Feeding Plants – What's the Best Fertiliser to Use?

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Gardener with different types of fertiliser

When we grow plants in containers, moisture and food are likely to run out more rapidly than in open ground, as the plants can’t send their roots beyond the container in search of extra sustenance. While a lack of water results in the foliage wilting and soft, sappy stems, a lack of food results in a gradual slowing of growth and a lack of lustre in the foliage. The plant will eventually become stunted.

The three main plant foods each have their own part to play in plant health and well-being. Nitrogen promotes healthy leaf and shoot growth, phosphorus helps root development, and potassium encourages flower and fruit production. A general fertiliser offers a balance of all three major nutrients, plus lots of minor ones and trace elements too – everything from magnesium and iron to manganese and molybdenum.

Organic and inorganic fertilisers

gardener adding slow-release fertiliser to compost — Gardener adding slow-release fertiliser to compost

The nutrients that we give plants can be supplied in organic or inorganic forms. Organic fertilisers are plant- and animal-derived products, such as hoof and horn, bonemeal, seaweed extract, and blood, fish and bone. These usually need to be broken down in the soil before the elements they contain can be absorbed in solution by the plants’ roots. This process makes soil bacteria ‘work’ to break down the organic material into simpler constituents, to which the plants have direct access.

Inorganic fertilisers are derived from minerals or are synthetic products which, once dissolved in water, are immediately available to plants. Their advantage is they can be absorbed immediately. However, because they don’t need soil bacteria to break them down, such bacteria are made redundant. Inorganic fertilisers, while providing instant nutrition, do nothing for the long-term health of the soil.

Some inorganic fertilisers are marketed as slow release or controlled release. Here, the effects of the fertiliser are delivered over a longer period. These products are handy for busy gardeners who are often absent, as they supply continuous nutrition over a period of time. Remember that even organic fertilisers do little to improve soil structure – only bulky organic matter (garden compost and manure) can do that, and both are vital ingredients of good earth.

When and how to use fertilisers

Gardener making tomato feed — Gardener adding tomato feed to a can of water

Plants slip into a semi-dormant or completely dormant state in winter, so it’s a waste to offer them any kind of food during this period. They’re most in need of nutrition just as growth is about to start, so it's a good idea to give plants a good helping of general fertiliser in March, so it can be readily absorbed by the time the strong growth spurt begins in April or May. Then again in June or July to see them through summer.

Don’t feed plants in cold, frosty weather, or in hot, dry summers. The compost or soil must always be moist at the time of feeding, whether with a liquid or solid fertiliser. Powdered or granular fertilisers are best sprinkled on to the surface of the soil and gently hoed in, to allow rain, or watering, to dissolve them and take them down to the roots. A mulch laid over the soil after application will seal in moisture and help to keep the fertiliser working.

Liquid feeds (in which the fertiliser is dissolved and applied in solution) are watered on and therefore act more quickly, but they – and foliar feeds, which are dissolved and sprayed on to leaves – provide benefits for a shorter period. Liquid feeds can be given weekly or fortnightly during the growing season and are especially useful for plants in containers. It is possible to overfeed plants, so always follow the instructions on the packet. Overfeeding can lead to foliage scorch and a ‘burnt’ appearance.

How often should I feed my plants?

Potting and seed composts already contain fertiliser. This is known as a base dressing, but will run out after six weeks, so you need to provide additional fertiliser. Pot on perennial plants that spend all their lives in a container into new compost in spring, either every year or when their size demands it. After a few years though, such a move becomes impractical and then the top few centimetres of compost can be removed in spring and replaced with fresh – a process known as top dressing. Most plants will enjoy an annual dose of general fertiliser, while leafy vegetables will appreciate a boost of nitrogen-rich feed.

Lawns are generally fed in spring with a high-nitrogen fertiliser and in autumn with a more balanced dressing that won’t encourage lush, frost-tender top growth, but will promote root growth. These products always give clear instructions on their time of use on the packet.