Plants need food, but the bags and bottles in garden shops can be confusing. Organic fertiliser, chemical fertiliser, NPK numbers, slow release, liquid feed: what do these terms mean, and which one should you use for your plants? This guide explains the difference between organic and chemical fertilisers in plain language, when each one works best, and how to use both safely without harming soil, plants, or the environment.
What Plants Actually Need
Plants need three main nutrients in large amounts: nitrogen for leaf growth, phosphorus for roots and flowers, and potassium for overall health and fruit. These are shown on fertiliser labels as NPK, for example 10-10-10. Plants also need smaller amounts of calcium, magnesium, and sulphur, plus trace elements such as iron and zinc. Good soil already contains many of these nutrients, but pots and raised beds lose them quickly through watering and harvesting, so feeding becomes important.
The best fertiliser is not always the strongest one. A plant that gets too much nitrogen grows soft, leafy, and weak, while a plant that gets too little stays small and pale. The goal is steady, balanced feeding that matches the plant and its stage of growth. Leafy vegetables need more nitrogen, flowering plants need more phosphorus, and fruiting plants need more potassium. Always start with healthy soil, because fertiliser cannot fix poor drainage, low light, or bad watering on its own.
Organic Fertiliser: Slow, Gentle, and Soil Friendly
Organic fertilisers come from living material, such as compost, well rotted manure, worm castings, seaweed, bone meal, and blood meal. They release nutrients slowly as soil microbes break them down. This slow release makes them gentle and hard to overuse, and it also improves soil structure, water holding, and microbial life over time. For most home gardeners, organic feeding is the safest default choice.
Compost is the best all round organic feed. It adds nutrients, improves soil, and helps roots grow. Well rotted manure adds nitrogen and organic matter, but it must be aged or composted first, because fresh manure can burn plants and carry weed seed. Seaweed feed gives trace elements and helps plants cope with stress, while worm castings are gentle enough for seedlings. For a simple way to make your own soil food, see our home compost guide.
When Organic Works Best
Use organic fertiliser for long term soil health, for leafy vegetables and herbs, for pots that you plan to reuse, and for gardens where children or pets play. It works well when you can plan ahead, because nutrients become available over weeks rather than days. It is also ideal for building new beds, for mulching, and for feeding plants that are growing steadily rather than showing an urgent deficiency.
Chemical Fertiliser: Fast, Precise, and Easy to Overuse
Chemical fertilisers, also called synthetic or mineral fertilisers, are made to supply specific nutrients in exact amounts. They dissolve quickly, so plants can use them within days. This speed is useful when a plant shows a clear deficiency, such as yellow leaves from low nitrogen, or when you need a quick boost during flowering and fruiting. They are also useful in hydroponics and for precise feeding of container plants.
The risk is overuse. Chemical fertilisers do not improve soil structure, and repeated heavy use can increase soil salts, harm microbes, and pollute water if excess runs off. They can also burn roots if applied to dry soil or at too high a rate. If you use them, always follow the label, use the lowest effective dose, water before and after feeding, and never feed a stressed, dry, or newly transplanted plant.
Organic vs Chemical at a Glance
Organic fertiliser feeds soil as well as plants, works slowly, and is forgiving, but it is less precise and slower to correct a deficiency. Chemical fertiliser feeds plants directly, works quickly, and gives exact nutrient ratios, but it does not build soil and is easier to misuse. Many experienced gardeners use both: organic matter as the base for soil health, plus a small amount of chemical or liquid feed when a plant needs a fast response. This combined approach gives you the benefits of both without relying too heavily on either.
Reading a Label
Check the NPK ratio first, then check whether the product is slow release, liquid, or granular. Slow release granules feed over weeks and suit pots and beds. Liquid feeds act fast and suit container plants during active growth. Granular feeds need watering in after application. Avoid products that promise instant results at very high doses, because they often cause soft growth that attracts pests. For potting mixes that hold nutrients well, our soil mix for pots guide explains a base mix that works with either feeding method.
When to Use What
Use organic feed when you are preparing soil, planting leafy greens, herbs, and long term perennials, and when you want to improve soil over time. Use a fast chemical or liquid feed when a plant is pale, slow growing, or fruiting heavily and needs nutrients now. Use a balanced feed for most vegetables during growth, a higher potassium feed for tomatoes and flowers during fruiting, and a low nitrogen feed for plants that are already leafy but not flowering. Stop feeding in winter when growth slows, and reduce feeding for succulents such as aloe vera, which need very little food, as explained in our aloe vera care guide.
For seedlings, use only very weak feed after true leaves appear, because strong feed burns roots. Our seed starting guide explains this timing in detail. For established vegetables in pots, feed every two to three weeks during the growing season, and watch leaves for signs of overfeeding, such as brown leaf tips, wilting after feeding, or a white crust on soil. If you see these signs, flush the pot with plain water and skip the next feed.
Safe Feeding Habits
Feed the soil, water first, and measure carefully. Mix compost into the top few centimetres of soil, water plants before liquid feeding, and use a measuring spoon rather than guessing. Keep fertiliser away from stems, because direct contact can burn tissue. Store fertiliser in a dry, labelled container away from children and pets, and wash hands after handling manure based products. Avoid feeding before heavy rain, because nutrients wash away and can pollute drains and waterways.
The simplest routine for most home gardens is this: add compost at planting, mulch with organic matter, feed lightly every two to three weeks in the growing season, and watch your plants. Healthy, steady growth is a better sign than rapid, soft growth. If you are unsure, use less rather than more, because you can always feed again, but you cannot easily undo root burn. For trusted, science based advice on feeding garden plants, the RHS fertiliser advice is a useful reference for types, rates, and safe use.