Plant Feeds for Pot Plants

Plants growing in fertile garden soil make most of their own food, but those grown in pots need supplementary feeding to stay healthy.

Using plant feeds

The manufacturers of plant feeds list the percentages of the three main ingredients on the label, either by their full name or by their chemical symbol. The figures next to the letters NPK are the percentages for each chemical. The ingredients are always listed in the same order, so it does not matter if the names or letters do not appear. For example, 8:6:12 indicates a feed with 8 per cent nitrogen, 6 per cent phosphorus and 12 per cent potassium.

Fertilizers that have an identical, or almost identical percentage of nitrogen, phosphorus and potassium are described as general, balanced or standard plant feeds. They are the most common types and can be used on a wide range of plants.

Fertilizers with a higher percentage of phosphorus or potassium are often used to promote a particular type of growth. A tomato fertilizer is high in potassium and is used when tomato plants need encouragement to make flowers and fruit.

Types of plant feed

Plant food, or fertilizer, is available in I several different forms. Slow-release granular fertilizers are worked into the potting mixture when the plant is repotted. Powder, granules or crystals are sprinkled on to the surface of the growing mixture and then watered in. Liquid fertilizers, available as liquid concentrates or concentrated soluble powders, are diluted with water before they are applied.

Liquid fertilizers are one of the best plant feeds as they are quickly absorbed by the plant’s roots. They are also inexpensive, economical to use and easily stored.

Liquid fertilizers are also available as foliar feeds, which must be diluted before they are sprayed on to the leaves, and as impregnated sponges, which are wiped across the leaves. Impregnated sticks or probes are pushed into the potting mixture. They release their nutrients when the moisture in the potting mixture dissolves the chemicals they contain. Tablets work in a similar way; they gradually dissolve in the compost.

Composition

Plant feeds have three main ingredients: nitrogen, phosphorus and potassium. Nitrogen (chemical symbol N) helps the plant to produce large rich-green leaves. It is vital for all plants in early stages of growth and is particularly important for foliage plants.

Phosphorus (chemical symbol P) also promotes the development of stems and leaves and encourages the plant to develop an extensive root system. It helps harden and ripen stems and leaves and encourages production of flower buds.

Potassium (chemical symbol K) gives flowers a good colour and makes the plants hardy and adaptable to adverse conditions such as dry air, wind or draughts. It also helps bulbs to build up growth for flowering the following year.

A healthy plant needs all of these chemicals. Apart from these three main ingredients, plants also need trace elements – minerals such as iron, copper and magnesium. These are only needed in very small quantities and are always included in a well-mixed plant fertilizer.

Liquid fertilizers are the easiest and most economical way to feed plants. As foliar feeds, they should be diluted with water and then sprayed directly on to leaves.

Slow release probes are simple to use. They should be pushed into the potting mixture, and the moisture will dissolve them and release the nutrients.

Another form of liquid fertilizer is the impregnated sponge, which is suitable for foliage plants. Wipe the sponge across the leaves, which will absorb the nutrients.

Liquid fertilizers come in concentrated form. Dilute them with water, then water directly into the potting mixture. Nutrients will be absorbed by the plant’s roots.

Do’s and Don’ts

DO

• Follow the manufacturer’s instructions for diluting concentrated feeds and feeding plants.
• Give a little food frequently in the right pro- portions of food to water.
• Feed plants on balconies or patios more frequently than plants in the house.

DON’T

• Feed plants during their resting period.
• Feed your plant if the potting mixture is dry. Water it first.
• Give food to sick plants. Treat the problem and wait until the plant has recovered before you begin to feed.

Specially prepared fertilizers for feeding African Violets are weighted in favour of phosphorus and many growers use nothing else. Yet it is sensible to use different feeds at different times, rather than rely on one.

FAQs

What happens to house plants when they are given too much plant food?

Feeding too often, or giving a stronger solution than is necessary, causes a build up of chemical salts that damages roots.

Can I always use a foliar feed instead of one that is applied to the potting mixture?

Foliar feeds can be used on plants that cannot absorb nutrients through their roots very well – such as air plants – but generally they should only be used when a plant needs a boost. Once the plant is healthy, resume a normal feeding routine with a standard liquid fertilizer.

Is it really safe to use tomato fertilizer on house plants?

Tomato fertilizers, which are high in potassium, are beneficial to some house plants, especially flowering ones such as Regal Pelargoniums, and most cacti. Otherwise it is best to use a standard fertilizer.