Fertiliser Types Explained: What Actually Suits Scottish Soil

Stand in front of the fertiliser shelf at any garden centre and you'll find the same wall of options: a sack of pellets that smells faintly of the farmyard, a tub of blue crystals promising instant results, a bottle of murky brown liquid labelled "seaweed extract," and half a dozen boxes stamped with three numbers separated by dashes, 7-7-7, 5-5-6, 4-3-2. None of it is labelled "for tomatoes" or "for your veg bed in Fife." You're left guessing, and the guess usually comes down to whichever bag is nearest the till.

The three numbers are the place to start, because everything else, organic or synthetic, granular or liquid, when to reach for which, follows from what they mean and how fast each type releases what it promises.

What the NPK numbers actually mean

Every fertiliser packet carries three numbers in a fixed order: N-P-K, for nitrogen, phosphorus and potassium. They state the percentage, by weight, of each nutrient in the product. A bag marked 7-7-7 is 7% nitrogen, 7% phosphorus and 7% potassium by weight; the rest is filler, carrier or, in organic products, the other organic matter the nutrients are bound up in.

Each nutrient does a distinct job:

• Nitrogen (N) drives leafy, vegetative growth, stems and foliage. It's the nutrient plants use fastest and, because it's so water-soluble, the one most often washed out of Scottish soil before a plant gets the chance to use it (more on that below).
• Phosphorus (P) supports root and shoot development, particularly useful when plants are establishing.
• Potassium (K) governs flowering and fruiting.

A high first number pushes leaf growth, good for salad crops and lawns, less useful for anything you want to flower or fruit. A high third number does the opposite: it's why tomato and fruit feeds lean potassium-heavy. A balanced ratio like 7-7-7 doesn't specialise in anything; it's a general maintenance feed rather than a tool for a specific job.

If growth still looks weak despite feeding correctly, the nutrients may be locked out by soil pH rather than absent, that's a separate question, covered in the soil type and liming articles. One deficiency NPK numbers don't cover at all is magnesium, a common problem in tomatoes and raspberries specifically, corrected with a diluted Epsom salts spray rather than more of a standard N-P-K feed.

Organic vs synthetic: not just a lifestyle choice

The organic/synthetic split matters less for ideology than for release speed, and release speed is where Scotland's climate actually changes the calculation.

Organic fertilisers, chicken manure pellets, blood fish and bone, well-rotted manure, comfrey feed, release nutrients as soil organisms break down the organic material. That process is gradual, temperature-dependent, and roughly tracks what a growing plant can use week to week. Little of the nutrient content is instantly water-soluble at the moment of application.

Synthetic (inorganic) fertilisers, Growmore, sulphate of potash, most blue crystalline "instant" feeds, are manufactured to be immediately soluble. Applied to soil or dissolved in a watering can, most of the nutrient content is available to plant roots within days, sometimes hours.

That speed is exactly the problem on Scotland's wetter, free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → sites. Nitrogen in particular is highly water-soluble, and Scotland's rainfall runs well above the driest parts of England, the Met Office's own regional rainfall figures put parts of western Scotland above 2,000mm a year against under 700mm on parts of England's eastern coast, the same west-to-east Atlantic weather pattern that shapes rainfall right across the UK. Apply a fast-acting synthetic feed to a sandy or light soil during a wet spell, and a meaningful share of that nitrogen leaches, gets washed down through the soil by rain, past the plant's roots and out of reach, before the plant has taken it up: money and nutrient lost to the drain, not the crop. A slow-release organic feed sidesteps this because it never dumps a large soluble dose at once: it trickles nutrient out at roughly the pace roots absorb it, so there's less to lose even when the ground is sodden.

This isn't a blanket case for organic-only gardening. Heavy clay is made of much finer particles than sand, with a larger surface area that holds onto dissolved nutrients rather than letting rain carry them straight through, so a clay-soil garden in a wet part of Scotland doesn't face the same leaching risk as a sandy one. The real variable is soil type combined with rainfall, not rainfall alone, free-draining soil in a wet climate is the specific combination where fast-release synthetic feeds lose the most to leaching, and where organic or genuinely slow-release products earn their keep.

