Controlled-Release Fertilisers – Which?

Decent fertilizers with controlled release will keep your plants fed for months, but bad fertilizers will prevent them from working. Our expert tests have found the best options for you.

Controlled discharge fertilizer is a perfect maintenance option when feeding containers when you add the grain to the compost when you plant.

Read on to find out which controlled release fertilizer performed beyond our test cycle. We also explain more about the difference between controlled release and irrigation.

Best fertilizer with comparative release

To help us in our task of finding the best controlled release fertilizer for your garden, we used fertilizer to feed pots of geraniums and potatoes. We mixed the fertilizer in the compost before planting the pots.

Our results table below shows how well our plants grew and flourished according to the feed they were given.

Which? Members can Login Now to open the results in our table. Not a member yet? Who should participate? to discover the best fertilizer by comparative release.

Table comments

  1. Based on suggested retail price for 1 kg package or size closest to the size sold.
  2. Ignores the price and is based on geraniums (50%) and potatoes (50%).

What is a fertilizer with a comparative release?

Usually in the form of small spherical granules, the fertilizer dissolves with controlled release when the compost in your pots is moist and warm enough for plants to grow. This fertilizer is as effective as liquid feed to keep your plants healthy, but is much easier to use as it only needs to be applied once a season.

Mix them into a compost when potting young plants in the spring. Read the instructions carefully as you risk overfeeding or overfeeding your plants and guess how much to add.

Some products are also known as slow release fertilizers. Technically, there is a difference between slow release and controlled release fertilizer. Slow release fertilizers gradually release their nutrients over time, regardless of the situation.

Controlled release fertilizer works when the soil temperature and humidity are high enough, feeds plants when they need them in the warmer months and releases less nutrients in the colder months when growth slows down. However, many products are still referred to as slow release as it is the term most commonly understood.

Want to create your own compost? Look at us the best compost

Controlled discharge against liquids

In recent years, we have tested Best Buy liquid feed against Best Buy controlled release fertilizers. We found that the floating plants grew larger but had fewer flowers – most of the growth was in the leaves.

The best results in the summer, on the other hand, were obtained from pots that were initially fertilized with controlled release and then filled with liquid feed when they stopped producing healthy growth. We recommend using controlled release feed, but provide additional liquid feed if your plants show signs of degradation.

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How we test fertilizers with comparative emissions

For our last test run, we selected 10 controlled release fertilizers designed to feed plants throughout the summer. Our shortlist included products from well-known brands such as Chempak, Garden Direct, Osmocote and Thompson & Morgan.

Eight of these controlled release fertilizers were loose granules and two tablets were extracted from the granules. To distinguish between using poor fertilizers and not using them, we cultivated plant sets with no fertilizer as a control. We mixed each feed together in a gravel-free Best Buy patio pot of compost according to the instructions in the package.

Each fertilizer was used to feed five 10-liter pots of geraniums (pelargoniums) and five pots of potatoes. We also planted five pots of geranium and five potatoes that we did not feed at all. We examined the geraniums three times in terms of leaf color, number and size of flowers, and plant size. When we cut the potatoes, we looked for tubers of sufficient size with smooth skin and no scabs.

Towards the end of our experiment, we collected compost samples from one of our most powerful pots and also from pots that did poorly. These were submitted for nutritional analysis to determine why the plants in the inactive pots were so unhealthy.

As our compost tests continue to show, the most expensive options are not always the most effective.

To see what compost we recommend, check out our chart the best compost