Soil research at the RHS

Most gardeners recognize the value of a healthy soil in order to support a healthy garden. However, the question of how best to maintain and improve healthy garden soils remains.

Organic matter

The addition of organic matter to soil helps aid nutrient provision, optimizes water management and potentially sequesters carbon thus helping mitigate climate change. 

Traditionally, a variety of organic materials have been used as mulches or soil conditioners in order to improve the performance of plants - largely through improved nutrient provision and improved soil water management.

Choosing the right material

The choice of material is largely determined by:

  • availability (a requirement for large volumes),
  • cost (cheap as possible but horticulturalists also recognise cost savings elsewhere, e.g. reduced use of fertilisers)
  • effectiveness (provision of nutrients, reduction in the weed population and improved soil structure).
  • aesthetics (For many larger public gardens the aesthetics of the organic material are also of value, the impression the material creates for visitors being of great significance, particularly if used as a mulch. Finally, horticulturalists are also “creatures of habit”; once they have identified a material they like, they tend to utilise it repeatedly.)

It is unlikely that all of these factors (availability, cost and effectiveness) are going to be met by any one material in any one situation. In addition, when examining the effects of repeated organic matter application on soil chemical and physical factors more closely, it is even less likely that they will all be ideal. 

Ongoing research at the RHS is examining management of soil organic matter in a horticultural context.

Current Projects

  • 10-year experiment investigating the long-term effects of repeated application of the same organic material to soil
  • PhD project: “The fate of carbon from organic amendments in a horticultural system” (University of Reading, 2014 – 2017)
  • Investigating the effects of long-term organic matter amendments on net primary productivity and greenhouse gas emissions in a horticultural context.

RHS Advice:

Soil care
Soil type
Organic gardening
Clay soils
Organic matter

Growing media research

Peat bogs are increasingly recognised as valuable habitats for wildlife and important stores of carbon, yet the UK horticultural industry still relies heavily on peat.

Environmentalists, government and horticultural businesses in the UK now recognise the environmental consequences of using peat in horticulture, and the industry is turning increasingly to sustainable raw materials.

This move also reflects increased societal awareness of sustainable production in general and more specifically the sustainable use of materials within horticulture itself.

Under the title of 'Project 4', a working group of industry stakeholders is attempting to balance a robust sustainability assessment with a pragmatic and cost effective approach that has been re-titled “responsibly sourced growing media”.

Whichever driver is pertinent, adapting to new growing media means revising the growers’ approach to managing plant quality. This includes management of water and nutrition and both remain a challenge.

Peat alternatives

Materials such as coir, green compost, composted pine bark and wood fibre are all widely used in growing media mixes. All have different properties in terms of water retention, water distribution and nutrient provision. When mixed together, as a product, this complexity is often compounded and chemical and physical properties can differ from mix to mix. Interestingly, this variability in properties could also offer potential benefits to growers, assuming growers can adapt their management practices with water use and nutrient savings potentially feasible, in turn improving the sustainability of production. 

Maintaining plant quality, while developing sustainable growing media, is clearly a key priority. 

Ongoing RHS research is examining water management in different mixes closely.

Current Projects

  • Agriculture and Horticulture Development Board - Sustainable resource use in horticulture: a systems approach to delivering high quality plants grown in sustainable substrates, with efficient water use and novel nutrient sources.
  • PhD Project: “Can we use soil microbes to help produce novel, sustainable growing media?” (Royal Holloway, 2013 – 2017)
  • PhD Project: “Developing non-peat growing media with microbial amendment” (Royal Holloway, 2017 – 2021)

Ongoing involvement in Defra projects:

  • Project 4 – Towards responsible sourcing and manufacture of growing media
  • Project 7 – Performance standard for amateur products

RHS Advice:

Peat free media


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