Choosing the right fertiliser can feel confusing, especially when every bag or bottle has different numbers on the label.
10-10-10.
20-5-10.
3-1-6.
What do they actually mean… and how do you know which one your plants need?
This guide breaks it down simply, so you can confidently choose the right NPK fertiliser for your garden, lawn, veggies or flowers, without guesswork or overfeeding.
What Does NPK Mean on Fertiliser Labels?
NPK refers to the three primary nutrients plants need to grow well:
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N – Nitrogen: Supports leafy, green growth
-
P – Phosphorus: Encourages strong roots, flowers and fruit
-
K – Potassium: Improves plant health, resilience and disease resistance
The numbers you see on fertiliser labels show the ratio of these nutrients by weight.
For example:
10-10-10 = a balanced fertiliser with equal parts nitrogen, phosphorus and potassium.
Understanding these ratios is the key to feeding plants what they actually need, instead of stressing them with the wrong nutrients.
Are NPK Numbers a Ratio or a Percentage?
You’ll often hear people say the numbers are a “ratio”, and that’s partly true. But technically they’re shown as percentages by weight.
For example, a fertiliser labelled 10-20-10 contains:
- 10% Nitrogen
- 20% Phosphorus
- 10% Potassium
The remaining percentage isn’t “missing”, it’s made up of other ingredients like trace elements, carriers, or sometimes additional nutrients.
So while we compare them like a ratio (because 20 is double 10), what you’re actually looking at on the bag is the percentage of each nutrient in the mix.
That’s why a 20-5-5 fertiliser isn’t just “higher nitrogen”, it literally contains four times more nitrogen than phosphorus or potassium.
Understanding this makes it much easier to compare products properly.
What If There’s a Fourth Number?
Sometimes you’ll see four numbers instead of three, for example 10-5-5-4.
The fourth number usually represents Sulfur (S), which helps with protein formation and nutrient uptake.
And if you’ve ever added up the numbers and noticed they don’t equal 100%, that’s completely normal.
The rest of the product is made up of:
- Organic matter or composted materials
- Fillers or carriers that help with application
- Secondary nutrients (like calcium or magnesium)
- Trace elements (like iron or zinc)
So NPK tells you the primary nutrients — not the entire story of what’s in the bag.
Why Choosing the Right NPK Matters
Plants don’t all grow the same way — and they don’t all need the same nutrients at the same time.
Using the wrong NPK ratio can lead to:
- Lots of leaves but no flowers
- Weak roots and poor fruiting
- Burnt plants from overfeeding
- Wasted fertiliser that never gets used
The right NPK, applied correctly, helps your garden:
- Grow faster and stronger
- Flower more reliably
- Produce better harvests
- Stay healthier through heat, cold and stress
How to Choose the Right NPK for Different Plants
Lawns & Leafy Growth
If your goal is lush green growth, nitrogen is the priority.
- Best NPK: Higher N, moderate K, low P
- Ideal for: Lawns, leafy greens, hedges
A steady feeding approach works best here, especially in Australian conditions where growth can surge quickly after rain.
Vegetables & Productive Gardens
Veggies need balanced nutrition, but their needs change as they grow.
- Early growth: More nitrogen for leaf development
- Flowering & fruiting: Increased potassium for yield and quality
A balanced NPK is ideal, with the flexibility to top up nutrients as needed.
Flowers, Fruit & Blooming Plants
If your plants are leafy but not flowering, the issue is often too much nitrogen and not enough potassium.
- Best NPK: Lower N, higher K
- Ideal for: Roses, ornamentals, fruit trees, flowering natives
Potassium supports bud formation, colour intensity and flower longevity.
👉 For targeted bloom support, explore
Our handpicked Flower Boosters & Enhancers
designed to promote stronger flowers and better fruit set.
Common Gardening Goals and Typical NPK Direction
If you’re unsure where to start, here’s a general guide:
Leafy growth (lawns, leafy vegetables):
Higher nitrogen (first number larger).
Root development (young plants, root vegetables):
Balanced fertiliser or slightly higher phosphorus early on.
Flowering and fruiting plants:
Moderate nitrogen, higher potassium to support blooms and fruit set.
These aren’t strict rules — but they give you a reliable starting point.
Liquid vs Slow-Release Fertilisers (Which Is Better?)
Both have their place — and the best gardens often use a combination of both.
Liquid & Foliar Fertilisers
Best for:
- Fast results
- Correcting deficiencies
- Supporting plants during stress or rapid growth
They’re absorbed quickly and are perfect when plants need nutrients now.
👉 Ideal options can be found in
Liquid & Foliar Solutions
Slow-Release Fertilisers
Best for:
- Long-term feeding
- Lawns and garden beds
- Low-maintenance gardens
They release nutrients gradually, reducing the risk of burn and nutrient runoff.
👉 A reliable option for Australian gardens is
GreenSpace Slow Release Fertiliser
Organic vs Synthetic Fertilisers
Release speed and nutrient source aren’t the same thing.
