Suspect you really ought to be feeding some of your plants, but feel confused by all the different terminology and advice? Here we explain in simple terms what fertilisers are, why they are used, and how and when your garden might need them.
What is fertiliser?
Let’s start by establishing the difference between organic matter and fertiliser. The term ‘organic matter’ refers to well-rotted animal or plant material such as manure, garden compost, or commercially produced blends. Gardeners use these to improve the condition of their soil, either by digging them in or spreading them over the surface as a mulch. With the help of worms, bacteria, and a whole host of other busy organisms, the organic matter then integrates with the soil to help achieve that ideal dark, crumb-like texture which indicates the perfect balance of air, water, and nutrients.
The addition of organic matter is a vital part of creating and maintaining a healthy soil, and often all that is necessary for successful plant growth.
Sometimes, however, extra nutrients are needed. This might be when:
- Plants are grown specifically for showy flowers or plump fruit;
- Regular pruning and removal of growth interrupts the natural nutrient cycle (e.g., lawns and hedges);
- Particularly fast-growing plants demand more sustenance than the soil has to offer;
- Plants are struggling to fight off pests or diseases;
- Plants are grown in containers and therefore limited in their ability to seek nourishment.
Situations such as these call for fertilisers, which offer a controlled and targeted boost of the three key nutrients (referred to as NPK):
- N = nitrogen: important for green, leafy growth.
- P = phosphorous: boosts healthy root and shoot development.
- K = potassium: essential for the formation of flowers and fruit.
These nutrients can be found in varying ratios depending on what the fertiliser is intended to be used for. We will talk more about this in the ‘How to read fertiliser labels’ section below.
What are the benefits of using fertilisers?
Us gardeners ask an awful lot of our soil, often using it to support far more growth than would naturally be the case. We cram perennials in cheek by jowl and are constantly plunging and whipping out annuals and biennials. We plant fast-growing, nutrient-hungry fruit and vegetables, and mow, trim, and tidy incessantly, expecting plants to regrow without the natural cycle of their leaves being left to break down into the soil and nourish roots.
Fertilisers allow us to repay some of what we take away, and better equip nature to meet our demands. In doing so we can encourage high flower and crop yields, along with strong, robust growth which is less susceptible to pests and diseases.
Types of fertiliser
Organic and inorganic
The nutrients in fertilisers can be derived from organic or inorganic sources.
Organic fertilisers come from material that was once living, and include bonemeal, chicken manure pellets, seaweed extract, and fish, blood, and bone. They usually need to be broken down by the elements and soil organisms before their nutrients can be absorbed by plant roots. In giving the soil bacteria a role to play, their use encourages a healthy and active soil ecosystem.
Inorganic fertilisers are either synthetically manufactured or derived from minerals. They usually dissolve immediately on contact with water, making their nutrients instantly available to plants. The downside of this is that soil organisms are rendered redundant and so gradually disappear. It can therefore be said that long-term use of inorganic fertilisers has a harmful effect on soil health.
Different forms of fertiliser
Fertilisers are available as the following product types:
- Compound: containing more than one nutrient, usually as a balanced, general feed, or in a ratio targeted to the needs of a specific type of plant. Can be organic or inorganic.
- Straight: single nutrient only, generally used to correct a deficiency, or as part of a tailored feeding programme. Usually inorganic.
- Controlled release: granules coated with a porous material, the thickness of which determines the rate of nutrient release. Usually inorganic.
- Slow release: gradually broken down with the help of soil organisms. Usually organic.
How to choose the right type of fertiliser
Choosing from the many different types of fertiliser available can be daunting, even for the professionals. The Hedgers have created the following ‘at a glance’ reference table to make the process as quick and easy as possible.
|Product||What is it?||Effects||Recommended use|
|Bonemeal||A powdered, slow release, phoshorous-rich byproduct of the farming industry.||Encourages healthy root growth.||Useful for mixing into planting holes. |
Less effective on alkaline soils.
|Chicken manure pellets||A dried and pelleted, slow release form of nitrogen-rich poultry waste.||Aids lush and healthy green growth.||Good for leafy edibles and foliage plants. |
Has a slightly alkaline pH, so should be avoided on acid-loving plants.
