Plant Nutrients Explained: NPK, Micronutrients, and Organic vs Mineral

NPK stands for Nitrogen, Phosphorus and Potassium — the three numbers printed on every bottle of plant nutrients. Nitrogen (N) drives leafy green growth, Phosphorus (P) supports roots and flowering, and Potassium (K) underpins overall vigour, flower quality and stress resistance. Your plants also need secondary nutrients (calcium, magnesium, sulphur) and micronutrients like iron — small amounts, but non-negotiable for healthy indoor growing.

This is a fundamentals guide for UK home and hobby indoor growers. We cover what each number means, how to read a real nutrient label, the honest trade-offs between organic and mineral feeds, and how your feed should change from veg to bloom. When you are ready to pick a specific nutrient line, see our companion guide on choosing a nutrient line; for getting the most from any feed, see pH and EC explained.

Unlike outdoor garden fertiliser, indoor and hydroponic feeds are usually split into separate Grow and Bloom bottles and dosed by EC — which is why getting NPK right matters even more under lights.

What does NPK stand for?

NPK stands for Nitrogen, Phosphorus and Potassium — the three primary macronutrients plants need in the largest quantities.

The three letters break down as:

• N — Nitrogen
• P — Phosphorus
• K — Potassium (the symbol K comes from kalium, the Latin-derived name for potassium)

These are the elements behind the names; the next two sections cover what the term is shorthand for and what it means on a fertiliser label.

What is NPK, and what does NPK mean?

When growers talk about a feed's "NPK", they are using shorthand for its balance of nitrogen, phosphorus and potassium — the three elements plants consume in the largest amounts. So in everyday use, NPK means the relative proportions of those three nutrients a feed supplies: how much nitrogen versus phosphate versus potash you are giving the plant. It is the quick way growers describe and compare feeds without quoting a full ingredient list.

What is NPK fertiliser?

NPK fertiliser is any plant feed whose label declares that three-number ratio. The figures state the percentage by weight of nitrogen, available phosphate (P₂O₅) and soluble potash (K₂O) in the product — for example 3-1-3 for a grow feed or 20-20-20 for a balanced all-purpose feed. On the shelf, every nutrient bottle (Canna, Biobizz, Dutch Pro and the rest) carries this ratio, and an NPK fertiliser can be a single complete bottle or a two-part grow-and-bloom system. What makes it an NPK feed is simply that it states those three macronutrients so you can match strength and balance to your plant's stage and growing medium.

What do N, P and K each do?

Each macronutrient has a distinct job, and each leaves a recognisable calling card when it runs short. Deficiency symptoms below follow Royal Horticultural Society (RHS) guidance.

Nutrient	What it does	Classic deficiency sign
Nitrogen (N)	Drives leafy, green vegetative growth and is a core building block of chlorophyll and proteins	Pale, yellowing older/lower leaves first; spindly, stunted plants, sometimes with pink tints
Phosphorus (P)	Supports root development, energy transfer, and flower and fruit formation	Stunted growth with purple discolouration on older leaves, eventually turning dull yellow
Potassium (K)	Regulates water, activates enzymes, improves overall vigour, bloom quality and stress/disease resistance	Yellow or purple-red tints with browning that begins at the edges of mature leaves; poor flowering

A useful rule of thumb: nitrogen problems show as overall paleness on old growth; phosphorus shows as purpling; potassium shows as scorched leaf margins. Because nitrogen, phosphorus, magnesium and potassium are all mobile in the plant, their deficiencies appear on older leaves first as the plant moves nutrients up to new growth.

Secondary nutrients and micronutrients

NPK gets the headlines, but a plant that only receives N, P and K will still struggle. Two more tiers matter.

Secondary macronutrients — needed in moderate amounts:

• Calcium (Ca) — cell-wall structure and new growth. A shortfall shows as a black sunken patch at the base of fruit (blossom end rot in tomatoes, peppers and aubergines).
• Magnesium (Mg) — the central atom in every chlorophyll molecule. Deficiency shows as yellowing between the veins of older leaves, sometimes with reddish-brown tints.
• Sulphur (S) — protein and enzyme formation.

Micronutrients (trace elements) — needed in tiny amounts but essential: iron, manganese, zinc, copper, boron, molybdenum. Iron deficiency is the one indoor growers see most: yellowing between the veins of the youngest, upper leaves while the veins stay green.

Two UK-specific notes. First, much of the UK is hard-water territory; high background calcium and magnesium in tap water affects how much extra Cal-Mag you need, especially when growing in coconut coir, which naturally holds onto calcium. Second, complete base nutrients from reputable brands already include the secondary and trace elements — you usually only add a separate Cal-Mag supplement when running coir, RO/soft water, or strong lighting. A good base feed plus the right water is most of the battle.

How to read a nutrient label: a worked example

Take a real product we stock: Canna Terra Vega, a base nutrient for the vegetative (growing) phase in soil and coco. Manufacturer and supplier figures put its ratio at roughly 3-1-3 (you may also see it quoted as 3-1-4, because labels sometimes express phosphorus and potassium as their oxides — P₂O₅ and K₂O — rather than as the pure element, which nudges the numbers).

Here is how to read it:

1. Find the three numbers. Roughly 3-1-3 means about 3% nitrogen, 1% phosphate and 3% potash by weight. The rest of the bottle is water and other ingredients.
2. Read the ratio, not just the size. A grow feed is high in N relative to P — exactly what you want for building leaves and stems in veg.
3. Check the secondary and trace list. Terra Vega also supplies magnesium, sulphur and a full set of trace elements, so it is a complete base feed, not a single-nutrient additive.
4. Match it to a stage. "Vega" is a veg feed; its sister product Terra Flores is the bloom feed with the ratio shifted towards P and K.