You can have organic slow-release, organic liquid, synthetic slow-release, or synthetic liquid fertilisers.
Here’s the difference:
Organic fertilisers
- Usually have lower NPK numbers
- Feed soil microbes as well as plants
- Improve soil structure over time
- Release nutrients more gradually
Synthetic fertilisers
- Deliver precise, measurable nutrients
- Act quickly
- Can burn plants if overapplied
- May leach faster in sandy soils
If you’re building long-term soil health, organic inputs play an important role.
If you need a fast correction, synthetic fertilisers can be useful when applied carefully.
👉 If you prefer feeding naturally while improving soil biology, explore our range of All Organic Liquid Fertilisers designed for Australian gardens.
When Should You Apply Fertiliser?
Even the right NPK won’t help if it’s applied at the wrong time.
Here’s a simple guide for Australian conditions:
During active growth (spring and early summer):
This is when most plants benefit from regular feeding.
During dormancy (late autumn and winter for many plants):
Avoid fertilising. Plants aren’t actively growing, so nutrients go to waste.
After heavy rain:
Nutrients can leach from soil, especially in sandy areas. A light top-up may help once the soil settles.
During extreme heat:
Avoid pushing growth during heatwaves. Plants are stressed and can burn more easily.
Pots vs in-ground:
Potted plants need feeding more often because nutrients wash out faster.
Timing matters just as much as the numbers on the label.
How to Apply Fertiliser Correctly (And Avoid Common Mistakes)
Even the right NPK can cause problems if applied incorrectly.
Key Tips for Best Results
- Always water plants after fertilising
- Follow label rates — more is not better
- Avoid fertilising stressed or heat-affected plants
- Feed little and often rather than all at once
Common Fertiliser Mistakes
- Overusing nitrogen and getting leafy plants with no flowers
- Applying fertiliser to dry soil
- Ignoring seasonal growth patterns
- Using one fertiliser for every plant
Matching NPK to plant type and growth stage prevents most issues before they start.
NPK Isn’t the Whole Story
If you’ve chosen the right NPK but your plants still don’t look right, the issue might not be nitrogen, phosphorus or potassium at all.
Plants also need:
- Calcium (strong cell walls)
- Magnesium (chlorophyll production)
- Iron (prevents yellowing between veins)
- Other trace elements in small amounts
For example, if new leaves are yellow but the veins stay green, that’s often an iron deficiency — not a nitrogen problem.
Container plants are especially prone to micronutrient deficiencies because they can’t draw nutrients from deep soil reserves. What’s in the pot is all they get.
So if growth looks odd despite feeding, it may be time to look beyond NPK.
If you’re growing in controlled systems or soilless media, precision feeding becomes even more important — our specialised Hydroponic Nutrients are formulated to deliver balanced nutrition where soil isn’t part of the equation.
Do You Need to Test Your Soil?
Soil testing can be helpful, but most home gardeners can get excellent results by:
- Observing plant growth
- Matching fertiliser to plant type
- Adjusting feeding during flowering or fruiting stages
If plants look healthy, green and productive — your NPK choice is working.
When Is a Soil Test Worth It?
A soil test is especially useful if:
- You’re starting a new garden bed
- Plants repeatedly struggle despite fertilising
- Leaves are yellowing but growth is stunted
- You’re growing heavy feeders like vegetables
pH also affects how nutrients are absorbed.
- Soil too acidic can limit availability of certain nutrients
- Soil too alkaline can lock out iron and other trace elements
The simple order is:
Test → adjust pH if needed → then choose your NPK.
Otherwise you might be feeding nutrients the plant physically can’t absorb.
Choosing the Right NPK: The Simple Rule
If you remember just one thing, remember this:
- Leaves = Nitrogen
- Roots & Flowers = Phosphorus
- Strength & Blooms = Potassium
Match the fertiliser to what your plant is trying to do right now — and your garden will reward you.
Special Considerations for Australian Gardens
Australia isn’t one-size-fits-all when it comes to fertiliser.
Phosphorus-Sensitive Natives
Many Australian native plants are sensitive to high phosphorus levels. Using a standard high-P fertiliser can damage roots.
If you’re feeding natives, choose a low-phosphorus or native-specific fertiliser.
Heavy Rain & Nutrient Leaching
In areas with intense downpours, nutrients can wash through sandy soils quickly.
Avoid fertilising right before heavy rain, and consider slow-release options for longer feeding.
Heat Stress
During extreme heat, plants slow down. Forcing growth with high nitrogen during heatwaves can cause stress or burn.
In very hot periods, reduce feeding strength or wait until temperatures stabilise.
Understanding your climate is just as important as understanding your NPK.
Final Thoughts
Choosing the right NPK doesn’t need to be complicated.
When you understand:
- What the numbers mean
- What your plants actually need
- When to use liquid vs slow-release fertilisers
You can feed with confidence — and grow healthier, more productive plants year-round.
Explore the full range of feeding solutions at Dr Greenthumbs to support every stage of growth:
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