Sustainable peat-free mixes for poinsettia by Neil Bragg
UK based soil scientist, Neil Bragg, shares his experience of working with peat-free substrates, in an article commissioned by the poinsettia specialists at Stars for Europe.
Around the 1930’s most nurseries had their own mixes for potting soils for container plants. The uniformity of the mixes was very poor and hence the results in terms of quality growth of plants reflected this. In the UK two scientists who were working at what became the John Innes Institute were asked to produce a uniform seeding and potting mix which allowed all the progeny of the breeding programs the opportunity to at least grow and display their characteristics. This led to the now famous ‘John Innes – seeding and potting mixes’ which were based on a sterilised loam, peat and sand mix with base fertilisers at the appropriate levels. These basic mixes became a standard to work with and later were added to, to provide increased levels of fertiliser for potting-on longer term crops.
The next big change in terms of potting soils came in the 1950/60’s with the introduction of all peat mixes – this meant all sorts of changes to irrigation regimes and fertiliser additions to mixes and in subsequent feeding. Plus, the challenge of interpretation of analytical results. The mixes were very much lower density and the ability to re-wet was often challenged compared to mixes containing soil and sand. There were variations in mixes; from the all peat, peat sand and or grit (UC Davis system mixes) to keep the density and radically from Penn state university the addition of perlite to give the peat-lite mixes for pot plants. Variations based on the mixes of the 1960’s have been the main mixes for container plants right up to the 2000’s.
In the UK from the 1990’s onwards there has been increasing pressure from both environmental NGO’s and from Government departments to reduce and eventually eliminate the use of peat in Horticulture. There have been, similar moves in other European countries such as Germany and Holland. And some countries such as Switzerland maintain they are ‘peat–free’. The reality is that there are serious moves across Europe to reduce the reliance on peat use in horticulture. Some sectors of the industry such as the ‘Soft- fruit’ producers have, since the early 2000’s, started moving away from peat-based substrates to the use of mainly coir-based bags and filled troughs for growing the fruits and this has proved very successful. Tomatoes, Pepper and Cucumbers moved to semi-hydroponic systems many years ago. The main areas still slowly moving to peat free potting soils are the ornamental container grown plant and some module raised plants such as vegetable transplants. Additionally, in the mushroom industry peat is still the most widely used material as the ‘casing’ layer on the compost to allow the mushrooms a fruiting zone.
The obvious question which needs answering is why has peat been so successful and what are the real challenges in moving away from it? Peat is an accumulation of various mosses, such as Sphagnum, over many hundreds of years in water, which is mainly only increased by rainfall. The conditions, for example, very low oxygen levels within the bog or mire, prevent the normal decay of the plant material and in fact preserve the structures of the mosses unlike any normal composting process. The result is that when peat is extracted from the mires and dried on the surface – yes, there is a release of CO2 from the breakdown products of the process and the resultant plant remains look like the original plant material, but are in fact preserved/stabilised remains. The properties which have gone on in the mire make the plant remain very stable then to further degradation and impart a low pH on the material, with additionally the ability to adsorb and retain water, rather like a sponge. Any aerobically composted materials can never behave in a similar fashion, nor do they have a similar stability. So, peat is unlike all the current batches of organic materials which are being or are likely to be used in potting soils.
Currently the range of materials available for using as peat-free ingredients in potting soils are:
- Barks, both fresh and composted,
- Coir, the pith removed from the outer husk of coconuts
- Composted Green wastes, mainly from selected collections of woody and green wastes, currently not well controlled in the UK.
- Woodfibres, almost entirely from pine species wood chip, and processed to varying degrees, hammer milling to steam and pressure extruded.
- Other materials such as separated solids from specific anaerobic processes (AD) and biochars from various pyrolysis processes are being used in some niche mixes, but volumes and price of the ingredients often prevent wider adoption.
With the introduction of an increasing array of new materials there are issues which need addressing in order to achieve quality plant production. The obvious areas to be considered are; water uptake and retention, nitrogen immobilisation by the microbial populations the materials bring to the mix, and frequency of additional feeds to ensure quality performance.
For suppliers of growing media and for growers the challenges are not just about a simple change of one ingredient in mixes but rather the adoption of often potentially quite complicated mixes which introduce a range of issues. The first consideration is that the crop cannot be assumed to grow well using the same watering and feeding regime as both previously and currently applied to any peat dominated mixes used on the nursery.
- The new mixes must be treated separately in terms of irrigation levels and frequency.
- The fertiliser choice for both the base fertilisers and the supplementary feeding of the crop will need adjusting. There will be issues particularly relating to very high background levels of Potassium and sometimes Chloride and Sulphates in the new mixes. Additionally, the normal supply of Calcium and Magnesium, which previously came from the limestone used to modify the natural acidity of the peat will no longer be in new mixes so alternative supplies of the elements are needed, various fertiliser companies recognise this and are working on new compound base fertilisers specifically for peat-free mixes.
- We must assume that the new organic constituents used in mixes will adsorb nitrogen to satisfy the inherent microbial populations that come with them. This can often be worse when additional materials are mixed, as it is simply not a case of 1+1=2 microbially. So it is necessary to feed additional nitrogen as water soluble feeds from the start of potting up until the quality of plants required is realised. This is generally achieved by the use of water soluble feeds for greenhouse crops but there may be the use of controlled-release fertilisers (CRF) and there has recently been interest in the use of organic type additional fertilisers.
In conclusion, we are all on a learning curve when using peat-free mixes. The best way to approach the subject is to take in mixes of one or two basic ingredients and to learn how to grow in them but to do so in a manner which allows the new mixes to be managed separately from any long-term existing peat-based mixes which have been used. The watering and fertiliser use will need adjusting and most important samples of the fresh growing media should be analysed. In the UK we have operated a ‘Poinsettia Monitoring Scheme’ for growers since 1998 and this has included a suite of analysis on a regular basis from the arrival of the new growing media, through to near sales of the final plants. The scheme usually starts with an analysis of the water to know particularly the alkalinity (bicarbonate content) of the water.
Interestingly, where in the past borehole and mains supplies of water were a problem with peat-based mixes for peat-free mixes such water supplies might well be very useful in terms of their Calcium and Magnesium supply. If, however pure rainwater is used as the irrigation supply then there will be a challenge to ensure the supply of Calcium and Magnesium are maintained to the plants, this can in part be satisfied by the use of Calcium Nitrate feeds but there is a need to consider specific water-soluble fertilisers with additional Calcium and Magnesium. The monitoring scheme also takes leaf samples every 2-3 weeks from pinching of the crop to ensure that the plants have received a full suite of the elements needed – with particularly attention paid to the leaf Phosphate level towards the end of August beginning of September as the flower bud development starts. The use of such a planned program of supporting analysis is vitally important as we move away from peat-based mixes to Peat free mixes.
Image credits: Stars for Europe: https://www.starsuniteeurope.eu/en/