What fertiliser do my plants need? The complete guide to a lush garden
You water them regularly, diligently pull out the weeds and make sure they get plenty of sunlight, but even so, your plants sometimes seem to need a bit of extra energy. Sound familiar? Just like us, plants need a varied diet of nutrients to stay strong, green and healthy. But anyone looking for plant food at the garden centre or online will soon come up against a wall of abbreviations, percentages and types.
Which fertiliser do your plants actually need? Is organic always better than chemical? And what does that mysterious NPK value on the packaging mean? In this comprehensive guide, we explain it all in detail, so you know exactly how to get your garden into top shape – and keep it that way.
- Why is it absolutely essential to fertilise plants?
- The NPK formula explained
- Organic fertiliser or artificial fertiliser?
- Liquid fertiliser or granules?
- General-purpose fertilisers vs specific fertilisers
- When and how often should you fertilise plants?
- Step-by-step guide: How to apply fertiliser correctly?
- Spreading lime and fertilising: the correct order
- The pros and cons of home-made fertilisers
Why is fertilising plants absolutely essential?
Some gardeners mistakenly regard fertilising as an unnecessary luxury. In reality, it is vital for your garden. Plants grow through photosynthesis: they absorb minerals from the soil and produce sugars using light, water and CO₂. In the wild, the soil is constantly nourished by dying plants and decaying leaves. In our meticulously maintained gardens, we often remove this natural waste (such as prunings and grass clippings). As a result, the soil becomes depleted over time. Furthermore, fertilising with prunings or mulch alone is rather one-sided. This is because it contains a great deal of nitrogen but, unfortunately, few other nutrients. Mulch alone cannot meet the plants’ full nutritional requirements. Fertilising replenishes the nutrient reserves. A well-nourished plant is much more resistant to drought, frost, diseases and pesky pests. For optimal growth, development and protection, all nutrients must be present. A deficiency in one nutrient cannot be compensated for by an excess of another. Quite the contrary, in fact. An excess of a particular nutrient can also have very harmful effects.
What fertiliser do my plants need? This depends on their specific requirements, but for most gardens, an organic all-purpose fertiliser is the best starting point. Green plants and lawns mainly need nitrogen (N) for leaf growth. Flowering plants and fruit trees need more phosphorus (P) and potassium (K) for strong root development, abundant flowering and tasty fruit.
The NPK formula explained simply: what exactly is in plant food?
On almost every box or bag of fertiliser, you’ll see the letters N-P-K, followed by a series of numbers (for example, 7-4-6). This forms the basis of all plant food. But what are these key components?
- N (Nitrogen): The growth engine. Nitrogen stimulates cell growth. It is essential for the development of strong stems and fresh green leaves. Leafy vegetables and hedges, in particular, need plenty of it.
- P (Phosphorus): The root builder. Phosphorus ensures the development of a sturdy and deep root system, and plays a role in bud formation.
- K (Potassium): The resilience booster. Potassium makes the plant more robust and improves its resistance to drought, frost and fungi. It also ensures abundant flowering and flavourful fruit in the vegetable garden.
- Mg (Magnesium): Magnesium is often added to the mix. This is the secret weapon for an intense, deep-green leaf colour.
| Nutrient | Function | Sign of a deficiency |
| Nitrogen (N) | Ensures effective photosynthesis and therefore good growth | Stagnant growth, yellow leaves, dying parts |
| Phosphorus (P) | Promotes flower and fruit formation, good root development and strengthens the immune system | Reduced frost resistance, red leaves |
| Potassium (K) | Resistance to frost and heat, good water management, and enhances the flavour of the fruit | Susceptibility to disease, reduced frost resistance, discoloured and/or cracking fruit |
| Magnesium (Mg) | Green leaves, promotes growth and enhances the flavour of the fruit | Leaf drop, yellowing of the leaves, spots on the leaves |
| Calcium (Ca) | Increases stress tolerance | Pale-coloured leaves, brown spots on the fruit and/or dented fruit |
Organic fertiliser or artificial fertiliser: which is the best choice for your garden?
The age-old dilemma for gardeners: do you opt for organic methods or go for chemical fertilisers? Both have their own specific effects.
Organic fertilisers: nutritious and environmentally friendly
Organic fertilisers are made from 100 per cent natural raw materials, of both plant and animal origin.
