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About plant nutrients

Most plants need three major nutrients to thrive; nitrogen, phosphorous and potassium, which are generally known as NPK (their chemical symbols). The three main nutrients are needed by plants for different reasons. Nitrogen promotes leaf growth, phosphorous is for the roots and potassium is needed for flower and fruits. The amount of each is written on fertiliser packets as a ratio, for instance 6:4:6. Note that the order of nutrients is always the same, ie N, P, K. If the ratios are about the same, it is a general-purpose fertiliser and will aid all round growth, but some fertilisers are higher in one or another nutirent. For instance, tomato fertiliser is designed to promote lots of plump fruit and will be high in potassium (K) and have a ratio of 4:5:8. Similarly a fertiliser for feeding grass in the spring will be high in nitrogen.

To remain healthy, plants also need other nutrients, such as calcium (Ca), sulphur (S) and magnesium (Mg), as well as zinc (Zn), iron (Fe), copper (Cu), boron (B) and manganese (Mn). Smaller amounts of these will be contained in most general fertilisers.

What to do

When to feed

• Give beds and borders a kick start by feeding in spring with a slow release fertiliser, before plants have put on too much growth. This is known as top dressing. Fertiliser applied to the soil and worked in prior to sowing or planting is called base dressing.
• Vegetables are hungry crops and will thrive if given a slow-release fertiliser two or three times a year.
• Other 'greedy' flowering plants, such as sweet peas, clematis and roses, will benefit from a mid-summer 'top up'. Sprinkle fertiliser around plants and water in. There's no need to feed in late summer. This only encourages a flush of late, lush growth that'll get hit hard by frosts.
• During the growing season, feed flowers in hanging baskets, pots and containers once a week, using a liquid feed applied from a watering can.

Improving soil

• Apart from providing fertilisers to the soil, it's a good idea to enrich soil before planting by adding plenty of organic matter, such as leafmould, garden compost or well-rotted manure. Not only will this boost the nutrient content of the soil, but it will improve its structure and help it to retain moisture.
• To do this spread a thick (about 5cm (2in) will do) layer of the material over the soil and fork into the surface to a depth of about 10cm (4in). To give existing beds a boost, mulch around plants with organic matter in the spring.

Different fertilisers

There are many different types of fertiliser available, including liquid tonics that can be applied from a watering can, granular fertilisers that are mixed into compost and powdered feed that is applied to the soil. These feeds work in three main ways:

• Controlled release fertiliser – ideal for containers, these come as granules that are mixed into compost and release their nutrients over a long period of time, some for up to 12 months. Plugs made from granules bonded together are also available - these can simply be pushed into the surface of the compost.
• Slow release fertiliser – good for feeding plants in the soil. Usually applied as a powder that can be scattered around perennials, trees, shrubs and vegetables.
• Fast acting fertiliser – for plants in need of a pick-me-up. These are ideal if a plant is suffering from a deficiency and are usually applied in a liquid form that can be used by the plant quickly.

Seaweed fertilizers are a good organic option. Alternatively try diluting the liquid from a worm composter.

Tips

• Always read the manufacturers instructions before applying fertilisers
• Don’t overfeed plants
• Wear gloves if handling powder or granular fertilisers