If you are growing crops under contract for specific customers, notably supermarkets and retail chains, it may be that the customer has already specified not only the growing medium but also the pot in which the plant is to be produced. If you have a free hand in selecting the growing medium, then you will need to consider several elements.
Obviously, you must choose a medium that is suited to the species under production and to the cultural techniques, including the system of irrigation. But you also need to look at what goes into a growing medium in terms of ingredients, their relative proportions and also at the service offered by the supplier — in other words the quality of the product, availability and timely deliveries.
A wealth of choice
At one time most nurserymen would have used loam-based products. Today the choice is huge. As well as there being products still based on peat, we have others that include bark, sand, rockwool, perlite, vermiculite and coconut fibre. And then there is the vast choice of products based on recycled botanical waste. However, the main requirements of a growing medium remain the same.
Growing media must be cheap and available in the quantities that you need, as well as in a form that is easy to transport, handle and store. In addition, any growing medium must be non-phytotoxic and pose no health and safety risks to users.
Down to the specifics: attention should be paid to the ratio of pore space to material for the holding of air and water, and for the uptake of water and nutrients by the plants. The growing medium must also be of the right chemical balance — not too rich for seedlings and cuttings, but sufficient for longer-term growth.
It is very important to understand the nutrient analysis regarding the concentrations and forms of the nutrients present in the medium. Check whether any of them are likely cause harm to the plants you intend to grow. Many growers request that the supplier incorporate specific fertiliser products.
The length of time the plants are to be on the nursery will determine which fertilisers are suitable and the most appropriate release patterns. Don’t forget that a longer-term fertiliser may be needed if the plants are destined for the retail shelves. It is also essential to check the pH levels and test the medium for re-wetting ability.
Depending on the crop, where it is to be grown and the time of production, it may be best to have a pesticide incorporated into the growing medium. The substrate should be guaranteed to be free of pests, diseases and weed seedlings.
In terms of physical properties, the medium needs to allow the roots easy penetration while at the same time ensuring sufficient stability for the plant. It also needs to be suitable for the method of irrigation used on the nursery and for any mechanised process, such as tray filling and potting.
Product consistency is another vital factor. Ready-mixed composts should always be supplied to a known and guaranteed standard. There should be little or no variability from one batch to another and from one month to the next.
Delivery is the other issue that must be absolutely right. Some growers, with suitable storage facilities and a sufficient cash flow, will order enough growing media to last them several months. Increasingly, however, nurserymen are seeking “just-in-time deliveries” and will opt for the supplier that promises to get the product there when needed.
It is also worth looking at delivery methods. If you are not able to handle big bales, then you will have to take smaller bales or buy bulk loads. Always consider how the product will be handled when it arrives at the nursery and remember that most growing media are likely to be best when fresh, shortly after purchase or mixing.
In recent years, the quality, consistency and delivery of growing media have improved. Many suppliers also offer technical advice and will assist in the formulation and mixing of prescription mixes to suit the needs of specific crops.
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