Once you can read one label, you can read them all. A bottle marked 20-20-20 is a balanced, all-purpose feed (20% N, 20% P, 20% K) — handy as a general feed but rarely optimal for a specific stage.

Organic vs mineral nutrients — which is better?

Neither is universally "better"; they suit different growers and setups. Organic feeds (like the Biobizz range) feed soil life, which then releases nutrients to the plant. Mineral feeds (like Canna and Dutch Pro base nutrients) deliver nutrients in immediately plant-available form.

	Organic (e.g. Biobizz)	Mineral (e.g. Canna, Dutch Pro)
Pros	Forgiving, hard to overfeed; feeds soil biology; often simpler in soil	Fast-acting, precise, repeatable; ideal for hydro/coco; easy to dial in with EC
Cons	Slower to act; less precise; needs living soil to work well	Easier to overfeed; less buffering; relies on correct dosing and pH
Best for	Soil growers, beginners, a hands-off "feed the soil" approach	Coco/hydro growers wanting control, consistency and faster response

In practice, many UK indoor growers run mineral nutrients in coco or hydro for control, and organics in soil for simplicity. You do not have to be purist about it — match the feed to your medium and how hands-on you want to be.

How feed changes by stage: veg vs bloom ratios

This is the centrepiece. Plants want different ratios at different stages: more nitrogen during vegetative growth to build leaves and stems, then a shift towards phosphorus and potassium during flowering to support buds, flowers and fruit. This is why nutrient ranges come as separate Grow and Bloom bottles.

Stage	Goal	NPK emphasis	Example ratio (illustrative)	Typical products
Seedling / clone	Gentle establishment, root start	Low, balanced, very dilute	~1-1-1 (quarter strength)	Mild start feed + root stimulant
Vegetative	Leaves, stems, structure	High N, lower P/K	~3-1-3 (high-N "grow")	Canna Terra Vega, Biobizz Bio-Grow
Early bloom / transition	Set flower sites	Balanced, P/K rising	~2-2-3	Grow + Bloom overlap
Mid–late bloom	Fatten flowers/fruit	High P and K, low N	~1-3-4 (high-PK "bloom")	Canna Terra Flores, Biobizz Bio-Bloom, + a PK booster
Flush / ripen	Clear excess salts	Minimal/none	Plain or near-zero feed	Water or a flush product

Exact ratios vary by brand and medium — treat the figures above as the direction of travel, not a prescription. The principle is what matters: high-N in veg, high-PK in bloom. Always follow your specific brand's feed chart, and pair stage-based feeding with sensible pH and EC monitoring so the plant can actually take up what you give it.

FAQ

What is a good NPK ratio? There is no single best ratio — it depends on the growth stage. Use a higher-nitrogen ratio (such as a "grow" feed around 3-1-3) during vegetative growth, then switch to a higher phosphorus-and-potassium "bloom" feed (the P and K numbers rise, N falls) once flowering begins. A balanced 20-20-20 is a reasonable general-purpose feed if you only want one bottle.

What NPK should I use for flowering? For flowering, use a bloom feed where phosphorus and potassium are emphasised over nitrogen — broadly a high-PK profile (for example something in the 1-3-4 direction), often topped up with a dedicated PK booster in mid-to-late bloom. High nitrogen in flower encourages leaf growth at the expense of buds, so it is dialled back.

What does NPK fertiliser mean? NPK fertiliser is any feed labelled with three numbers showing the percentage by weight of nitrogen, phosphate and potash it contains. It tells you the strength and balance of the three primary macronutrients so you can match the feed to your plant's stage and medium.

What is the meaning of 20-20-20 NPK? A 20-20-20 product contains 20% nitrogen, 20% phosphorus and 20% potassium — an equal, balanced ratio. It is sold as an all-purpose feed because it supplies all three macronutrients in equal measure, making it versatile across veg and early bloom, though stage-specific feeds usually give better results.

Do I still need Cal-Mag if my feed already contains calcium and magnesium? Often not in soil. But if you grow in coconut coir, use RO or soft water, or run intense lighting, a Cal-Mag supplement is commonly needed because coir holds onto calcium and soft water supplies little. In hard-water areas your tap water may already cover much of the need.

Nutrients at The Grow Superstore

Once you understand NPK, choosing a feed is far easier. Browse our full Nutrients & Boosters collection — the single best starting point — or jump to a specific brand:

• Canna Nutrients — mineral base feeds for soil, coco and hydro, including Canna Terra Vega (grow) and Terra Flores (bloom).
• Biobizz Organic Nutrients — 100% organic feeds such as Bio-Grow and Bio-Bloom for soil growers.
• Dutch Pro Nutrients — mineral grow and bloom feeds plus boosters like Explode.

These are examples to illustrate the fundamentals, not a brand recommendation. For a head-to-head on which line suits your setup, see choosing a nutrient line. We offer free UK mainland delivery on orders over £50 (exclusions apply — see the live site).

---

Why trust The Grow Superstore

We have been supplying UK indoor and hydroponic growers since 2009, from our physical store in Peterborough. Our team gives in-store expert advice and consultations every day, helping growers from first grow to expert match the right nutrients to their medium, water and lighting. If you are unsure which feed suits your setup, pop in, call, or message us — practical, honest guidance, never hype.

Last reviewed: 13 June 2026. This guide covers fundamentals; always follow your nutrient manufacturer's feed chart.