Our ultimate guide to feeding plants

How to feed plants

Understanding how plants take up nutrients can help us work out how to feed them better. Plants take up nutrients from the soil through their roots, and to allow this to happen, the nutrients need to be dissolved in water. This is why it’s important to water plants regularly – as well as keeping the plants hydrated, it also enables them to get the nutrients they need from the soil.

Soil pH (this is a measure of how acidic or alkaline the soil is) also affects plants’ ability to take up nutrients. Elements such as iron, manganese and phosphorus are less easily absorbed by plants growing in alkaline soils, especially ericaceous plants like heathers, rhododendrons and azaleas.

To get the best results when feeding your plants:

• Water regularly in dry periods to ensure the soil remains moist
• Check that the soil pH is suitable for the plants
• Ensure the food you are using for your plants contains the right nutrients for the plants’ needs.

The good news for gardeners is that most healthy soils already contain the nutrients plants need, so many plants grown in a garden only need occasional feeding once established. However, for plants grown in containers, it’s a different story. Although potting composts typically contain enough nutrients to feed the plants initially, these nutrients will be used up after about 6 weeks. In addition, some of the nutrients are washed out of the compost every time the plants are watered. For this reason, plants in containers need to be fed regularly to replenish the nutrients in the compost.

What is plant food?

To grow strong and healthy, plants need three key nutrients. These are:

• Nitrogen (chemical symbol N) to promote leafy growth
• Phosphorus (chemical symbol P) to develop strong roots and shoots
• Potassium (chemical symbol K) to encourage the growth of plenty of flowers and fruits

Most plant foods contain one or more of these nutrients. The proportion of the three chemical elements, known as the NPK ratio, will be listed on the packaging of the plant food. As an example, Tomorite, a tomato food high in potassium compounds which promote the development of flowers and fruit, has an NPK ratio of 4-3-8, meaning that it contains 4% nitrogen compounds and 3% phosphorus compounds plus a higher level (8%) of potassium compounds.

Note: Plant food high in potassium compounds is often referred to as ‘high-potash’ fertilisers or feeds. This comes from the days when wood ash, a form of potassium carbonate also known as potash, was used as plant food.

In addition to the three main elements, plant food also often contains smaller amounts of other elements such as magnesium, iron, boron, calcium, copper and zinc. Although they are essential for plant health, plants only need small amounts of each of these elements, so they are known as trace elements.

Types of plant food

Plant food is divided into two categories: organic plant food, which are made from natural materials such as seaweed or bone meal, and inorganic plant food, which are man-made using chemicals.

• Compound plant food can be either organic or inorganic. They contain several different nutrients, either in a balanced mix or high in a specific nutrient to meet plant requirements. Tomorite is an example of a compound plant food.
• Straight plant food is usually inorganic and contain one single nutrient. They can be used to correct specific nutrient deficiencies in the soil or applied at certain stages of the plant’s growth to meet specific needs. For example, Sulphate of Ammonia, a straight plant food containing nitrogen, is used to promote leafy growth for crops like lettuce and spinach.
• Ericaceous plant food is designed specifically for acid-loving plants like rhododendrons, azaleas, heathers and blueberries, and contain a combination of elements such as iron and manganese in a form that is easier for these plants to absorb even if the soil pH is not ideal.
• Slow release plant food is usually organic. They are gradually broken down in the soil by micro-organisms, releasing their nutrients. Slow release plant food is usually dug into the soil at the start of the growing season, giving them time to break down over several months. Bone meal is a typical example of a slow release feed.
• Controlled release plant food is inorganic, consisting of granules with a coating that allows water in. As damp soil warms in spring and the plants start to grow, the granules release their nutrients. If there is a sudden cold snap or dry period, the release of nutrients will slow down as plant growth also slows. Apply controlled release feeds at the start of the growing season, and they will release their nutrients over several months as plants need them.

When to feed your plants

Different plants have different feeding requirements, so it’s always a good idea to research the specific feeding requirements for a plant, but here’s a general guide on when to feed your plants.

1. In late winter or early spring, rake a slow release plant food like Fish, Blood and Bone into vegetable beds or scatter it over borders so that it has time to break down and feed the soil before the growing season starts.
2. When planting new plants in borders, add a measure of controlled release plant food granules to the planting hole to give the plants a good start. Make sure to check the packaging to find out how much to apply and apply the feed safely.
3. Container plants need feeding regularly during the growing season. New potting compost contains enough food for 6 weeks, but once that has been used up, feed flowering container plants weekly during spring and summer with a high-potash feed like Tomorite.
4. Feed houseplants fortnightly during spring and summer with a high-nitrogen liquid houseplant feed to encourage lots of leafy growth. Reduce plant feeding to once a month during winter.
5. Watch for discoloured leaves, which are often a sign that plants are lacking in certain nutrients. Yellowing leaves can indicate a lack of iron, purple tinges can mean a potassium deficiency and yellowing between the leaf veins can be a sign of low magnesium levels. If you notice these symptoms, feed with a plant food high in the missing nutrients such as Sulphate of Iron (for iron deficiency), Levington Superphosphate (for potassium deficiency) or Epsom salts (for magnesium deficiency).

As a general rule, don’t apply liquid plant food to plants in dry soil, as this can damage their roots. Water the plants first and allow them to rehydrate for a few hours before applying plant food. If you’re not sure about how often to feed your plants, it’s best to start by underfeeding them rather than overfeeding.