The best fertilizer for tomatoes is a balanced feed at planting, followed by a potassium-rich feed once the first flowers and fruit appear. Tomatoes need nitrogen to build a strong plant early, but too much later gives lots of leaves and little fruit. Potassium drives fruit size, flavour and colour, and steady calcium supply helps prevent blossom end rot.
In short: feed for the stage the plant is in.
What tomatoes need, stage by stage
| Stage | Main need | What to use |
|---|---|---|
| Planting | Roots and a strong start | Balanced fertilizer or compost worked into the soil, with phosphorus near the roots |
| Early growth | Leaves and stems | Balanced feed; moderate nitrogen |
| Flowering and fruit set | Flowers and fruit | Lower nitrogen, more potassium |
| Fruiting and ripening | Fruit size, sugar and colour | High-potassium tomato feed |
Which NPK ratio is best?
There is no single magic number, because needs change through the season:
- At planting: a balanced ratio such as 10-10-10, or a starter with more phosphorus
- From first flowers: a ratio with more potassium than nitrogen, such as a typical liquid tomato feed
- Commercial growers: often use a balanced base fertilizer at planting, then water-soluble grades through drip, such as 19-19-19 early and 13-0-45 (potassium nitrate) during fruiting
See NPK ratios for how to read the numbers.
A simple feeding schedule
- At transplanting: mix compost and a balanced fertilizer into the planting area.
- Two to three weeks later: a light balanced feed if growth looks pale.
- When the first fruits set: switch to a high-potassium tomato feed.
- Through fruiting: feed regularly according to the product label, often every one to two weeks for liquid feeds in containers.
- Late season: reduce feeding as the last fruits ripen.
Preventing blossom end rot
Blossom end rot, a dark sunken patch at the base of the fruit, is caused by a shortage of calcium in the developing fruit. Most of the time the soil has enough calcium; the problem is that uneven watering stops the plant moving it to the fruit. To prevent it:
- Water deeply and regularly, and mulch to keep soil moisture even
- Avoid very high nitrogen and excess potassium, which compete with calcium
- Add calcium only if a soil test shows it is low, or use a calcium nitrate feed in commercial systems
Other deficiency signs
| Symptom | Likely cause |
|---|---|
| Lush leaves, few flowers | Too much nitrogen |
| Pale older leaves | Nitrogen shortage |
| Purple-tinged leaves, slow early growth | Phosphorus shortage, often in cold soil |
| Scorched leaf edges, uneven ripening | Potassium shortage |
| Yellowing between veins on older leaves | Magnesium shortage |
Organic options
- Compost and well-rotted manure at planting
- Bone meal for phosphorus and calcium at planting. See bone meal vs blood meal
- Comfrey or seaweed feeds as liquid potassium sources
- Blood, fish and bone as a general organic feed
For commercial growers and distributors
Commercial tomato production usually relies on a base NPK fertilizer, then water-soluble grades and calcium through drip irrigation, guided by soil and leaf tests. Agromer supplies NPK and other fertilizers in bulk to importers and distributors, including custom NPK formulas through our European producer partner. Send us the product, quantity, packaging and destination. See NPK fertilizer and our fertilizer range.
Frequently asked questions
What is the best fertilizer for tomatoes?
A balanced fertilizer at planting, then a high-potassium tomato feed from the first fruits onwards.
What NPK is good for tomatoes?
Balanced, such as 10-10-10, at planting. Once flowering starts, a feed with more potassium than nitrogen.
How often should I fertilize tomatoes?
Liquid tomato feeds are often used every one to two weeks during fruiting. Follow the product label.
What causes blossom end rot?
A shortage of calcium in the fruit, usually because of uneven watering rather than a lack of calcium in the soil.
Can too much fertilizer hurt tomatoes?
Yes. Too much nitrogen gives leafy plants with few fruit, and excess fertilizer can burn roots.
Is bone meal good for tomatoes?
It can help at planting by supplying phosphorus and calcium, unless your soil is alkaline or already high in phosphorus.