What to feed plants: fertilisers explained

Val Bourne / 17 August 2017

Find out what you need to know about the N-P-K ratio of fertilisers, plus top tips for feeding plants.

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
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

7-7-7 is an equal mixture of nitrogen, phosphorous and potassium.

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.

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.

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.

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.

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.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.