What to Look for When Buying Fertilisers
Buying fertiliser can feel confusing, but once you understand the basics, it becomes much easier. Fertilisers are simply mixtures of elements that plants need to grow. The key is learning to look past the headline claims and understand what’s actually inside the bag or bottle.
What Do Plants Really Need?
Plants need many elements, but the most important are:
Carbon, Hydrogen & Oxygen
These make up most of a plant’s structure.
✔️ You don’t need to buy them – plants get these free from air and water.
Nitrogen (N), Phosphorus (P) & Potassium (K)
These three nutrients are included in almost all fertilisers and are shown on the label as N-P-K.
Understanding N-P-K
Nitrogen (N) – Growth
Nitrogen makes up 78% of the Earth’s atmosphere and is essential for:
- Leaf growth
- Chlorophyll
- Proteins and enzymes
Nitrogen is often in short supply in soil, so plants grab as much as they can. Too much nitrogen causes lots of leafy growth but poor flowering or fruiting.
Think of it like pick-and-mix sweets: once you start, you don’t stop.
Too much nitrogen can actually stunt growth by blocking uptake of other nutrients.
Phosphorus (P) – Roots, Flowers & Energy
Phosphorus is mined naturally and is vital for:
- Root growth
- Buds and flowers
- Energy transport inside the plant
- Seed size
Plants absorb phosphorus best at a soil pH of 6.5–6.8. Organic matter and healthy soil life greatly improve phosphorus availability.
Potassium (K) – Strength & Health
Potassium (potash) helps with:
- Root and tuber growth
- Water regulation
- Disease resistance
- Starch and sugar formation
It improves plant strength, resilience, and overall quality.
Secondary Nutrients (Needed in Smaller Amounts)
Calcium
- Important for cell walls and growing tips
Magnesium
- Essential for chlorophyll
- Without it, plants can’t process sunlight
Sulphur
- A building block of proteins
- Found in most fertilisers made from sulphates
These usually come from soil, compost, or fertiliser mixes.
Trace Elements (Needed in Tiny Amounts)
Plants also need very small amounts of elements such as:
- Iron
- Boron
- Copper
Healthy soil with plenty of organic matter usually supplies these. Good-quality compost and fertilisers often include them.
How to Read a Fertiliser Label
All fertilisers show an N-P-K number, for example:
- 20-10-10 = 20% nitrogen, 10% phosphorus, 10% potassium
Some labels show:
- A compound (oxide) rating
- And an actual usable nutrient rating
Example:
A fertiliser labelled 7-7-7 may actually supply 7-3.1-5.8, making it a high-nitrogen feed, not truly balanced.
This is why Growmore works well in spring—most soils lack nitrogen.
Organic vs Synthetic Fertilisers
Organic Fertilisers
- Lower N-P-K numbers
- Slow release over time
- Improve soil structure
- Often contain trace elements and beneficial microbes
Don’t be put off by lower numbers—the nutrients last longer and work naturally.
Synthetic Fertilisers
- Higher N-P-K
- Fast acting
- Useful for quick results
- Need careful dosing
Fertiliser Quality Matters
Two fertilisers with the same N-P-K may not be equal:
- One may use cheap ingredients (like potassium chloride)
- Another may use higher-quality sources (like potassium sulphate)
Some “plant foods” aren’t regulated and don’t list nutrients at all.
Stick with trusted brands that clearly state N-P-K.
Simple Rules: What Should I Buy?
1. Stick to a Balanced Feed
If you’re unsure, choose:
- 2-1-1 or 3-1-2 ratios
They all work if used at the recommended rate.
You don’t need lots of specialist feeds—many are marketing-driven.
Example:
- General feed 6-3-6
- Tree & shrub feed 6-3-6
- Vegetable feed 3-2-5
You could use the same fertiliser for all of them.
Exception:
Tomato feed (e.g. 4-3-8) – extra potassium helps fruiting.
You can safely use tomato feed on other plants too.
2. Check Value for Money
Compare how much nutrient you’re actually buying.
Example:
- 1kg powder 24-18-6 at £3.95
- 1L liquid 6-3-6 at £2.95
You’d need 3 litres of liquid to match the nitrogen in the powder—making the powder much better value.
Fast vs Slow Fertilisers
Fast-Acting Fertilisers
- Water soluble
- Quick results
- Short-lived
- Can cause soft growth and pest problems
- Easily washed out
Think fast food: quick energy, but not long lasting.
Slow-Acting Fertilisers
Organic Based
- Rely on soil microbes
- Steady, gentle growth
- Healthier plants
- Best combined with organic matter
Think slow cooking: better long-term results.
Slow-Release (Coated)
- Nutrients released gradually
- Feed once and forget
- Ideal for containers
What Am I Feeding?
Plants in the Ground
- Trees, shrubs, perennials:
Use slow-release or organic fertilisers - Fruit & veg:
Higher rates needed
Replace nutrients removed at harvest
Plants in Containers
- Bedding plants:
Slow-release plus liquid feeds - Shrubs & trees in pots:
Organic + slow-release
Liquid feeds in spring
Top Tip
You don’t need lots of fertilisers.
Choose one good balanced feed, understand how it works, and apply it correctly. Healthy soil and sensible feeding always beat chasing miracle products.
