Vegetables are hungry plants. A bed that grows a full season of tomatoes, cabbage, courgettes and beans removes a large amount of nitrogen, phosphorus, potassium and trace elements with every harvest, and those nutrients must be replaced or the next crop will be smaller and paler. Fertilizing vegetables is the routine of putting those nutrients back — through compost, manure, and fertiliser — at the right time and in the right amount. This guide explains what vegetables need, when to feed them, which fertilisers to use, and how to avoid the mistakes that burn plants or waste money.
The foundation is always compost: a 5–8 cm layer worked into the bed each year replaces organic matter and a good share of nutrients, and it improves the soil itself. Fertiliser sits on top of that foundation, supplying the extra nutrients that heavy-feeding vegetables need through the season. Neither one replaces the other. A bed with compost but no fertiliser will grow decent vegetables; a bed with fertiliser but no compost will grow fast for a season and then decline as the soil itself degrades.
What vegetables need
Plants need three nutrients in the largest amounts, shown as N-P-K on fertiliser packets: nitrogen (N) for leafy growth, phosphorus (P) for roots and fruiting, and potassium (K) for general health, flowering and fruit quality. They also need calcium, magnesium and sulfur in smaller amounts, and trace elements such as iron, manganese, zinc and boron in tiny amounts. A balanced fertiliser supplies all three major nutrients; a specialist fertiliser supplies more of one than the others.
Different vegetables need different balances. Leafy vegetables such as lettuce, spinach and cabbage need plenty of nitrogen for leaves. Fruiting vegetables such as tomatoes, cucumbers and peppers need more potassium and phosphorus once they start flowering and fruiting, and too much nitrogen at that stage gives lush leaves and little fruit. Root crops such as carrots and potatoes need potassium and phosphorus for root and tuber development, and too much fresh nitrogen gives forked roots and leafy tops with small roots. Legumes such as peas and beans fix their own nitrogen, so they need little nitrogen fertiliser and mainly benefit from phosphorus and potassium.
A soil test is the only way to know what your soil is actually missing. Test every one to two years through your local extension service, and fertilise on the basis of the results rather than habit. A bed that already has plenty of phosphorus does not need more, and adding it wastes money and can lock up iron and zinc.
When to fertilize vegetables
Feed in three stages. Before planting, work compost and, if needed, a balanced granular fertiliser into the bed two to three weeks before sowing or planting. A typical rate for a balanced granular fertiliser is roughly 70–100 g per square metre, but always follow the packet rate for the product you use — rates vary widely between products. At planting, add a small amount of fertiliser to the planting hole for heavy feeders, or water seedlings in with a half-strength liquid feed. Through the season, feed heavy feeders every two to four weeks with a liquid feed, or side-dress with granular fertiliser scratched into the soil around the plant and watered in. Stop feeding late in the season — from late summer onwards — so plants harden off before winter instead of pushing soft growth that frost damages.
Timing within the season matters. Feed leafy vegetables early, when they are growing leaves fast. Feed fruiting vegetables with a high-potassium feed once the first flowers appear and through fruiting, not before — early nitrogen gives leaves at the expense of fruit. Feed root crops lightly and early, and avoid fresh manure or high-nitrogen feed close to harvest, which can cause forked roots and poor storage. Water before and after feeding: fertiliser applied to dry soil can burn roots, and watering it in carries it to the root zone.
Organic and synthetic fertilisers
Organic fertilisers come from plant or animal material and release nutrients slowly as organisms break them down. They include compost, well-rotted manure, blood meal (high nitrogen), bone meal (phosphorus and calcium), fish emulsion (balanced, fast-acting), seaweed (potassium and trace elements), and wood ash (potassium and calcium, strongly alkaline). They improve the soil as well as feeding the plant, and they are hard to over-apply badly, though they can still burn if used in large amounts. Our guides to fish emulsion fertilizer, bone meal and blood meal cover the main ones in detail.
