The foundation of potting soil often consists of peat, supplemented with a mixture of various natural materials. Photo: WUR
Plants thrive in the potting soil that’s available from any gardening store, but the peat used in this mix is bad for the environment. That’s why the Greenhouse Horticulture Business Unit of Wageningen University & Research, in collaboration with potting soil companies, is looking for sustainable alternatives. Biochar, a type of charcoal, could be a likely candidate. But is replacing peat with biochar really that easy?
Potting soil is a mix of various natural materials, with peat as a common base ingredient. This base is then mixed with other natural materials, like tree bark, shredded coconut, sand, compost, manure, perlite or lime. Since peat is a base ingredient, it is used in significant quantities, especially in the Netherlands. ‘Consumers and growers use about 5 million m³ peat per year to grow their plants’, says Chris Blok, researcher of plant nutrition and rooting media at the Greenhouse Horticulture Business Unit of Wageningen University & Research. The peat used for potting soil is imported from countries like Ireland, Germany, Estonia, Livonia, Lithuania, Sweden and Finland. Digging up peat layers results in the decomposition of the harvested peat over many decades after the fact. The harvested peat thus releases large quantities of CO₂ when the plant remains in the peat decompose. This released CO₂ contributes to climate change. Blok: ‘This highlights the need to replace peat in potting soil.’
'We need alternatives for peat in potting soil'
WUR is working with potting soil companies to find alternative materials to replace peat in the mixtures. Photo: WUR
Strict requirements for peat alternatives
Existing alternatives, like coco peat, cannot fully match the demand for peat. There is simply too little of them, Blok says. He and his colleagues are working together with potting soil companies to find other candidates to replace peat in potting soil mixes. ‘Finding alternatives for peat in potting soil is a difficult process, since the material is subject to a wide range of requirements. We are considering 20 properties in detail, but we know there are many other characteristics that might come into play. The presence of heavy metals, for instance, or plant, animal or human diseases, pesticides or the water retention qualities of the soil’, Blok lists. Based on scientific literature, Blok compiled an extensive list of about 130 materials. The majority of the items on the list were materials that couldn’t replace peat on a large scale or had other obvious disadvantages. ‘In the past, research was conducted into materials like plastic and car tyres, as well as ash from general waste incineration, which is slightly radioactive, and the bark of ferns, which would result in selective cutting the species into extinction in its jungle environment.’ Due to their serious disadvantages, Blok eliminated these options and selected others for which he considered a production scope of at least 100,000 m³ feasible. ‘New materials like these need to be processed before they are suitable for use in potting soil. That requires serious factory adaptations, which means large quantities are required to make this interesting from a financial point of view.
Every year, consumers and growers use 5 million m3 of potting soil. To make this more sustainable, we need an alternative for peat, the base ingredient of potting soil. Researcher of plant nutrition and rooting media Chris Blok of Wageningen University & Research is testing biochar, a kind of charcoal. Can biochar claim its place in the potting soil market?
The optimal recipe for biochar in potting soil
One of the materials Blok tested was biochar, a lightweight material consisting of carbon and ash. Biochar is already produced in various factories, just not for making potting soil. The wood used for biochar production is derived from factories that manufacture planks and furniture. If this is briefly heated at temperatures over 500 degrees in anaerobic circumstances, you are left with a very fine material. Part of this is currently used in the production of steel. This leaves a lot of biochar, most of which is not suitable for use in potting soil. Blok: ‘Sometimes, the material is so fine that it forms dust. On top of that, the heating process can also result in a material that’s poisonous to plants. These are some examples of types of biochar that we can’t use for potting soil.’ That’s why Blok and his colleagues first mapped out the requirements that the production of biochar for potting soil should meet. This way, they want to mitigate the current disadvantages, like dust formation, and improve the price-performance ratio to the point where biochar becomes an attractive alternative for peat. Moreover, the composition of the added fertilisers in the potting soil required adjustment. ‘In the past, the sector used a fixed composition of fertilisers, but now, we need to reassess this for each individual material.’ The biochar project has resulted in composition models that manufacturers can use to calculate the optimal component combinations based on the ideal properties of the end product.
'With biochar, there are still concerns about the safety, price and viability'
Left: The sandy soil is enriched with biochar, which is mixed with moist compost to prevent dust. Photo: WUR
Right: Growing blocks made from compressed hemp fibers are used for growing lettuce and herbs. Photo: WUR
Blok’s research shows that biochar is a suitable alternative for peat in potting soil. However, this does not mean that this ingredient will soon become a staple of potting soil in garden centres or in the operations of growers. The market proves slow in picking up the new alternative, according to Blok. ‘With biochar, that last step isn’t a given. There are still concerns about the safety, price and viability.’ To tackle these issues, follow-up research is called for. Blok envisions growing trials in collaboration with biochar and potting soil producers. Those trials should show what measures are needed to enthuse the market.
Multiple alternatives needed to replace peat
Meanwhile, Blok and his colleagues are continuing their search for a variety of alternatives that, combined, may replace peat in the potting soil market. An interesting development occurred at Den Ouden Organic, a partner in the biochar subproject. This company conducted large-scale experiments with wood fibre. Blok: ‘Composting this material in combination with plant remains results in high-grade potting soil. This idea was immediately picked up by the market, and other companies were enthusiastic. That is a wonderful development. We need multiple alternatives in order to replace peat in potting soil. Each alternative helps, if it meets the various requirements and can be produced on a large scale and at an affordable price. We will continue testing various materials, including biochar.’
Share this article