- Advantages: Not only do they nourish the plant, but they also improve the overall soil structure. Soil organisms (such as bacteria and worms) must first break down the fertiliser, causing the nutrients to be released gradually over a period of about three months. This means there is no risk of root burn.
- Disadvantages: They work more slowly. You therefore need a little more patience before you see results.
Mineral fertilisers: for a quick boost
Artificial fertilisers are made up of synthetic salts and minerals.
- Advantages: The nutrients are immediately available to the plant. You’ll often see spectacularly green results within a few days. They can also be perfectly tailored to your needs.
- Disadvantages: They are only effective for a short time (often just a month) and, unfortunately, do nothing to benefit soil life or soil structure. If you use too high a dose or apply them in direct sunlight, you run a high risk of burning your plants.
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Our recommendation? Choose organic fertilisers wherever possible. You’ll be building long-term soil health, which is the foundation of any successful garden.
So what exactly are compound organic-mineral fertilisers?
Organic raw materials are naturally rich in nitrogen. To provide plants with all the necessary nutrients, organic fertilisers are often supplemented with other nutrients. This results in an organic-mineral fertiliser that offers a broader range of nutrients.
Some mineral nutrients, such as calcium and sulphur, occur naturally and do not require further processing. As a result, many organic-mineral fertilisers consist of 100 per cent natural raw materials.
Liquid fertiliser or granules
As well as the source, you also need to choose the form.
- Solid fertiliser (granules or crumbles): Ideal for ornamental gardens, lawns and vegetable gardens. You simply scatter them and they provide long-lasting nutrition.
- Liquid fertiliser: You add this to the watering can. It is absorbed extremely quickly. Liquid fertiliser is particularly perfect for houseplants, potted patio plants or as a quick rescue treatment for a weakened plant.
Fertiliser sticks are also a handy way to easily provide your plants with nutrients. You simply insert them into the potting compost, after which they gradually release nutrients over several months. Ideal for houseplants and balcony plants that could do with a bit of extra nourishment!
Bear in mind that a nutrient deficiency is not always due to reduced uptake of these substances. Garden soil contains phosphorus, but effective uptake depends on soil conditions. However, damp or compacted soil with a low pH means that plants can absorb very little, if any, of this phosphorus. Applying a phosphorus-rich fertiliser to such soil would have the opposite effect. In this case, it is much better to loosen the soil, enrich it with humus and apply lime. It is always a good idea to carry out a soil analysis to determine the soil’s pH level. This way, you can avoid over-fertilisation.
All-purpose fertiliser vs. specialised fertiliser: what do your plants need?
You can easily look after your whole garden with a good, organic all-purpose fertiliser. However, some plants have specific requirements:
- Acid-loving plants (hydrangeas, rhododendrons, azaleas, heather): These plants cannot tolerate lime and require acidic soil. A specialised fertiliser for acid-loving plants helps to maintain that low pH level.
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- Roses: They really are voracious eaters! They need a huge amount of energy (potassium and magnesium) for their lush blooms.
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- Boxwood and hedges: These require extra nitrogen in particular to ensure continuous foliage growth and recovery after pruning.
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- Tomatoes: Fruiting vegetables need plenty of potassium to develop firm, juicy and flavourful fruits. Too much nitrogen will simply result in lots of leaves and few tomatoes!
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- Strawberries: Strawberries require a high dose of potassium. This nutrient gives the fruit a rich flavour and improves its resistance to diseases and pests.
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When and how often should you fertilise your plants?
As a rule of thumb, you should fertilise your garden two to three times a year, as an organic fertiliser remains effective for around a hundred days.
Make sure you only fertilise during the flowering and growing seasons. Plants that are fertilised whilst in dormancy will produce new shoots that do not harden off sufficiently. This makes them particularly susceptible to frost.
You can use a nitrogen-rich fertiliser until mid-August at the latest. After this time, you can apply a potassium-rich fertiliser to shrubs, perennials and other perennial plants. This protects the plants against frost.
Granular fertilisers should be scattered directly into the planting hole at the time of planting, or worked into the soil during the annual main fertilisation in spring. Liquid fertilisers are ideal if you wish to fertilise your plants during the growing season.
Fertilising plants in spring
- Trees, shrubs, hedges: Fertilise with mixed fertilisers for ornamental gardens. Fertilise boxwoods with boxwood fertiliser, and acid-loving plants such as rhododendrons or hydrangeas with specific fertilisers designed for them.
- Roses: Lightly work organic fertilisers or bone meal into the soil.