Synthetic fertilisers are manufactured salts that supply nutrients in exact, fast-acting forms. They work quickly and predictably, and they are useful when a soil test shows a specific deficiency. But they do not improve the soil, they can burn plants if over-applied, they can leach into groundwater, and heavy use can suppress soil life. Many gardeners use a combination: compost and organic feeds as the base, and a synthetic feed for a quick correction when a plant shows a clear deficiency.
Whichever you use, follow the packet rate. More fertiliser does not mean more growth — beyond a point it means burnt roots, lush soft growth that attracts pests, and nutrients washing into waterways. A soil test, a measured dose, and a watering can are worth more than a heavy hand with the bag.
How much fertiliser to use
| Vegetable group | Feeding approach |
|---|---|
| Leafy (lettuce, spinach, cabbage) | Compost before planting, nitrogen-rich feed every 2–3 weeks |
| Fruiting (tomatoes, cucumbers, peppers) | Balanced feed early, high-potassium feed from first flowers, every 2 weeks |
| Root crops (carrots, potatoes) | Compost before planting, light balanced feed early, avoid fresh nitrogen later |
| Legumes (peas, beans) | Compost before planting, little or no nitrogen, light potassium feed if needed |
| Brassicas (broccoli, cabbage) | Compost plus lime if acidic, balanced feed every 3–4 weeks |
| Alliums (onions, garlic) | Compost before planting, light feed early, stop feeding by midsummer |
These are starting points, not prescriptions. Your soil, your climate and your varieties all change the answer, and a soil test through your local extension service is the best guide to how much your beds actually need. Use the fertiliser calculator to work out quantities for your bed size and the product you are using.
Common mistakes
The most common mistake is over-fertilising, especially with nitrogen, which gives lush, soft, pest-prone growth and little fruit. The second is fertilising dry soil, which burns roots — always water first. The third is feeding at the wrong time: nitrogen late in the season pushes soft growth that frost kills, and high-nitrogen feed on fruiting plants gives leaves instead of fruit. The fourth is using fresh manure on growing crops, which burns roots and can carry pathogens — compost manure for several months first, and never apply fresh manure to edible crops near harvest. The fifth is fertilising without a soil test, year after year, so phosphorus builds up to levels that lock up other nutrients. And the sixth is relying on fertiliser alone while neglecting compost, so the soil itself declines even as the plants are fed.
Safety notes
Wear gloves when handling fertiliser and manure, and wash your hands before eating. Store fertiliser in a dry place away from children and pets, and keep it in its original labelled container. Do not mix different fertilisers together unless the labels say it is safe — lime and ammonium fertilisers react and release ammonia gas. Wash all vegetables before eating, and observe the harvest interval on any product label. If you are unsure about a product or a rate, check your local extension service for advice.
Frequently asked questions
How often should I fertilize vegetables?
Heavy feeders such as tomatoes, cucumbers and cabbage benefit from feeding every two to four weeks through the growing season. Light feeders such as peas, beans and root crops need less — compost before planting and a light feed early in the season is usually enough. Always follow the product rate and stop feeding late in the season.
Is compost enough, or do vegetables need fertiliser too?
Compost is the foundation and supplies a good share of nutrients, but heavy-feeding vegetables usually need extra feeding through the season as well. Think of compost as the soil improver and fertiliser as the top-up: use both, and let a soil test tell you how much top-up your beds need.
What is the best fertiliser for tomatoes?
Tomatoes need a balanced feed early for growth, then a high-potassium feed once the first flowers appear and through fruiting. Too much nitrogen after flowering gives lush leaves and little fruit. Feed every two weeks through the season and water before and after feeding.
Can I over-fertilize vegetables?
Yes, easily. Over-fertilising burns roots, produces soft pest-prone growth, and washes nutrients into waterways. Follow the packet rate, water before and after feeding, and do not feed more often than the product recommends. A soil test helps you avoid adding nutrients your soil already has.
Should I use organic or synthetic fertiliser?
Both work. Organic fertilisers release nutrients slowly and improve the soil, and they are hard to over-apply badly. Synthetic fertilisers work quickly and are useful for correcting a specific deficiency shown by a soil test. Many gardeners use compost and organic feeds as the base and a synthetic feed for quick corrections.