Fertilising plants: a guide for gardeners

Gardening is a wonderful activity that positively impacts your physical and mental health while beautifying your outdoor living space. It’s a great way to get in touch with nature — you can even grow your own food. But for excellent results, you need to feed your soil. 

Because what you produce is only ever as good as the earth you grow it in, and if you’ve let your soil go to rack and ruin, expect poor results. 

This article is all about fertilising plants so that they thrive. We’ll cover why it’s necessary, what the information on the packet means, and how and when to apply the correct fertiliser for the best results.

And just for balance, we’ll also look at the common mistakes people make when fertilising their gardens. 

Ready? Let’s get started. 

Why plant fertilising is essential for healthy growth

Plants need essential nutrients to help them thrive — drawing nourishment through their roots from the earth. And while these nutrients are naturally occurring, heavily-used flowerbeds, turf, and veg patches become depleted over time. 

And that’s where fertilising comes in to save the day. Fertilisers are an excellent way to re-enrich the soil, replacing the nourishing compounds that your plants need. 

Additionally, fertilising improves the soil structure to facilitate better water and oxygen absorption for your plants.

NPK — The 3 Essential Active Ingredients of fertiliser

Not all fertilisers are the same, so there are very few one-size-fits-all approaches to feeding your soil. And while some products claim to be All-Purpose fertilisers, you’ll only get optimal results if you address the needs of the specific plants you’re aiming to feed.   

The most essential nutrients contained in most fertilisers are: 

• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)

And while other ingredients, such as lime, magnesium oxide, and sulfur, are contained in many fertilisers, it’s the NPK content that you should really look out for. 

Because:

• Nitrogen (N) is an essential component of chlorophyll and promotes foliage growth
• Phosphorus (P) supports root growth and flowering
• Potassium (K) contributes to general plant health and disease resistance

So, look at your fertiliser packet: it will display an NPK ratio (or it might just say something like 15-9-12, which would be 15% nitrogen, 9% phosphorus, and 12% potassium). 

So, a nitrogen-rich fertiliser is ideal for a growth boost in the spring. However, use a high-potassium feed if you’re preparing your plants for wintering. 

Understanding plant fertilisers

As well as understanding the NPK composition of your chosen fertiliser, there are some other points you’ll need to consider. 

Firstly, consider the plant species you’re feeding because each plant requires specific active ingredients to thrive. 

That’s not to say you’ll need a different fertiliser for every plant in the garden. An All-Purpose Fertiliser will suffice for most of the flowering plants in your garden. Still, you’ll need specialist fertilisers for ericaceous (acid-loving) plants, such as rhododendrons, or a high magnesium/potassium-rich formula for roses. 

Additionally, consider whether your fertiliser is organic or inorganic.

Organic vs inorganic fertilisers for plants

There are two principal approaches to feeding your plants:

• Organic — using a plant/mineral/animal-based composition
• Inorganic — using a chemical-based composition

Organic fertilisers

Organic fertilisers are usually slow-release formulas derived from plant, animal, and mineral sources. They improve soil structure and encourage microbial activity within the earth. 

Examples of organic fertilisers are:

• Compost — homemade or shop-bought
• Well-rotted manure — never use raw manure because the ammonia content is too high and will burn your plants
• Algae
• Bone meal, such as Fish, Blood & Bone

Inorganic fertilisers

Inorganic fertilisers are factory-produced chemical compounds offering a quick nutrient boost — especially effective after planting or transferring. 

These fertilisers are more highly-concentrated than organic fertilisers and can be used to increase soil nutrient levels quickly. 

Examples of inorganic fertilisers are:

• Synthetic fertilisers
• Chemical fertilisers

Avoid universal fertilisers when planting

Choose a fertiliser specific to the plant you’re growing, transferring, or replanting. 

As a rule of thumb:

• Vegetables need a high-nitrogen fertiliser to encourage leaf growth
• Flowers need phosphorus-rich feed to encourage root growth and flowering
• Trees and shrubs require high levels of potassium to promote plant health and disease resistance. 

When to Fertilise your plants — An Annual Plan

Several variables dictate how you approach your annual fertilising plan. 

Before diving straight in, consider:

• Is your plant currently under-fertilised or well-fed?
• Are you preparing for a specific season?
• Are you helping a plant recover from disease?

Fertilising at the right time of year

Once you’ve found the right fertiliser for your plants, the next question is how often to apply feed. Bear in mind that, just like us, plants go through different growing phases, so choose the right fertiliser appropriate to the season. 

The three main feeding windows are:

• Early spring and simmer
• Late summer/early autumn
• Late autumn

In general, plants need a specific concentration of active ingredients to survive each season. And remember, plants react to weather conditions, so might not need feeding as much when it’s cold. 

However, this all sounds super-complicated, doesn’t it? Don’t tear your hair out — here’s a seasonal overview to make things a little simpler:

Spring

In the spring, your plants wake up from their winter dormancy and need an extra dose of potassium. This strengthens their disease resistance and prepares the plant for the coming season.

