What to feed plants: fertilisers explained

One of the hardest things in gardening is to know whether to feed, or not, and there’s no simple answer because some plants prefer poor soil.

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What plants not to feed

Aromatic plants with a Mediterranean provenance often have deep-root systems that seek out food and water from the depths, so silvers are often best left to their own devices. British wildflowers also prefer low-nutrient soils, so concentrate fertilisers in certain areas and feed in the growing season - including September.

Leguminous plants, such as sweet peas, beans and garden peas, make their own nitrogen through root nodules. If you apply lots of nitrogen you’ll produce lots of leaf at the expense of flower. Brassicas often follow legumes in vegetable gardens, because the soil has been enriched.

Ericaceous plant foods have added iron to help the absorption of nutrients.

What’s in most packeted fertilisers

Shop bough fertilisers contain an N-P-K formula consisting of N for nitrogen, P for phosphorous and K for potash. A good fertiliser will also contain trace elements essential for plant growth.

Nitrogen (N) produces green leafy growth and foliage
Phosphorus (P) helps root and shoot growth
Potassium (K) is for flower, fruit and general hardiness

The ratio of N-P-K varies and is recorded numerically by numbers relating to the weight of the added ingredients. The order of nitrogen, phosphorous and potassium (N-P-K) is always the same and the numbers can be on the front, side or back of a packet.

For example

Different fertilisers contain different ratios of the three major ingredients recorded as a series of a numbers. 5-10-5 is twice as rich in phosphorous as it in nitrogen and potassium, 7-7-7 is an equal mixture of nitrogen, phosphorous and potassium, and 4-7-6 refers to 4 of nitrogen, 7 of phosphorous and 6 of potash.

A balanced slow-release fertiliser will often have equal amounts of each and this mixture will feed a plant for several months without producing a noticeable surge of growth.

Choosing your fertilisers

Tomatoes, strawberries and bedding plants need a potassium-rich feed to encourage flowers, such as Tomarite.

Brassicas tend to be given a nitrogen-rich fertiliser to promote leafy growth.

Some fertilisers release their nutrients quickly while others are much slower. For the gardener, the slow- to medium-release balanced fertilisers with equal amounts of the three major ingredients are best for general use. They do not promote a surge of fast sappy growth but they do encourage general growth and vigour. Nitrogen-rich feeds promote leaf at the expense of flower.

Do packeted fertilisers work on their own?

The short answer is no, because soil structure is also highly important and artificial powdered and pelleted products will not improve the structure of your soil.

Their efficiency relies on you having good soil structure with plenty of air gaps for root development and water dispersal etc. Garden compost, well-rotted manure (either dug in or used as a mulch that’s pulled down by worms) is also vital for your plants. It aerates the soil and adds nutrients. Garden centres sell sterilised well-rotted manure, or you can make garden compost.

Find out how to make your own compost heap

Common fertilisers and their N-P-K ratios

Vitax Q4 (5.3- 7.5 -10)

A general purpose balanced slow release fertiliser for productive and ornamental plants. Can be applied around roses, hellebores and woody plants.

Chempack 3 (20-20-20)

With 7 trace elements, it’s recommended for bedding plants, houseplants, chrysanthemums, roses, container plants, salad vegetables, dahlias, soft fruit, ericaceous plants, vegetables, sweetcorn and hanging baskets.

Growmore (7-7-7)

Useful for vegetables and potatoes but the 7-7-7 figures are misleading because this is actually a high nitrogen feed - so don’t use this on fruit (and that includes tomatoes) as it won’t encourage flower or fruit. The discrepancy is due to the chemical make-up of the compounds.

Chempak Rose Food (10-12-24)

The high potash ratio will boost the number of flowers and toughen up the foliage on roses and many other flowering plants. All rose feeds are high in potash and they can be used with lots of flowering plants - although they do tend to be expensive.

Westland Gro-Sure Tomato Food (6-3-10)

A seaweed-enhanced tomato food with extra potash and magnesium. A Which Best Buy.

Tomorite (4-3-8)

Although this is a water-on liquid feed for tomatoes, it’s also excellent for all container-grown bedding plants, agapanthus, dahlias etc. Use once a week once the first truss begins to set.

Dilute a capful (20ml) in 4.5 litres (1 gallon) of water. The potash content is high compared to the nitrogen and phosphorous. Homemade comfrey tea is very similar in content.

Gro-Sure All Purpose Soluble Plant Food (12.5-5-25)

Heavy on the nitrogen and far less potash, so best used for vegetables or leafy plants such as box or yew.

Westland Chicken Manure (4.5-3.5-2.5)

Ratios vary according to the maker, but generally pelleted chicken manures, which release nutrients slowly, are high in nitrogen and relatively inexpensive. Often used to feed potatoes, courgettes, winter squashes and brassicas. It can attract the interest of foxes and dogs do eat the pellets. It's easy to store and handle, and easy to apply. Rich in nitrogen and lower in potassium so good for promoting leafy crops and feeding hellebores.

Fish, Blood and Bone (3-9-3)

Often used on established plants, because it contains all three elements. Keep away from plant stems and leaves and fork it in. Fish, blood and bone is good for adding to the ground to raise fertility or increase soil nutrients following a hungry crop. Does not affect flower and fruiting as nitrogenous feeds can do.

Bonemeal (3.15.0)

The high phosphorous content encourages root development and strong growth so bonemeal is often added to the planting hole when planting bare root roses or other plants.

Liquid seaweed (ratios vary)

Liquid seaweed is concentrated liquid that needs diluting. It's easy to water on to seedlings and releases nutrients slowly. High in potash but needs continual applications to improve growing conditions.

Tips for using branded fertilisers

Keep the products to the bare minimum. You do not need a separate fertiliser for every vegetable, for instance.

Wear gloves when handling any fertiliser.

Always adhere to the instructions on commercial, packeted fertilisers. Don’t exceed the dose because this can do more harm than good.

High nitrogen feeds can contaminate water courses. Avoid these near ponds and streams etc.

Pelleted fertilisers should be applied to damp soil where possible.

Only add fertilisers to plants before and during the growing season, because these additions tend to be washed out of the soil during winter.

Light sandy soils tend to be hungrier. Clay soil is much more fertile, so needs less fertiliser.

Garden compost

This is often low in nutrients although well-balanced otherwise. The bacteria involved keep nitrogen low. But this organic material improves soil structure when dug in, creating a friable mixture that encourages root growth.

The process of adding green waste to your own heap is very eco-friendly and, added over many years, it will improve your soil.

If you have a preponderance of grass clippings lay them on a sheet for 24 hours in a sunny place and either add them to your heap (once brown) or mulch raspberries etc.

Comfrey tea

Certain leaves left to rot in water form a nutritious liquid - although the process smells rather evil. It’s possible to make a wall-mounted comfrey tea maker using a drainpipe sealed at both ends with removable pieces. Feed the leaves into the top and drain the brown tea from the base after two to three weeks. Dilute before using one part tea to twenty parts water.

Chopped comfrey leaves can also be used as an accelerator on the compost heap and put into potato trenches, so planting Comfrey Bocking 14 by a compost heap is an excellent idea and the flowers are rich in nectar.

Find out how to make comfrey tea, or read more guides on soil improvement

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