The three numbers on a fertiliser pack are the proportions of nitrogen, phosphorus and potassium (N-P-K). Match them to what the plant is doing: nitrogen for leaf and stem, phosphorus for roots and shoots, potassium for flowers and fruit. Then match the release speed to your ground. On light, free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → soil in a wet part of Scotland, a fast synthetic feed can lose much of its nitrogen to rain, and a slow organic feed is the safer base.
What the NPK numbers mean
Every packet gives the three numbers in a fixed order: nitrogen, phosphorus, potassium. They are the proportions of each nutrient in the product, and a high number means a higher concentration of that nutrient, not a better feed. The jobs differ:
- Nitrogen (N) supports vegetative growth, leaves and stems. Plants use it fastest, and being very water-soluble it is the nutrient most easily washed out by rain or watering.
- Phosphorus (P) supports healthy root and shoot growth, which helps when plants are establishing.
- Potassium (K) supports flowering, fruiting and cold hardiness.
A high first number pushes leaf growth, good for salad crops and lawns but wrong for anything you want to flower or fruit. A high third number does the opposite, which is why tomato feeds lean potassium-heavy. A balanced feed is general maintenance, not a tool for a specific job.
If growth is weak despite feeding, nutrients may be locked out by soil pH rather than missing; the soil type and liming articles cover that. N-P-K also does not cover magnesium, a common deficiency in tomatoes and raspberries, which a spray of diluted Epsom salts can help with.
Organic or synthetic: release speed
The split matters less as a matter of principle than for how fast nutrient reaches the plant, and that is where Scotland's climate changes the choice.
Organic feeds (chicken manure pellets, blood fish and bone, well-rotted manure, comfrey) are made from living material and are usually less concentrated and slower acting. They depend on soil organisms to break them down, so release follows temperature and roughly tracks what a growing plant can use week to week. Synthetic feeds such as Growmore and sulphate of potash are manufactured to be soluble, so most of the nutrient is available within days.
That speed is the problem on wetter, free-draining sites. Nitrogen is easily washed out of soil by rain, and sandy soils need extra care for that reason. Rainfall across Scotland ranges widely: the Met Office puts the west Highlands at over 4,000mm a year, while the coasts of East Lothian, Fife and the Moray Firth get under 700mm. A fast synthetic feed on light soil in a wet spell can lose a meaningful share of its nitrogen below the roots before the plant uses it, which wastes money and sends nitrogen towards groundwater and watercourses. A slow-release organic feed trickles nutrient out at roughly the pace roots absorb it, so there is less to lose.
This is not a blanket case for organic only. Heavy clay is made of much finer particles than sand and holds on to dissolved nutrients better, so a clay garden in a wet area faces less leaching risk. The combination to watch is free-draining soil with high rainfall, where fast synthetic feeds lose the most and slow-release products earn their keep.
The trade-off runs the other way on cold, wet clay. Organic feeds rely on soil organisms, which slow in cold soil, so on heavy, slow-to-warm ground an organic feed may release little just when new growth wants it. A synthetic starter feed at planting, rather than waiting for the soil to warm and organic feeds to start working, can close that gap. The RHS suggests slow organic feeds go on in February to March because they act slowly, and fast-acting Growmore in March to April.
Granular, liquid or slow-release
| Format | How it works | Best for | Watch out for |
|---|---|---|---|
| Granular (dry) | Scattered on soil or worked in; breaks down over weeks | Base feeding at planting, top-dressingAdding fresh compost, feed or grit to the soil surface around an established plant, rather than digging it in. Full definition → beds, borders and veg plots | Can scorch roots or foliage if over-applied; slower to correct an acute deficiency |
| Liquid | Diluted in water and taken up through roots, or sprayed on leaves (foliar feedingA liquid fertiliser applied directly onto a plant's leaves rather than the soil, absorbed faster than a root feed. Full definition →) | Containers and pots, quick correction of a visible deficiency, regular feeding through the season | Short-lived, so needs repeating; easy to overdose containers; spray foliar feeds on dull days to avoid leaf scorch |
| Slow-release (organic or coated synthetic) | Nutrient locked in organic matter or a coated pellet and released gradually | Base dressing at bed prep, hungry perennials, where you cannot feed weekly | Slower to show results; apply earlier so release matches peak demand |
Liquid feeds are the safer choice for pots and containers, because solid feed on the compost surface is easy to over-apply and a container has no soil reservoir to buffer an excess. Granular rates depend on the product's strength, so follow the rate and interval on the bag; many granular organic feeds are applied at planting and then again every six to eight weeks through the growing season. Chicken pellets, blood fish and bone, Growmore, comfrey and seaweed feed are all used on edible crops, but keep fresh, un-composted manure away from crops close to harvest.
Two terms are worth knowing. Top dressing is scattering fertiliser on the surface around an established plant, and base dressing is working it into the soil or compost before planting.
