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, including anyone new to hydroponics (start with our beginner's guide to hydroponics). 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, EC and water quality.

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

What is NPK fertiliser, and what does NPK stand for?

NPK fertiliser is any plant feed whose label declares nitrogen (N), phosphorus (P) and potassium (K), the three primary macronutrients plants use in the largest amounts. The symbol K comes from kalium, the Latin-derived name for potassium. Add calcium (Ca), magnesium (Mg) and sulphur (S) and you have the six macronutrients for plants; "NPK" is shorthand for the first three.

The label 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, whether it is a single complete bottle or a two-part grow-and-bloom system.

What types of NPK fertiliser are there, and how do you apply them?

The RHS splits fertilisers into a few types:

• Straight fertilisers supply one nutrient, or mostly one, such as sulphate of potash or Epsom salts. They fix a specific shortfall.
• Compound fertilisers carry several nutrients in one product and are sold as all-purpose or general-purpose feeds. This is where a full NPK ratio lives.
• Slow-release (controlled-release) fertilisers are granules or pellets that let nutrients out gradually, which suits potted compost better than a recirculating reservoir.

Liquid feeds act quickly and are measured into the watering can or reservoir. Granules and powders work more slowly and go into the medium. Liquid feed is how indoor growers apply NPK, and the same idea underpins hydroponic nutrients; our guide to choosing a nutrient line compares the ranges.

More is not better. The RHS lists dark green leaves and stunted growth as signs of too much nitrogen. If you suspect overfeeding, flush and check pH: see pH, EC and water quality.

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 on nutrient deficiencies.

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. For a wider symptom check, see our troubleshooting guide to yellow leaves, brown spots and root rot, and for lockout and flushing, deficiencies, lockout and flushing.

Do plants need more than NPK?

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

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, so your tap water often carries calcium and magnesium already. Second, complete base nutrients from reputable brands include the secondary and trace elements, so you add a separate Cal-Mag supplement for plants mainly when growing in coconut coir (coco) or using RO or soft water, and usually not in soil with hard tap water. Options include Canna Calmag Agent and Dutch Pro Calmag; pH, EC and water quality covers how to dose it. A good base feed plus the right water is most of the battle.

How do you read a nutrient label?

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 3-1-4).

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. Suppliers quote slightly different figures because phosphate and potash are expressed as oxides.
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 Canna Terra Flores is the bloom feed with the ratio shifted towards P and K.

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; our guide to root-zone additives covers enzymes, microbes and silicon. Mineral feeds (like Canna and Dutch Pro base nutrients) deliver nutrients in immediately plant-available form.

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: match the feed to your medium (our guide to soil or coco helps you choose) and how hands-on you want to be. For a simple one-part feeding system, see our FloraMax nutrient guide.

How should feed change by stage, from veg to bloom?

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.

Exact ratios vary by brand and medium, so treat the figures above as the direction of travel, not a prescription: 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 (a pH and EC tester makes it easy) so the plant can actually take up what you give it. For timing and dosing, see PK boosters and the full-cycle feeding schedule.

FAQ

What is a good NPK ratio? There is no single best ratio; it depends on the growth stage. Use a higher-nitrogen "grow" feed (around 3-1-3) in veg, then switch to a higher-PK "bloom" feed 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? Use a bloom feed with phosphorus and potassium emphasised over nitrogen (something in the 1-3-4 direction), often topped up with a PK booster in mid-to-late bloom. High nitrogen in flower encourages leaf growth at the expense of buds.

Is liquid or granular NPK fertiliser better? It depends on how you grow. Liquid feeds act quickly and are measured into the watering can or reservoir, which is why indoor and hydroponic growers favour them. Granules and powders work more slowly and suit potted compost.

What is the meaning of 20-20-20 NPK? A 20-20-20 product has equal parts nitrogen, phosphate and potash, so it is sold as a balanced, all-purpose feed. It works 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? Usually not in soil with hard tap water, which already carries calcium and magnesium. If you grow in coconut coir or use RO or soft water, a Cal-Mag supplement is usually needed because coir holds onto calcium and soft water supplies little. Our pH, EC and water quality guide has the detail.

Nutrients at The Grow Superstore

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

• Canna Nutrients — mineral base feeds for soil, coco and hydro, including Canna Terra Vega (grow) and Canna 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 Dutch Pro 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.

Why trust The Grow Superstore

We have been supplying UK indoor and hydroponic growers since 2009, from our physical store in Peterborough (6 Wulfric Square, Bretton, PE3 8RF). 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.