economic and environmental impacts of topsoil loss, in the UK and throughout the world, are significant. Across the UK, rainwater is washing soil from the land into rivers, lakes and seas; this process is ‘soil erosion’. Under natural conditions, soil erosion occurs very slowly, however, farming requires the clearing of forests and woodlands, resulting in a lack of plants to anchor soil to the ground. This speeds up soil erosion; the speed of soil loss dramatically outpaces the accumulation of new soil.

The experiment

The fabricated soil samples, composed entirely from waste materials, contained:

  • 25 per cent sand waste from Cornish china clay mines
  • 32.5 per cent composted tree bark
  • 32.5 per cent composted green waste
  • 10 per cent coal rich clay (lignite).

During the experiment, soil was compacted into four, one metre tall opaque tubes with a sandy layer at the bottom. As artificial rainwater dripped continuously onto the soil, the water flowed through the soil and reached the base to be collected as leachate. At weeks 26 and 48, the team added a common agricultural fertiliser, made of Nitrogen, Phosphorous and Potassium (NPK), to two of the samples.

The quantity of lost soil nitrogen was determined by measuring the nitrogen content in the leachate over time. Ideally, the soil would retain most of its nutrients; this is required for healthy plant growth. Additionally, if the soil retains nitrogen poorly it would potentially pollute local water sources, through a process called ‘eutrophication’. Nitrogen-rich leachate could fertilise local water sources, forcing algae to bloom on the surface; this blocks sunlight and damages ecosystems below.

The results

  • nitrogen in the leachate steadily decreased until week 27, as expected in natural soils under these conditions
  • from week 27 onwards the nitrogen content of the leachate began increasing
  • the addition of fertiliser did not alter the results in terms of trend, but it exaggerated nitrogen leakage
  • week 26 was the key point in the experiment as, at this time, carbon in the soil became limited. This condition causes microorganisms in the soil to retain more carbon, and therefore, excrete more nitrogen
  • after week 26, inorganic nitrogen levels in the leachate came very close to the safe threshold level set by the European Union. This implies that carbon limitation does reduce nitrogen retention in soils.

These results highlight the need for careful management of fertilised soils, by adding carbon rich materials before carbon becomes limited.

Artificial soil has potential as a sustainable solution for mitigating soil erosion problems, and reducing the quantity of waste sent to landfill. However, in order to be environmentally beneficial in the longer term, the carbon component of the soil may require attention to reduce water pollution, especially when used in the agricultural industry.

Robot arm picking cauliflowers

Agri-Tech Cornwall

The University of Plymouth is proud to be a partner on the Agri-Tech Cornwall project, a £10 million initiative to help the United Kingdom become a world leader in agricultural technology and sustainability.
Researchers from the University will share their expertise and collaborate with small and medium-sized Cornish companies to research the future sustainability of the sector.

European Regional Development Fund (ERDF)

The University of Plymouth is proud to be supported by the European Regional Development Fund. As one stream of funding under the European Structural and Investment Funds (ESIF) Growth Programme 2014–2020, the ERDF focuses on smart, sustainable and inclusive growth.
The main priorities involve contributions to research and innovation, supporting and promoting small and medium size enterprises (SMEs), and the creation of a low carbon economy.
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