- Border plants: Fertilise in early spring with mixed organic fertilisers.
- Potted and container plants: From April onwards, fertilise with special fertilisers for patio and container plants.
- Vegetable garden: A few days before sowing or planting, work organic fertilisers into the topsoil.
Fertilising plants in summer
- Roses: Fertilise in July with special rose fertilisers
- Border plants: Top-dress with a mixed fertiliser for ornamental gardens
- Potted and container plants: Until mid-August, add liquid fertiliser to the watering can every two weeks, or use slow-release fertilisers such as sticks or cones.
- Vegetable garden: To maintain nitrogen levels, you can top-dress with, for example, blood meal.
Fertilising plants in autumn and winter
- Trees, shrubs, hedges: In autumn and winter, maintain the soil with a layer of compost mulch.
- Roses: Roses also benefit from mulch. From November onwards, pile the soil up around the base of the plants.
- Vegetable garden: Fertilise using mulch from October to March. At the end of winter, you can apply organic fertilisers to the vegetable garden to prepare it for the coming spring. You can also sow green manure crops.
Step-by-step guide: How to apply fertiliser correctly?
Spreading fertiliser is simple, but these tips will help you get the most out of it:
- Measure out the correct dose: Check the packaging carefully. More isn’t always better!
- Spread evenly: Distribute the granules evenly around the base of the plant.
- Work in lightly: Rake the fertiliser granules lightly into the top few centimetres of soil. Take care not to damage the shallow roots.
- Check the weather: It’s best to fertilise just before a rain shower, or water generously straight after spreading. This helps the fertiliser dissolve and allows it to sink straight into the soil.
- Avoid strong sunlight: Never spread fertiliser over wet leaves when the sun is shining brightly, as this causes scorch marks.
Spreading lime or applying fertiliser: what is the correct order?
An absolute classic amongst gardeners’ questions! Lime and fertilisers aren’t in competition with each other, but they do require careful planning. Lime raises the acidity (pH level) of your soil, which helps roots to absorb fertiliser more effectively. However, never apply lime and fertiliser at the same time. They react chemically with each other, causing the vital nitrogen to escape into the air rather than being absorbed into your soil. Always apply lime first (preferably in late autumn or early spring) and wait at least 3 to 4 weeks before applying the fertiliser.
What about homemade fertilisers?
Do you really need to invest in scientifically formulated fertilisers at all? Can’t you just use your kitchen waste as fertiliser, as you read everywhere online? The answer is clear: yes, proper fertilisers are definitely better than recycled kitchen waste! Although coffee grounds or eggshells, for example, can be useful additions!
Coffee grounds have a high concentration of nitrogen. Your plants can therefore benefit from it for healthy, vigorous growth. However, phosphorus and potassium are present only in very limited quantities. If you fertilise using only coffee grounds, there is a risk of nutrient deficiency. You should therefore always regard coffee grounds as a bonus. Please note: coffee grounds are slightly acidic. Hydrangeas, rhododendrons and other acid-loving plants therefore thrive on coffee grounds. Tomatoes, courgettes and leeks also tolerate slightly acidic soil. For plants that require alkaline, i.e. lime-rich, soil, it is best not to use coffee grounds. These include, for example, lavender, rosemary, apple and pear trees.
Eggshells, on the other hand, contain a lot of lime. Clean the eggshells and scatter them around plants that like lime. These include, for example, boxwood, lavender, roses, butterfly bush and yew.
Conclusion: give your garden the energy it deserves
Healthy, strong plants start with healthy, rich soil. By choosing a high-quality (preferably organic) fertiliser and applying it at the right times of year, you can transform a lacklustre garden into a lush, colourful paradise. With the right NPK ratio in mind, you’re now all set for the growing season! Does your garden deserve a nourishing boost too? Discover our extensive, carefully selected range of organic fertilisers and plant food. If, after reading this, you’re still unsure which product is the perfect match for your green paradise, please get in touch with our gardening experts; we’ll be delighted to help you (with a good dose of gardening know-how)!
Find out much more in these blogs
- Renewed vigour: the importance of lime
- Five uses of lava meal: THE natural soil improver and fertiliser
- Lime cyanamide a wonder product for the (vegetable) garden or not
- The pH value of your kitchen garden
- Autumn garden chores: your checklist!
- Prepare the soil for planting
- Potting soil and soil improver, what's the difference?
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