Gardening Questions

All You Need to Know About Fertilisers

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A thriving garden starts with getting the basics right. Plants need suitable light, the right amount of water and a steady supply of nutrients to support healthy growth.

You have chosen the right spot and worked out when to water, but how do you know your plants are getting the nutrients they need? That is where fertiliser comes in. The right fertiliser can supplement the nutrients available in the soil or potting mix, helping plants grow healthy roots, foliage, flowers and fruit.

Choosing a fertiliser does not need to be complicated. Once you understand what is on the label and what your plants need, it becomes much easier to find the right option and apply it with confidence.

What this fertiliser guide covers

In this guide, we will take a closer look at:

The main types of fertiliser and how they work
How to understand NPK and fertiliser ratios
How to choose the right fertiliser for your plants
When and how to apply fertiliser
How soil pH can affect nutrient availability

What you need to know about fertilisers

Types of fertiliser

The most common types available:

Organic based - either certified 100% organic or including organic matter.
Composts and soil improvers.
'Instant hit' powder, granular and liquid fertilisers.
Slow-release fertilisers.
Controlled release fertilisers.

Learn more about the frequency, pros and cons of each.

The benefit of Osmocote slow release fertiliser

The benefits of Scotts Osmocote Controlled Release Fertilisers, include:

Easy to apply
Feeds for up to 6-months
Releases nutrients only when plants need them - more when it's warm and plants are growing and less when it's cold and growth is slow
Scotts Osmocote® uses advanced prill technology that ensures even and controlled nutrient release with no wastage
Wetting agent improves water absorption, helping plants take up nutrients
Less excess and leaching, helping keep our environment healthy

Osmocote Controlled Release Fertiliser Benefit

Understanding fertiliser ratios and NPK fertilisers

Fertilisers are combinations of essential elements (nutrients) that have been mixed at specific ratios to encourage plants to grow, flower or fruit.

Fertiliser manufacturers list these ratios on the packets, usually shown as N:P:K or Nitrogen (N), Phosphorus (P) and Potassium (K). These are the macro or major chemical elements that directly impact plant growth and development.
Plants also require micro nutrients and trace elements to maintain optimal health. To see what these are and why they are important to plants, look at Fertiliser Components and their Uses.

Most fertiliser packaging will include a Guaranteed Analysis that lists all major, minor and trace elements included and their relevant percentages or parts per million.

The importance of soil pH

The pH of your soil determines the availability of some nutrients
Test your pH every three to four years and adjust accordingly
pH should be between 5.5 and 6.5 for most flowering trees and shrubs.

To find out more about soil pH and healthy soils read our story How to establish a new garden bed.

What fertiliser and when

Choosing the best fertiliser for your plant type

High nitrogen fertilisers for young and newly planted trees

What: Use a high nitrogen fertiliser to help develop strong growth.

Note: Additional applications of chicken manure will also be of benefit.

Acidic fertilisers for acid-lovers like gardenias, camellias, roses and blueberries

What: Use a fertiliser like Scotts Osmocote Controlled Release Fertiliser for Roses, Gardenias, Azaleas & Camellias which has been specifically designed to give acid-loving plants the nutrients they need to flourish.

Note: Use a controlled release fertiliser to ensure the right nutrients are available at the right amount for your plant.

Native fertilisers high in phosphorous for native trees and shrubs

What: Use a fertiliser designed for phosphorus sensitive plants.

Note: Native plants may be hardy, but they still need feeding. As they are accustomed to phosphorus-deficient soils, ensure that you feed them with low P fertilisers.

High potassium fertilisers for flowering plants

What: To encourage flowering, most plants benefit from a fertiliser high in potassium, like Scotts Osmocote Pots & Planters Controlled Release Fertiliser. This is best applied at the start of Spring and Autumn. Use a liquid

Note: Use an all-purpose controlled release fertiliser twice a year, and apply liquid feeds throughout spring and summer for maximum wow!