Understanding Fertiliser Labels (NPK)
Fertiliser labels can look technical, but they are simply a guide to the main nutrients a product supplies. Understanding NPK helps you choose sensibly for the job, avoid overfeeding and remember that compost, manure and good soil care remain the foundation of a productive garden.
Read the three main numbers on a fertiliser label as N-P-K. They describe the relative amounts of nitrogen, phosphorus and potassium in that order. Nitrogen supports leafy growth, phosphorus supports root development, and potassium supports flowering and fruiting. The numbers do not tell you everything about a product, but they help you compare its emphasis. A fertiliser with a higher first number is generally aimed at leafy growth, while a higher final number is often more useful when plants are forming flowers and fruit. Check the whole label too, because it may explain where and when the fertiliser is intended to be used.
Match the fertiliser to the stage and crop rather than applying the same thing everywhere. Leafy vegetables and plants making vigorous spring growth may benefit from a nitrogen emphasis, but too much can produce soft leafy growth at the expense of flowers or fruit. Young plants establishing roots may benefit from phosphorus, although many garden soils already contain sufficient amounts. Fruiting crops and flowering plants use potassium as they develop buds and produce harvests. Soil type, existing fertility, weather and the plant itself all affect what is needed. When in doubt, improve the soil first and feed modestly rather than treating fertiliser as an automatic answer.
Look for instructions describing how and when to apply the product, then follow them exactly instead of guessing at a stronger amount. More fertiliser is not better: excess can damage roots, create weak growth and wash away into drains or watercourses. Apply only to moist soil where instructed, and water afterwards if the label recommends it. Keep fertiliser off leaves unless it is specifically designed for that use, and do not apply before heavy rain. Store it dry, labelled and out of reach of children and animals. A small hand tool or dedicated scoop helps you apply it evenly without handling it unnecessarily.
Organic materials can supply nutrients more gently while improving the soil itself. Well-rotted manure adds organic matter and a modest nutrient contribution, especially useful before hungry crops. Garden compost improves structure and supports soil organisms, though its nutrient content varies. Seaweed can provide a useful broad supplement, and comfrey leaves can be composted, used as a mulch or made into a diluted garden feed. These are not identical replacements for every fertiliser, because their nutrient levels vary, but they are valuable parts of a balanced approach. Use peat-free composts and avoid relying on any single input as a cure for every plant problem.
Consider whether a plant actually needs feeding before opening a packet or bottle. Pale leaves can result from cold soil, waterlogging, drought, shade, compacted ground, root damage or unsuitable pH, not just a nutrient shortage. A soil test is useful for persistent problems, especially in a vegetable plot where amendments are added regularly. Observe which plants are affected and whether they are in the same bed. Healthy, compost-rich soil often reduces the need for routine feeding. Container plants are different because nutrients wash out more quickly, so use a suitable feed according to its instructions and refresh potting compost when repotting.
Create a simple feeding record for your garden. Note what you used, where, when and how plants responded, alongside additions of compost or manure. This prevents repeated applications and helps you spot beds that are being overfed. Rotate crops on an allotment, mulch beds annually and return healthy plant material to the compost heap where appropriate. These habits build a steadier nutrient cycle than frequent corrective feeding. If a label is unclear, ask at the RHS or a local garden centre before applying it. Choosing the mildest suitable option and using it sparingly is usually safer than trying to force fast growth.
Frequently asked questions
What do the NPK numbers mean on a fertiliser label?
They show the relative emphasis on nitrogen, phosphorus and potassium. Nitrogen supports leafy growth, phosphorus supports roots, and potassium supports flowers and fruit. They are a comparison tool, not a complete feeding plan. Check the label's crop guidance and consider your soil, season and plant stage before applying anything.
Can garden compost replace fertiliser?
Garden compost is an excellent soil improver and supplies some nutrients, but the exact amount varies with its ingredients and maturity. It can meet many general garden needs when applied regularly. Hungry crops, long-season container plants or a confirmed deficiency may still need a suitable additional feed.
Should I feed plants if their leaves are yellow?
Not automatically. Yellowing can be caused by poor drainage, dry roots, cold conditions, shade, compacted soil or unsuitable pH. Check the plant's growing conditions first and look for patterns across the bed. Feed only when there is a likely nutrient need and use the label guidance carefully.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.