Fertilisers are concentrated sources of plant nutrients, usually in compact form such as pellets, granules, powders or liquids. They are used to improve plant growth and yields.

A selection of fertilisers in a garden centre. Image: RHS/Tim Sandall
A selection of fertilisers in a garden centre. Image: RHS/Tim Sandall

Quick facts

  • There are a wide range of fertiliser available for home gardeners
  • Fertilisers can correct plant nutrient deficiencies
  • Fertilisers are not a substitute for good soil care using organic mulches and soil conditioners

Why use fertilisers?

Fertilisers are used to improve plant growth. The faster growing the plant, the more it will benefit from fertiliser application. If you have a healthy soil, it is often not necessary to use fertilisers, but using them may produce a showier display of blooms or a higher yield of produce from edible crops.

Fertilisers are also used where plants are showing signs of nutrient deficiency, usually shown by leaf yellowing or discolouration in varying patterns for different nutrients.

Remember that healthy soil structure and

pH are just as important as fertiliser application in the prevention of plant nutrient deficiencies. Soil conditioners such as manure and compost help the soil to form into crumbs with spaces for air and water between them, making nutrients, water and air all more available to plant roots. Lime is added to remedy acidity.

What are fertilisers?

Fertilisers contain concentrated sources of plant nutrients in chemical or organic form. Most contain major plant nutrients, which plants need in relatively large amounts. Some also contain trace elements, which plants only need in tiny amounts.

Most fertilisers are based on the three major plant nutrients:
Nitrogen (N): For green leafy growth
Phosphorus (P): For healthy root and shoot growth
Potassium (K): For flowering, fruiting and general hardiness

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.

Product choice

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

Inorganic fertilisers: These are synthetic, artificial forms of plant nutrients or naturally occurring mined minerals. Inorganic fertilisers are usually more concentrated and faster acting than organic fertilisers. Examples of inorganic fertilisers include: Growmore, Miracle-Gro All Purpose Soluble Plant Food, Phostrogen All Purpose Plant Food, Sulphate of Ammonia, Sulphate of Potash, and Superphosphate and Tomorite Concentrated Tomato Food.

Organic fertilisers: These are derived from plant or animal sources and contain plant nutrients in organic form. Organic products tend to be slower acting, as large organic molecules have to be broken down by soil organisms before the nutrients within them are released for plant use. Examples of organic fertilisers include: seaweed, hoof & horn, dried blood, fish blood & bone, bone meal, poultry manure pellets and liquid comfrey or nettle feeds. Other widely available products include Miracle-Gro Performance Organics All Purpose Concentrated Liquid Plant Food and Levington Tomorite Organic Concentrate.

Inorganic and organic fertiliser can be found as the following types of products:

Compound fertilisers: These contain a mixture of different nutrients, and may be balanced (containing similar proportions of all the major plant nutrients) or may supply more of some nutrients than others, as per the requirements of different crops. They may be organic or inorganic, or contain both.

Straight fertilisers: These contain only one or mainly one nutrient. They are usually used to provide different nutrients at different times of the year, or to correct particular nutrient deficiencies. They are usually inorganic.

Controlled release fertilisers: These are almost always granules of inorganic fertilisers coated with a porous material such as sulphur or synthetic resin. Water enters the granule and the fertilisers leach out into the surrounding soil. The warmer the soil, the faster the leaching; this corresponds to plant growth which is faster in warm weather. By varying the thickness of the coating granules can be designed to feed plants for different periods of time.

Slow release fertilisers: These degrade slowly, usually under the influence of soil micro-organisms to release their nutrients and again are dependant on soil temperature. These are usually organic and include hoof & horn and bone meal.

How to use fertilisers

There are many ways to apply fertilisers, and the method you choose will greatly depend on the product you are using. Here are some of the most common methods of application, along with examples of when you would use this method.

Top dressing: This is the application of quick-acting fertilisers to the soil surface around plants to stimulate growth, and is usually carried out in spring at the start of the growing season. Take care to avoid leaf contact, which can cause scorching, and to protect against over application, which could cause root damage and pollution of ground water.

Base dressing: This is the incorporation of fertiliser into the soil or potting compost before sowing or planting.

Watering on: Liquid fertilisers or soluble powders and granules can be dissolved or diluted and watered onto plant roots during the growing season to give them an instant boost. They are mainly used for feeding glasshouse crops, pot plants and bedding. The nutrients in liquid fertilisers are instantly available. Care must be taken to avoid leaf contact, which can cause scorching.

Foliar feeding: This is the application of a dilute solution of fertiliser to the leaves of plants, useful as an emergency treatment for correcting nutrient deficiencies or for providing quick supplementary feeding. The absorption of liquid fertiliser is greatest where leaf surfaces are tender, particularly on the under surfaces of leaves or on young leaves that are just expanding. Foliar feeds should not be applied in bright sunlight because the foliage may be scorched.

How to make your own fertiliser

Free liquid fertilisers can be made at home by collecting the liquid component of rotted vegetation. Comfrey, nettles and liquid from wormeries all make effective liquid fertilisers if used in large amounts. They are much weaker than chemical fertilisers, but also much safer and more environmentally friendly. 

