others include bark, sand, rockwool, perlite, vermiculite and
coconut, while a wide range based on recycled botanical waste is also available. But despite the ever-changing list of ingredients, the criteria for choosing a growing medium remain the same.
Clearly, the medium must suit the species under production and the cultural techniques being used, including the system of irrigation. It needs to be non-phytotoxic and pose no health and safety risk to the user. But attention must also be paid to the relative proportions of the ingredients.
Evaluating the chemical balance
The ratio of pore space to material is crucial because this will affect the medium’s air content and the ability of the plant to take up water and nutrients and the roots to penetrate through the medium. The medium must also have the correct chemical balance — not too rich for seedlings and cuttings, but sufficient for longer-term growth.
It is essential to check the pH of the growing medium and to test its re-wetting ability. You must also ask for, and understand, the nutrient analysis of any given growing medium — particularly with regard to the concentrations and forms of nutrients present. Check whether any are likely to cause harm to the plants you intend to grow.
Most manufacturers will incorporate specific fertiliser products on request; the best type being determined, at least in part, by the length of time the plants are expected to remain on the nursery. Don’t forget that a longer-term fertiliser may be required for plants destined for the retail shelf.
The substrate should be free of pests, diseases and weed seedlings. Depending on the crop, where it is to be grown and the time of production, it may be a good idea to have pesticides incorporated into the growing medium.
You are in business to make a profit, so the price has to be right, but with margins ever-more slim it can be difficult to source a product that will guarantee a decent return.
It also needs to be available in the qualities you need, and at the time you require. Always bear in mind that different growing media will have different storage properties and will react differently to being handled. Growing media should be matched to any mechanical processes on the nursery, such as tray filling and potting.
It is essential you investigate the quality of the product and its consistency. If you are ordering ready-mixed composts, they should be supplied to a known and guaranteed standard and the product should be consistent, with no variability between batches or from one month to the next.
It is equally important that you are able to take delivery of the product when you need it. With sufficient cash flow, and suitable storage facilities, you can buy enough to last several months but, increasingly, nurserymen are looking for “just in time” deliveries. It’s important to remember that most growing media are best when fresh — shortly after purchase or mixing.
Thought should also be given to the method of delivery. If you do not have the equipment to handle large bales then take smaller bales, bags or bulk loads.
Partly because of price, but also for the sake of convenience, the trend today is to buy a reduced number of mixes — perhaps one for liners, one for finished plants and one for ericaceous subjects — and literally make the plant fit that environment within the limitations of the product. But although it is becoming less popular, there are advantages to mixing your own composts on nursery. It will require handling and mixing equipment but it does mean you can mix the quantity required and grow the plants in the ideal environment. You can tweak nutrient content and pH to precisely match the growing medium to the crop.
Confidence in green composts
The establishment of the PAS100 standard, plus its amendment for growing media, means growers can have confidence in green compost. Weed seeds, conductivity levels, contaminants etc are covered and chemical constituents measured. E. coli and Salmonella are tested and checked. And while there will be variations in green compost, it is likely to be no more than for peat. It also has the advantage that the nutrient content is high, especially magnesium and iron. Having a full physical and chemical analysis of the product allows you to decide how to amend the product and which nutrients you can cut back on.
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