There's a separate timing trade-off worth knowing, on the opposite kind of ground to the free-draining soil above. Organic feeds rely on soil organisms to break the material down, and those organisms slow down in cold, wet soil, which describes heavy clay or poorly-drained ground more than it does light, free-draining soil: waterlogged ground holds onto winter cold and takes longer to warm in spring than soil that sheds water freely. On heavy, slow-to-warm ground, an organic feed can sit largely inert for longer than the label suggests, releasing little just when new growth wants it most. A synthetic starter feed at planting, applied a little earlier than the packet's general guidance suggests, closes that gap without abandoning organic feeds for the rest of the season.

Granular, liquid or slow-release: matching format to the job

Gardener's World notes that liquid feeds are the safer choice for pots and containers specifically because solid granular feed applied to the compost surface is easy to over-apply and can scorch roots, whereas containers also have no soil reservoir to dilute or buffer an excess the way open ground does.

Granular feeds don't come with a universal "how much" figure the way the table above might suggest, the right rate depends entirely on the specific product's strength, so follow the rate printed on the bag rather than a general rule, and re-apply only at the interval it states (typically once at planting, then again every 6-8 weeks through the growing season for most granular organic feeds). Chicken pellets, blood fish and bone, Growmore, comfrey and seaweed feed are all sold and commonly used on edible crops as well as ornamentals, though fresh, un-composted manure (not the same as pelleted chicken manure) should be kept off crops close to harvest.

There's also a straightforward application-method vocabulary worth knowing: top dressing (scattering fertiliser on the surface around an established plant) and base dressing (working fertiliser into the soil or compost before planting).

Five common feeds, compared

This is where the packet numbers get concrete, actual ratios vary by brand and batch, so treat the figures below as a guide and check the bag for the exact one you've bought.

Coastal gardeners have a sixth option the table above doesn't cover: fresh seaweed gathered from the shore, dug straight into soil over winter or spring at a spade or two deep without needing to rot down first, much as manure would be worked in. There's no automatic right to take it, though, check with the landowner and local council before gathering, since not every beach permits it.

Comfrey feed's exact nutrient content is genuinely variable, Garden Organic's own trials found meaningful differences between growing sites, leaf age, and even whether the leaves were soaked or compressed, so treat any precise percentage quoted online with scepticism; what's consistent is that it behaves like a potassium-rich liquid tomato feed once brewed for around a month, and that letting it brew longer than 30 days doesn't add anything. Seaweed products, meanwhile, are valued less for their NPK numbers, which are modest and inconsistent across brands, than for the trace elements they supply that Growmore and blood fish and bone typically lack.

Putting it together for a Scottish plot

None of these compete head-to-head; gardeners typically layer them through the season rather than picking just one.

The same leaching that can waste a fast feed on light soil has a knock-on downstream: excess nitrogen that isn't taken up by plant roots doesn't just vanish, it leaches into groundwater or runs off the land into rivers, lochs and burns, harming water quality. Scotland's Nitrate Vulnerable Zone rules on keeping fertiliser and manure away from watercourses are written for farms rather than back gardens, but the same principle is worth keeping in a domestic plot: don't apply granular or liquid feed right at the edge of a garden pond or a burn running through the ground, and clean up any spill on a hard surface rather than hosing it toward a drain.

Nitrogen feeding also has a cut-off point worth building into the calendar rather than feeding right up to autumn. Nitrogen applied too late in the season pushes soft, sappy new growth that hasn't had time to harden offGradually getting young plants raised indoors used to outdoor conditions over 7–14 days before planting out, so cold, wind and direct sun don't shock or damage them. Full definition → before the first frost, worth building extra margin into on a plot with an early first frost date (check your own area against the frost dates page). Stop nitrogen-heavy feeding by mid-to-late summer on shrubs, perennials and fruiting crops, switching to a potassium-led feed if anything, rather than continuing a balanced or nitrogen-heavy feed into autumn.

Common questions

Sources