Q. Who benefits from the use of a controlled-release fertiliser (CRF) and how?
A. Putting a CRF into a growing medium is a bit like giving the plant the key to the larder. Once the CRF is activated it will last, approximately, for as long as it states on the bag. From the grower's point of view it increases efficiency and boosts profitability by reducing the number of applications that might be required throughout the plant's life on the nursery. Indeed, for some plants and some production systems it may be the only application that is needed. CRFs also reduce storage requirements and there is less packaging for disposal.
An additional advantage to the grower is the nutrient securing offered by CRFs. Outdoor plants can be subjected to heavy rainfall in the UK, causing leaching and making the application of liquid feeds difficult. With a CRF, nutrients continue to be released at a controlled rate.
With an appropriate longevity of CRF, plants can be dispatched from the nursery in the knowledge that they are taking food with them. The grower need not worry about the plant going hungry at the garden centre - and the garden centre manager need not be concerned about feeding the plants. With sufficient longevity, the CRF will see the plant safely into the hands of the consumer, who may then still benefit from the plant continuing to feed on the reserves.
For products such as planted hanging baskets and troughs, the CRF provides a win-win situation. The consumer, perhaps a local authority putting on summer displays, can rest assured that the plants have access to fertiliser that is delivered safely over a long period of time.
Q. Are there any disadvantages to the application of CRFs?
A. Care needs to be taken in selecting the right CRF. Once it has been mixed into a growing medium, it is difficult to take out again. While a shortfall of nutrients can be made up for with liquid feeds, too much can result in over growth and possibly even damage.
Q. What is the mechanism behind CRFs and what gives them their longevity?
A. CRFs are coated. The coating and the way in which it works are likely to differ from brand to brand and perhaps even within brands. There may be more than one coating on a product but inside there will be a granule containing the nutrients - usually NPK plus trace elements. The trace elements may form a coating over the NPK but then the whole granule is wrapped in a semi-permeable membrane through which water passes by the process of osmosis.
As the water is absorbed by the granule, so the nutrient content is dissolved. As more water enters the granule, pressure builds up until the dissolved nutrients are forced out.
It is a combination of the permeability of the coating plus temperature that controls longevity. Like all reactions, there is more activity at higher temperatures. When the outdoor temperature is low, as in early spring, the molecules are less energetic and they respond slowly. As the temperature increases, molecular activity increases and nutrient release is accelerated.
Effective release temperature is around 5 degsC upwards and you can expect roughly a five per cent increase in response per degree increase in temperature - but products will vary.
To give a level playing field and allow product comparison, manufacturers usually state release at 21 degsC. This is not meant to reflect growing conditions.
Q. Can you offer any guidelines to help choose the best CRF for my crops?
A. You should first choose the most suitable longevity. If you are growing a short-term herbaceous perennial, then you might only need a fiveor six-month product. But if you are growing long-term nursery stock subjects, you might require a 12- or 18-month CRF to suit that cultivation period.
Next you should look at the analysis and decide what is most suitable. Woody plants, for example, are likely to need a high N, so a typical analysis might be 16-8-12. For flowering plants you are likely to want something with more potassium and less nitrogen. In this example a typical analysis might be 15-6-16 or 14-3-19. Of course, it is vital that the product is used at the correct rates for your crops.
Finally, select a reliable product with proven performance to ensure the timely delivery of nutrients and always try to keep up to date with trials and research.
Download table of supplier and product information for controlled-release fertilisers