Before we discuss slow release fertilisers, let us first understand what is a fertiliser. A fertiliser is a plant food of a natural or synthetic origin that is used to enhance the growth of plants. To be classified as a fertiliser, plant foods need to contain one or more of the following elements (or nutrients): nitrogen (N), phosphorus (P) or potassium (K). These elements exist in a compound form, for e.g. N might be present as urea, P might be present as monoammonium phosphate and K might be present as potassium nitrate.
There are many types of fertilisers and these can be bio-based, mineral-based and or a combination. These are further classified as fast release, slow release or controlled release.
The most traditional type is an NPK fertiliser where the N, P and K are present in varying ratios and these are typically made from inorganic sources. Being inorganic, the compound forms of these elements are readily available making them fast release fertilisers.
We will now dive into Slow Release Fertilisers to understand them a little better.
What is a slow release fertiliser?
A slow release fertiliser is a fertiliser that releases its nutrients gradually over time. These fertilisers can be bio-based whereby the microbes in the soil break down the bio-based fertiliser into a form that the plant can absorb. This process takes time and depends on a number of factors including the richness of the microflora in the soil, temperature, humidity and others. A slow release fertiliser can also be inorganic in nature. For example, plants absorb nitrogen in the form of ammonium ions and nitrate ions. Providing nitrogen nutrients in a form that is more complex than that such as coated urea is considered slow release. The dissolution of the coating and the transformation of urea to a useful source of nitrogen depends to a large extent on the temperature with higher temperatures increasing the rate of reaction.
What are the benefits of using this kind of fertiliser?
Inherent to slow release fertilisers are their ability to release nutrients gradually over time. This reduces the leaching of nutrients into groundwater and nutrient run-off reducing the impact fertilisers have on eutrophication. At the same time, more of nutrients are available to the plants as they grow, reducing waste.
When is it recommended to use a slow release fertiliser?
Just like any other fertiliser, you can use a slow release fertiliser when your plants require a feed. This can be any time of the year and will depend on the type of plant. Generally speaking, a few months before summer is a good time to apply fertiliser to get your garden ready for the warmer summer months.
What is the best way to use it?
This depends on whether the slow release fertiliser is solid or liquid. Follow the instructions on the product to get the dose right and make sure you regularly apply as per the instructions so your plants don’t go hungry.
And remember to give your plants a good dose of water, especially during the warmer months.
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