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  3. The Definitive Guide to Fertiliser for Plants

The Definitive Guide to Fertiliser for Plants

Fertilisers are used to improve the plant growth and strength. Cultivated plants are not able to survive on water alone, therefore many garden plants become weak in growth, have poor colourisation, and often lack flowers or fruits. But given fertiliser, they can grow how they are meant to be and have a better chance of withstanding attacks from pests and diseases. It is vital that container grown plants receive fertiliser, as moisture and food are more likely to run out of the soil quicker.

What is a fertiliser?

Fertilisers are concentrated sources of plant nutrients, usually supplied in pellets, granules, powders, and liquids. They all contain a balance of the three main nutrients plants need to grow:

– Nitrogen (N) to help promote healthy leaf and shoot growth

– Phosphorus (P) to help with root development

– Potassium (K) to encourage flower and fruit production

Fertilisers can also contain traces of other elements like magnesium and iron as well.

As reference by the RHS “All fertilisers should quote their N:P:K ratio on the product packaging. For example, a ratio of 20:20:20 indicates a balanced fertiliser (in this instance Chempak formula 3), but a ratio of 10:12:24 would indicate a high potassium fertiliser (in this case Chempak Rose Food). However the N:P:K can be misleading if they refer to phosphates and potash, rather than phosphorus and potassium, see our advice in fertiliser labels explained for more information.”

Nitrogen, Phosphorus and Potassium are the main elements in fertiliser

Why use a fertiliser?

If you have noticed your plants showing signs of nutrient deficiency, yellowing leaves or decolourisation in varying patterns for different nutrients, then you would benefit with using a fertiliser. Along with nutrient deficiency, any fast-growing plant would benefit from fertiliser application as they are likely using more nutrients than other plants.

If you have healthy soil, then it may not be necessary to use fertiliser but using them can promote a brassier display of flowers or increase the number of edible crops products.

Types of fertiliser:

There are two main types of fertilisers: man-made and organic (made from plant or animal). 

Man-made fertilisers

More concentrated and faster acting than organic fertilisers, man-made fertilisers are made of synthetic, artificial forms of plant nutrients.

Organic fertilisers

Made from plant or animal sources, these fertilisers are slower acting due to the time it takes for the large organic molecules to be broken down by the soil before the nutrients can be released for the plant.

Inorganic and organic fertiliser can be found as:

Compound fertilisers

Containing a mixture of different nutrients, these may be organic or inorganic fertilisers or contain a balance of both.

Controlled release fertilisers

This works by water entering the granule, and the fertilisers seep out into the surrounding soil. The warmer the soil, the faster the release. By varying the thickness of the coating around the granules, they can be designed to feed plants for different periods of time.

Slow-release fertilisers

These organic, often bone meal, fertilisers degrade slowly under the influence of soil micro-organisms. The degrading process is dependent on the soil temperature.

Straight fertilisers

Containing one nutrient, these man-made fertilisers are ideal for providing different nutrients at different times of the year.

How to use fertilisers

There are various ways to apply fertilisers, including: top dressing, base dressing and watering on. The way to fertilise your plants will mainly depend on the product you are using. Here is how all of these methods work:

Top dressing:

Top dressing is when you apply a quick-acting fertiliser to the soil surface around a plant. This stimulates the plants growth and is usually applied in spring at the start of the growing season.

Base dressing:

This is when a fertiliser is mixed into the soil or potting compost before planting.

Watering on:

These are fertilisers (liquid, powder or granules) that can be diluted with water and watered onto the plant roots at the beginning of the growing season. The watering on method is commonly used for greenhouse crop, potted plants and bedding.

Feeding around the garden

(lawn, acid-loving plants, roses, established plants in ground, container grown plants (annuals/bedding, long-term plants in pots, houseplants) fruit and veg)

How to make your own fertiliser


When should you fertilise a plant?

There is no strict rule on when to fertilise your plants, so long as you do not over fertilise a plant. Some gardeners like to fertilise their plants every 2-3 weeks during growing season, others only apply fertiliser twice over the growing season. We recommend following your fertilisers instructions as each fertiliser is different.

Should you water plants before fertilising?

Don’t apply fertiliser to dry soils, the soil should be moist before adding the fertiliser.

When should you not apply fertiliser?

Fertilising plants at the end of the growing season is the worst time. “We recommend starting to fertilise your plants in spring and finish fertilising once their growing season has ended” says Chris Bonnett, gardening expert for the Express

Updated on August 21, 2023

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