Plants also enjoy a good portion of phosphorus and nitrogen to strengthen root growth and encourage leaf growth and flowering. It’s good to use a fertiliser with added magnesium oxide in spring, which will add a beautiful green to foliage. 

Apply iron sulphate to your lawn to strengthen your turf, kill off the winter moss, and prepare your grass for scarification. 

Summer

Prepare your plants for the (hopefully) hot, dry days ahead at the end of spring/early summer with a slow-release fertiliser enriched with nitrogen and potassium. 

Late summer

After a (hopefully) hot, dry summer, your plants need a little recovery time. So, support them with a potassium-rich fertiliser enriched with magnesium oxide. This stimulates cell renewal, which helps your plants fight off lurking autumnal diseases.

Early autumn

A potassium-rich fertiliser during the early days of autumn will prepare your plants for the milder temperatures and more rain. Use a potassium-rich fertiliser to give your plants a good health boost. 

Late autumn

This is the time to prepare your plants for the winter. Lime is perfect for this time of year because it helps reduce soil acidity, discouraging weed growth. It also makes it easier for your plants to absorb more nutrients in the spring. 

Winter

It’s too cold to fertilise because most plant roots become inactive and don’t need to absorb nutrients. 

So, put your feet up!

Choose the right time of day for fertilising

Early morning or evening are the best times for fertilising your plants if you’re using a granulated or slow-release feed because you’ll need to water immediately afterwards to activate the feed.

If you water during or just before intense sun, your plant’s foliage can burn, and the water required to activate the fertiliser evaporates too quickly. 

Don’t over-fertilise!

Over-fertilisation can lead to root- and foliage burn, nutrient imbalance, and poor plant growth. So, always check the instructions of your specific fertiliser to understand how regularly you should apply your feed. 

Bear in mind that the nutrient demands of each plant species could vary, so research your garden plants before you buy your fertiliser. 

Identify plant needs with the nitrate test

Conducting a nitrate test is the fastest way to determine whether your plants need feeding. So, take a soil sample and use a nitrate test strip to determine the nitrogen content of your soil. If it’s too low, it’s time to fertilise.

If the nitrogen levels are still within the target range, there’s no need to fertilise. 

However:

If the leaves are well-above target levels, dilute the soil by adding additional soil. Alternatively, you can flush out the excess nutrients with increased watering. But this isn’t without its risks: overwatering can lead to compacted soil and root rot. 

Use the eye test for plant fertilising 

An experienced eye can tell just by looking at a plant whether it needs feeding.

Look out for:

• Limp, yellowish leaves
• Leaf discolouration 
• Discoloured spots on leaves or stem

When transplanting plants into your flowerbeds, check the roots for rot. If you find unhealthy-looking roots, prune off the rotten sections before planting.

Methods of fertilising your plants

There are three principal forms of shop-bought fertiliser:

• Liquid fertiliser
• Granular fertiliser 
• Powder fertiliser

Personally, I prefer granular fertilisers because they tend to offer slow-release feeding and are easier to target around the plant stem. And they don’t get so easily blown away in the wind. 

This is the best way to apply the various fertilisers:

Liquid fertiliser

Spread liquid fertiliser with a pressure sprayer, such as MOOWY’s high-quality sprayers, with volumes of 4/7/10 litres.

Granular fertiliser

Distribute granular fertiliser by hand. 

If you’re fertilising large areas, you might opt for a hand spreader, ensuring even nutrient distribution.

Powder fertiliser

Apply powder fertiliser either by hand (make sure you wear gloves!). Alternatively, you could use a hand shovel. 

Avoid applying powder fertilising when it’s windy — the powder will blow away in the wind, and you could end up fertilising the wrong plants or even poisoning fish ponds, etc. 

Fertilising your plants — a step-by-step guide

Your fertiliser needs to get down into the roots — otherwise, it won’t be effective. 

So, ensure the soil is well-aerated, loose, and crumbly, like this:

1. Clear weeds, stones, twigs, and other debris from the soil surface
2. Loosen the surface soil with a rake or hand fork.
3. For beds: work a small amount of organic matter, such as well-rotted compost, into the soil. This helps improve the soil structure and improves the nutrient potential. For lawns: you could use a scarifying rake, which will also remove the thatch layer that could suffocate the roots.  
4. Apply your fertiliser of choice. 
5. Water well. This helps remove any fertiliser that may have landed on leaves, preventing leaf burn. It also helps activate the fertiliser. 
6. Wait. You should see results within a few days. 

FAQs

Why fertilise your plants?

Fertilising your plants is an essential part of your gardening regime, ensuring your plants get the nutrients they need to thrive. 

Choose the right feed for each plant, and apply it at the right time of day. Always use the recommended amount, and wear protective gear. 

With the correct fertilising regime, you’ll have a lush and healthy garden all year round! 

Ready to get started? Or do you need more info?

We hope we’ve provided all the info you need to become an expert fertiliser! But if you have questions, please don’t hesitate to get in touch.

We’ll get back to you as soon as possible. 

Thanks for reading!