Most potting soils contain a large amount of peat. But clearly a turning point has been reached. More and more often growers, customers and consumers are asking for peat-free mixtures. The reasons are the large amount of CO2 that is released during the excavation of peat and the loss of ancient wetlands. This was one of the reasons to start searching for new raw materials for potting soils. A lot of experience has already been gained with coco peat, and biochar, which appears to offer added value, has the potential to become a key ingredient.
Various links in the chain are participating in the search for new raw materials for potting soil. Product manager Karel de Bruijn, of manufacturer Van der Knaap of Kwintsheul, the Netherlands, is one of those closely involved. Another is Chris Blok, project manager root media and plant nutrition of Wageningen UR Greenhouse Horticulture. Separately, both specialists give their views on the developments.
Coco peat is naturally airy
Van der Knaap has been using coco peat in its substrates since the 1990s. It quickly proved to be a good alternative to peat as an organic and recyclable product with many possibilities for each crop. De Bruijn: “Now many of our mixtures consist of forty per cent coco peat and I certainly expect that share to increase. When the English ‘peat-free’ lobby gets its way, only plants that have been raised on a peat-free substrate will be able to enter the country from 2020. Major customers and large retail stores play with the idea of limiting the use of peat. In itself this is not a problem: Coco peat is as a sustainable and workable alternative. It is naturally airy and absorbs water easily.”
Absolutely everything from the coconut is used. The coconut milk and the flesh find their way into the food and pharmaceutical industry, the hard casing of the inner nut is burned without oxygen to form carbon for chemical applications and the outer husk is excellent starting material for substrate slabs and/or mixtures.
Different cultivation strategy
The company’s R&D department is examining the characteristics and possible applications of this product. “Coco peat behaves as a material that can be easily rewet how ever much we let it dry out; it always takes up water again and in doing so regains its original physical properties. For the fine substrates we use coir, a by-product from the production of fibre from the outer husk. The husk is cut up for the coarser substrates.”
The company has its own production sites in Sri Lanka, the Dominican Republic and India and therefore has tight control over the quality of the coco peat substrate and coconut products. “When it arrives we test the quality again in our laboratory. We treat the coco peat here as well.”
Coco peat has a naturally lower CEC-complex than peat. In peat this complex takes up H+ ions, contrary to coco peat in which the complex takes up potassium and sodium. During processing the potassium and sodium ions are exchanged for calcium. The chemical characteristics of coco peat are therefore completely different to those of peat and therefore cultivation with this potting soil mixture requires another strategy. De Bruijn: “In terms of fertilisation generally you need to have higher levels of calcium and iron and a lower level of potassium.”
Each plant has its own needs and every grower has his own method of cultivation. Therefore a specific mix is created for each client that best suits his cultivation method. “In addition, we develop complete cultivation concepts. By growing crops in our trial greenhouse on coco peat mixtures we explore experimentally the best fertilisation and watering strategies. The grower has to arm himself with new ‘coco peat skills’ and we help by providing specially customised pre-fertilisation and cultivation advise. For that we monitor growth and yield of crops in the trial department. We also take soil samples to follow the ratio of the elements and if necessary to steer them.”
Good results have been achieved for example with dracaena and pot roses. The final result for both crops on peat and coco peat was the same. “We did see a difference in growth phases, in coco peat for example the fertilisers are available sooner. And the shelf life of dracaena grown in coco peat increased. Last year we started a peat-free crop of pot anthuriums. In addition we are continuously testing the possibility of growing phalaenopsis on a one hundred per cent coco peat mixture. This could be a less attractive medium for Lyprauta larvae.”
Increase plant resilience
There are more alternatives according to Wageningen UR Greenhouse Horticulture: Wood-fibre and wood-pellets; compost; bio-foam; sugar cane chips; and rice chaff. These are all products, which just like coco peat can replace peat in potting soil mixtures. However, the suitability of the products does vary. According to researcher Chris Blok the ‘production process’ in particular has the largest impact on their usability.
One of the processes that can be controlled is pyrolysis. This process decomposes organic material by heating it to very high temperatures (600-900ºC) in the absence of oxygen. The end products are: energy (bio-oil), gas, tar and a large amount of a by-product called biochar. And thanks to the rise of the ‘Bio-based Economy’ pyrolysis is currently a hot topic. “It would be nice if the horticulture sector could also join in. Trade in biochar is already fully underway in Australia, for example.”
But the current method of producing biochar is unsuitable for Dutch pot plant production. That’s logical; the entire decomposition process is focused on the production of bio oil and not enriching the value of the by-product. “Therefore we now have a partnership with the Energy Centre Netherlands, Petten, where we are running pilots in which we can change factors that will make it advantageous to horticulture. In addition to shapeless, porous carbon, the material contains nutrients and has a high pH. We have to process the product before it is suitable as a substrate.”
Blok certainly believes in the potential of biochar as a growing medium. By-products of pyrolysis increase the resilience of plants to diseases such as Botrytis and mildew. They also bind nutrients and root fungi and most importantly, trap CO2 from the air. Biochar also seems to be a good product for the establishment of specific microorganisms. “We don’t know exactly how all the properties work. We are investigating that now. We have already seen good production results from the gasification of materials at low temperatures. This is called torrefaction. You have to operate the process very well otherwise toxic substances are produced.”
The low temperature gasification of rice chaff, for example, is already used to a great extent for horticultural crops in South-East Asia. “This is knowledge we can use here. I am experiencing good results with biochar from wood pellets. In contrast biochar produced from horticultural waste is, after gasification, a very alkaline product that reacts very powerfully. Therefore the search is still on-going.”
Compost is also a good alternative to peat in potting soil. Just like biochar it is important that the processing is steered to obtain the desired horticultural characteristics. “Together with Attero and Pokon Naturado we are researching the possibilities of making further adjustments with compost. Current techniques include covering it against rain, blowing oxygen through it and sieving. New techniques are hydrolysis, digestion composting, washing and other knowledge-intensive steps.”
Treatment results in a ‘new’ compost that has a lower EC, is more stable and requires a lower pH buffering. “Currently at our trial location we are using pot begonias to investigate how far we can go with blending this compost. Actually the ongoing research is much wider than only compost and we are also researching combinations with other alternatives such as wood fibre, coco peat and biofoam.”
The torrefaction research has been running since 2005 and the joint compost testing was launched more than three years ago. Even so Blok reckons it will be another five years before any ingredients can be used in commercial mainstream potting soil products.
As a replacement for peat, the amount of coco peat in potting soil mixtures can be increased. Coco peat, however, has different chemical properties to peat and therefore cultivation methods need to be adapted. Potting soil producers treat it to make it as horticulturally friendly as possible. The reseachers keep to the same strategy. They firstly identify the needs of the crop and then incorporate these conditions into the production process. Compost and biochar, a residual product of the oxygen-free combustion of natural materials, can in time partly replace some of the peat used.
Text: Jojanneke Rodenburg. Photos: Studio G.J. Vlekke