Five common feeds compared
Ratios vary by brand and batch, so treat these as a guide and check the bag.
| Feed | Type | Approx. NPK | What it is for |
|---|---|---|---|
| Blood, fish and bone | Organic, granular, slow-release | Roughly balanced; varies by brand, check the bag | General-purpose base feed at planting; suits fruit trees and soft fruit |
| Pelleted chicken manure | Organic, granular, slow-release | Westland Organic Chicken Pellets 4.5-1.5-2.5 | Nitrogen-led feed at bed prep; improves soil structure as it breaks down |
| Growmore | Synthetic, granular, fast-acting | 7-3.1-5.8 | Quick general top-up when soil needs a fast, even boost, best on soil that will not leach it away |
| Comfrey feed (home-brewed) | Organic, liquid, fast-acting | Not precisely quantified; similar to a commercial tomato feed in Garden Organic’s trials, high in potash | Fortnightly liquid feed for tomatoes, peppers, cucumbers and other heavy fruiters through summer |
| Seaweed feed | Organic (marine), liquid or granular | Often no guaranteed ratio unless fortified; the label gives one if so | Trace-element top-up with modest N-P-K; a supplement to, not a replacement for, a base feed |
Fresh seaweed gathered from the shore can be dug into soil over winter or spring without rotting down first, as a substitute for farmyard manure. There is no public right to collect seaweed from the beach, so check with the landowner and local council first.
Comfrey feed's strength varies between sites, leaf age and how it is made, so treat any precise percentage with scepticism. Garden Organic's trials found a feed brewed for about a month had a nutrient content similar to a commercial tomato feed, with high potash, and that brewing longer than 30 days added nothing. The trial found homemade feeds generally did not need diluting, though Garden Organic's practical guide advises diluting a thick, dark concentrate, so judge by the strength of your batch. Seaweed products are valued less for their modest N-P-K than for the trace elements (iron, manganese, zinc, copper, boron) that Growmore and fish, blood and bone often lack.
Putting it together for a Scottish plot
These do not compete; gardeners usually layer them through the season.
| When | What to reach for | Why |
|---|---|---|
| At planting / bed prep | Pelleted chicken manure or blood, fish and bone | Slow, steady base feed that will not leach away before roots are established |
| Mid-season top-up on soil that drains moderately | Growmore or another fast synthetic | Quick, even boost, best on ground less prone to washing it through |
| Fortnightly once fruiting crops are established | Comfrey or seaweed liquid feed | A liquid’s speed suits a hungry, fruiting plant, with little time in the soil to leach |
| Pots and raised beds, especially under cover | Liquid feeds, or any of the above at label rates | Nutrients are less likely to wash through a container than an open, free-draining border |
Nitrogen the plant does not take up can leach into groundwater or run off into burns, lochs and rivers. Nitrate vulnerable zone rules in Scotland are written for farms, but the principle holds in a garden: do not apply granular or liquid feed at the edge of a pond or burn, and clean up spills on hard surfaces rather than hosing them to a drain.
Nitrogen also has a cut-off. Nitrogen-rich feeds late in the season stimulate soft, sappy growth that is especially vulnerable to frost, and the RHS advises not fertilising late in the year, after September, since the new growth will not toughen up before winter. That includes high-nitrogen lawn feeds. Check your area against the frost dates page, and switch to a potassium-led feed if you feed fruiting crops late.
Common questions
Is a higher NPK number always a stronger fertiliser?
A higher number means a higher concentration of that nutrient, not a better product. A lawn feed high in nitrogen is the wrong choice for a fruiting tomato that needs potassium. Match the ratio to what the plant is doing, not the size of the numbers.
Can I use Growmore instead of a tomato feed?
Growmore will support general growth, but it will not push flowering and fruiting the way a potassium-heavy feed will. It is a reasonable base feed, not a substitute for a fruiting-stage feed such as a tomato feed or comfrey feed.
Does “organic is slower” mean it is weaker?
No. Total nutrient can be just as high; the release speed differs. On heavy clay or in a container, where leaching is less of a risk, a fast synthetic feed’s speed is an advantage.
Can I over-feed with organic fertilisers?
Less easily, because the nutrient is not immediately soluble, but fresh, un-composted manure or over-applied pellets can still cause root scorch or excess nitrogen. Follow the stated rate.
Is seaweed feed worth using if its NPK is low?
Its value is the trace elements that Growmore and fish, blood and bone often lack, not the N-P-K line. Treat it as a supplement alongside a base feed.
Sources
Show the 7 sources
- RHS: Fertilisers, types and uses: NPK ratio; nitrogen, phosphorus and potassium functions; organic feeds less concentrated and slower acting; balanced, straight and slow-release feeds; Growmore 7-3.1-5.8; Westland Organic Chicken Pellets 4.5-1.5-2.5; slow organic feeds in February to March and Growmore March to April; liquid feeds faster acting; foliar feeds on dull days; diluted Epsom salts for magnesium deficiency in tomatoes and raspberries
- RHS: Seaweed products for gardening: Rich in trace elements; fortified products quote an NPK ratio; fresh seaweed as a substitute for farmyard manure; no public right to collect seaweed from the beach, check with landowner and council
- RHS: How to feed plants: Do not fertilise late in the year, after September; new growth will not toughen before winter; applies to high-nitrogen lawn feeds
- RHS: Nutrient deficiencies: Nitrogen easily leached by rain; sandy soils more prone to deficiency
- Garden Organic: Does comfrey liquid feed live up to its claims?: Comfrey feed brewed for a month similar to a commercial tomato feed; no gain beyond 30 days; high potash; results vary between sites
- Met Office: Eastern Scotland climate: West Highlands over 4,000mm a year; coasts of East Lothian, Fife and the Moray Firth under 700mm
- GrowScotland: Scottish frost dates: First autumn frost bands by region