Add about 1kg (2 lb) nettles to 10 litres (2 gal) of water, leave for about two weeks and use at a dilution rate of 10 : 1. Add 1kg comfrey leaves to 15 litres (3 gal) of water and leave for six weeks in a sealed container then use undiluted. Wormery liquid should be diluted with water until it is the colour of weak tea, usually at a rate of 10:1.

How to make homemade natural plant feed

Almost any soft green vegetation can be used to make natural fertilisers. Suitable plant material is most readily available from mid summer to early autumn, but the resulting liquid can be kept until the following summer. In other seasons hay or lucerne pellets, sold for pet and pony feed, can be used, or coarse leafy ‘hay’ of favoured plants can be prepared at home and stored
Plants often used to make liquid feed include bracken, groundsel, lawn mowings, nettles and comfrey, ideally the non-flowering ‘Bocking 14’ variety. Comfrey is especially prized as it extracts an unusually high proportion of potassium from soils that are well supplied with potassium (this is common in clay soils but very rare in sand), making it rich in this essential nutrient. Organic sources of potassium are scarce and relatively expensive – waste products of sugar production are the commonest. When available, wood ash (3% potash) can also be used in sparingly liquid fertilisers. Many fertilisers are ‘organic based’, which often means potassium from mined sources has been included.
The process is simple – a barrel or similar container, ideally with a drainage tap or stopcock, is crammed with freshly harvested green material, water added (optional), covered, and left to rot. The natural bacteria and fungi on the plants and in the environment break down the plant material. This results in a murky ‘soup’. Although the material is often foul-smelling, little nutrient is lost as vapour.
The rotting process can take from a few days to several weeks, depending on temperature and how soft the greenery was. When the material is well decomposed, the liquid can be drawn off and used or stored in a container in the dark until needed. Residues can be composted.

Using homemade fertilisers

 As the raw material will be highly variable depending on species, age of plant material and underlying soil fertility, the nutrient content of the liquid will be variable. This does not affect use – simply dilute to a pale straw or tea colour and apply every few days until the desired plant growth is seen. Plants typically contain 3-4% nitrogen, 0.5-1% phosphorus and 1-2% potassium dry weight.  For comparison a synthetic liquid fertiliser might contain 15% nitrogen, 30% phosphate and 15% potash. Therefore use homemade fertiliser generously.and prepare large amounts.
Homemade fertilisers are unlikely to cause plant damage, unlike synthetic ones, which must be applied as per the manufacturer’s guidance.
Gardeners use a wide range of recipes, but as the raw material is highly variable and plants are over 90% water anyway, the method is not crucial. Typical recipes include:

Nettle liquid feed
  1. Gather the freshest, greenest nettle shoots available (with gloved hands)
  2. Cut shoots roughly into 12-25cm lengths
  3. Place nettles in a container and add water to half cover the nettles
  4. Weight nettles with a brick or similar
  5. Allow to rot for 1-4 weeks until the nettles are decomposed and a murky, smelly liquid is present
  6. Drain or decant for use or to store
  7. Dilute 1:10 for use
  8. Add any solid residues to the compost bin and prepare the next batch
Comfrey liquid fertiliser
  1. Cut fresh green comfrey leaves, leaving some to sustain the plant to help it recover
  2. Remove coarse stems and any flowers to the compost bin
  3. Cut up material so it can be readily packed into a container
  4. Weight down material with a brick or similar weight
  5. Leave for at least two weeks until the comfrey has rotted
  6. Tip or drain off the resulting smelly, brown liquor to use or store
  7. Dilute to a pale tea colour, typically 1 part liquor in 10 parts water
  8. Discard solid residues to the compost heap and start the next batch

Comfrey feed can also be bought. A typical analysis is NPK 1.7% nitrogen, 0.5% phosphates and 5.3% potash. Home made feed is likely to be less concentrated.
Other homemade liquid fertilisers include:
Wormery liquid: Wormeries are a highly effective, and fascinating, way of disposing of garden and household wastes. The action of the worms releases a liquid that makes effective fertiliser after diluting with water at a rate of one part liquid to ten parts water. Its nutrient composition will vary depending on what the worms have been fed, but is likely to be slightly stronger than liquid fertilisers derived from plant material.
Steeped rotted manure: In former times head gardeners would fill a small hessian sack with fully decomposed manure, which remains solid when soaked, and place the sack in a tank or barrel of water. After a week or two the resulting smelly liquid was used as liquid fertiliser, diluted to a pale tea colour. Its nutrient content will probably be slightly higher than plant derived fertilisers but less than synthetic ones. Scrupulous hygiene is vital.
Compost tea: Steeping garden compost in water for 2-3 days produces, after straining, a fertiliser which also contains microbes and chemicals secreted  by these microbes. In addition to fertilising effects, the other constituents may help to improve plant health. There are many recipes, including ones where air is bubbled through the brewing tea, which is said to produce a superior product. The benefits are not entirely proven but compost tea is unlikely to do harm.

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