Feeding Plants

All plants need food in order to stay healthy and grow well. If you have a healthy soil then feeding is not usually required because the plant should be able to get all the nutrients it needs. However, using a feed can produce a better display of flowers or a higher yield of fruit. Fertilisers are also used when a plant shows signs of nutrient deficiency.

The three major nutrients that most plants need are nitrogen, phosphorous and potassium. Fertilisers contain concentrated sources of these nutrients (as well as minor nutrients) in chemical or organic form. You will see this presented as NPK (their chemical symbols). 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 example 6:4:6. If the ratios are about the same, it is a general-purpose fertiliser and will aid general growth, but some fertilisers are higher in one or another nutrient. For example, tomato fertiliser will be high in potassium (K), with a ration of 4:5:8 in order to produce lots of juicy fruit. Similarly, a fertiliser for feeding grass in the spring will be high in nitrogen. Plants also need other minor 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.

Fertiliser can be applied as either a top dressing to beds and borders or it can be worked into the soil as a base dressing prior to planting or sowing.

Containerised plants need regular feeding, as they only have what you give them. Plants in beds and borders, by contrast, are able to use the resources present in the garden soil, and may not need feeding. Ornamental trees and shrubs in garden soil may not need regular feeding by fertiliser. Some crops that do benefit from regular fertiliser are fruit, vegetables and bedding plants.

　Types of Fertiliser

There are two main types of fertiliser: organic and inorganic.

Inorganic fertilisers are synthetic, artificial forms of plant nutrients. They are usually more concentrated and faster acting than organic fertilisers. Examples of inorganic fertilisers include: Growmore, Miracle-Gro, Phostrogen, Sulphate of Ammonia, Sulphate of Potash, and Superphosphate and Tomorite. Organic fertilisers are derived from plant or animal sources and contain plant nutrients in organic form. Organic products tend to be slower acting, as they need to be broken down by soil organisms before the nutrients 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.

Solid feeds need to be dissolved before the nutrients become accessible to plants, therefore water them in well if the soil is dry. Working out how much solid feed to apply can be tricky - too much will make a very concentrated solution and burn the roots; too little and the plants will go hungry! Therefore, always follow the instructions carefully.

Powders and liquids that are dissolved or diluted with water are more suitable for plants in containers and pots. The nutrients are immediately available to the plant and are applied in the right concentrations. The right amount to apply is what you would pour as if you were just watering. Any excess will run out of the bottom of the pot. Never feed plants that are wilted, always apply water first and wait until they have perked up again.

Feeds work in the following ways:

• Compound fertilisers contain a mixture of different nutrients, and may be balanced or may supply more of some nutrients than others. They may be organic or inorganic, or contain both.
• Straight fertilisers contain only one or mainly one nutrient and 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 are usually granules of inorganic fertilisers coated with a material of varying thickness. Water enters the granule and the nutrients are released into the soil. The warmer the soil, the faster the release. These granules are ideal for container plants. The thickness of the coating determines the length of time it takes for the nutrients to be released - some can remain in the soil for up to 12 months. Plugs made from granules bonded together are also available to push into the surface of the compost.
• Slow release fertilisers degrade slowly and are good for feeding plants in the soil. They are usually organic and include hoof & horn and bone meal. Slow-release feed granules can be mixed in with compost when you are potting up, or added to fresh compost when you are top dressing established plants.
• Fast acting fertiliser is used for plants that require 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.

How to Apply Feed

There are several ways to apply fertilisers, and the method you choose will depend on the product you are using. The following is a list of some of the more common methods of application:

• Top dressing 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. It is important that one avoids contact with the leaf as fertilisers can cause scorching. Also avoid applying too much as this can damage roots.
• Base dressing is the incorporation of fertiliser into the soil or potting compost before sowing or planting.
• Liquid feeding involves diluting liquid fertilisers or soluble powders and watering them onto plant roots during the growing season to give them an instant boost. They are mainly used for feeding greenhouse 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 is the application of a dilute solution of fertiliser to the leaves of plants. It is useful to treat nutrient deficiencies or to provide quick supplementary feeding. The leaf surface absorbs the nutrients into the plant. Foliar feeds should not be applied in bright sunlight because the foliage may be scorched.

When to Feed

• Feed plants if they are showing signs of nutrient deficiency, such as wilting, distortion, spindly growth
• Feed plants if they are producing lower than expected yields of flowers or fruit, or are putting on less growth than expected (but appear otherwise healthy and are growing in suitable conditions)
• A top dressing of slow release fertilisers can be added to beds and borders in spring before plants have put on too much growth.
• Vegetables are hungry crops and will thrive if given a slow-release fertiliser two or three times a year
• Sweet peas, clematis and roses, will benefit from a mid-summer top-up. Sprinkle fertiliser around plants and water in. There is no need to feed in late summer because this will only produce a flush of late, lush growth that will suffer when the frosts arrive
• Hanging baskets, pots and containers can be fed once a week during the growing season, using a liquid feed applied from a watering can

Do I Need to Feed?

Feeding a plant when there is no need can cause problems. If a plant looks unhealthy it is best to check for diseases or pests as the cause before reaching for the feed. Poor growth can also be due to environmental conditions, such as water logging, drought or weather issues. The soil type can also have an impact on nutrient levels. Sandy soils and chalky soils tend to be lower in nutrients than clay or loam soils. Soils also vary in the availability of nutrients. Soils that are dry, waterlogged, very acid or very alkaline may not allow plants to access existing nutrients. Correcting these factors (where possible) may be more effective than giving fertiliser, and in fact may be necessary for fertilisers to be effective.