|Liquid seaweed||A fast-acting natural extract containing high concentrations of the three main nutrients, plus smaller trace elements often lacking in other fertilisers. |
Seaweed has been used as a soil improver for millennia.
|Improves soil fertility and boosts overall plant performance.||Particularly useful for container-grown plants, though can be applied throughout the garden.|
|Blood, fish, and bone||A powdered, balanced fertiliser made from byproducts of the food industry.||Improves soil fertility and boosts overall plant performance.||An effective annual spring top-dress for established plants and lawns. |
Can also be used to give exhausted soil an extra boost after a season of hungry crops.
|Ericaceous feed||Can be organic (often derived from pine needles or bracken), or inorganic. Liquid, powdered, or granular.||Boosts overall plant performance while maintaining a low soil or compost pH.||Suitable for acid-loving plants such as blueberries, rhododendrons, and azaleas.|
|Specialist feeds (e.g., tomato, rose, houseplant, orchid, cacti)||Specially formulated to meet the needs of a particular type of plant.||Often focussed on encouraging flowers, fruits, foliage etc.||Can be useful, particularly on edibles, though beware of ‘over-specialising’. Many plants are fine with one of the more general options listed above.|
How to read fertiliser labels
All fertilisers should have an NPK number displayed on their label. Once you have grasped the basic concept of this, it can be a useful tool in deciding which one to buy and how to use it.
The NPK number shows the percentages of nitrogen (N), phosphorous (P), and potassium (K) within the fertiliser, and usually takes the form of three numbers separated by dashes.
When these are roughly the same (e.g., 20-20-20), it can be assumed the fertiliser is balanced and therefore general purpose. When one of the numbers is significantly higher, you know it has a more specific use. An example is tomato feed – its NPK rating of 4-3-8, tells you it is high in potassium, and therefore used to encourage flowers and fruit.
How to use fertilisers
There are several different ways you can apply fertiliser:
- Top dressing is when powdered, granular, or pelleted fertilisers are sprinkled onto the surface of the soil around established plants. This is usually done in spring before growth starts, and often again in midsummer for a boost. Lightly hoe or rake in, and in the absence of rain, water with a hose or watering can. A finishing mulch of organic matter helps to lock in the moisture needed for fertilisers to work effectively.
- Base dressing is the mixing of powdered or granular fertiliser into the soil or potting compost when planting (or sowing).
- Watering is how liquid and soluble feeds are applied and is done using a watering can or special hose pipe attachment. This method provides instantly available nutrients and is commonly used for container plants and bedding.
- Foliar feeding is the least common approach, though can be useful as an emergency measure for ailing plants, or to quickly get nutrients into plants when the soil is very dry. It involves the spraying or misting of a dilute solution directly onto leaves.
When to use fertilisers
Granular, powdered and pelleted fertilisers for top dressing
Apply just before plants come into active growth in early spring (so late March). Another application in June will maintain strong growth throughout summer.
Granular and powdered fertilisers for base dressing
Mix through planting holes or in potting compost when planting or sowing in autumn or spring. Reapply to container-grown plants with each repotting.
This type of fertiliser is fast-acting but shorter lasting, so will need to be reapplied every week or two during the growing season.
We have also compiled a handy quick reference guide for key plants in the garden:
|Type of plant||What kind of fertiliser to use||When to apply|
|Lawn||Specialist spring lawn feed, or blood, fish, and bone|
Specialist autumn lawn feed
|Roses||Specialist rose feed, or any high potassium feed||March, then again in June/ July|
|Shrubs and hedges||Young shrubs and hedges, or those in need of a boost can benefit from a once-a-year application of general balanced feed. |
Otherwise, an annual mulch should suffice.
|Container plants||General purpose liquid feed||Every one to two weeks from May to September|
|Fruit trees||High potassium feed, or specialist rose feed||March, then again in June/ July|
We hope this has helped make sense of fertilisers, and you now feel able to browse our Plant Feed and Fertiliser page with knowledge and confidence!
Remember to keep an eye on our regularly updated Hedges Direct blog for more helpful guides like this.