Liquid, Slow Release, Organic, NPK, Macro/Micro…
If you’re new to plant care then stepping into the world of fertiliser might seem a little overwhelming at first. With so many options on the market, understanding what you need and what will work best for your plants is even more of a challenge.
But don't let that stop you - Fertilising is an important part of any Plant Parent's plant care routine. In fact, we created ourIndoor Plant Food after talking to so many of our customers and realising that many people did not know how important it is. A good fertilising regime will keep your plants looking happy and healthy, and if we can make it easier to do so then great.
This page is to help educate and answer everything and anything fertiliser related. We’ll be updating constantly, so if there is anything missing or that we’ve left unanswered, please reach out to us.
Fertiliser is not food
It is much easier for us to think of it like food as the term denotes a requirement for life. Food provides us with nutrients just like fertiliser does for plants. But plants actually create their own food through photosynthesis. Fertiliser can be considered a supplement, providing plants with the nutrients they would receive themselves in their natural environments by sending out roots and drawing them up from the soil.
Our indoor plants are not able to find those nutrients themselves. Instead, they require us to provide them in the form of fertilisers. Yes, plants can survive with just the right water and light requirements, but their health would suffer, their growth would slow, they may start to lose some of their colour and dull out, or become vulnerable to pests and diseases.
When it comes to choosing a fertiliser, it is essential to look at the N:P:K ratios. NPK are the symbols for the three most important (but not only!) nutrients our plants require.
They stand for:
(N) Nitrogen, (P) Phosphorus, (K) Potassium
They are the essential three macronutrients for plants. Each of those letters will have a corresponding number next to it. The higher those numbers are, the greater percentage of that nutrient is in the fertiliser.
Examples might be:
N:P:K 10:10:10, NPK 12:2:12, or even just 30:0:10.
These are all different percentage ratios of N:P:K designed for different purposes and plants.
A good fertiliser (what is often referred to as a ‘Complete Fertiliser’) will also contain a number of micronutrients (sometimes referred to as Trace Elements). These micronutrients are additional nutrients that plants still require, just in smaller doses than the macronutrients listed above . They all have a role and will aid plant development.
Macronutrients often included in fertilisers include:
(B) Boron, (Fe) Iron, (Mn) Manganese, (Zn) Zinc, (Cu) Copper, (Mo) Molybdenum, (Mg) Magnesium, (Ca) Calcium
Purchasing a complete fertiliser is key. While some fertilisers will promote the NPK aspect, if it doesn’t add the macro (or trace) nutrients then your plants will still be missing out on everything they need.
If you can’t find the NPK or macros on the label, consider it a red flag and avoid!
Picking a NPK ratio that works for you and your plants will obviously require some degree of understanding in terms of the function of each nutrient. We’ve covered the big three below:
Nitrogen- Foliage. This nutrient is primarily responsible for leaf growth as it is a major component of chlorophyll (the compound used by plants to photosynthesize). Plants absorb more nitrogen than any other nutrient. Plants that are deficient in nitrogen will experience stunted leaf growth.
Phosphorus - Roots, flowers and fruits. You'll notice fertilisers for flowering and fruiting plants (think citrus feeds) will have a higher number for phosphorus in it. This is because phosphorus is primarily associated with general plant health and vigour. Plants deficient in phosphorus will see new shoot and root growth inhibited with reddish colouring developing on older leaves first.
Potassium - Staying strong and powering on. Potassium helps strengthen the plant's ability to fend off pests and diseases, as well as protecting it against cold or dry weather and build up reserves for dormancy. It keeps the plant's functions performing as they should and works by aiding the movement of water, nutrients and carbohydrates around the plant tissue. Potassium deficiency will show up as brown scorching and curling on leaf tips and yellowing in between the leaf veins (otherwise known as chlorosis)
There are a number of different ways to fertilise your plants, and understanding how they work will help you decide what's best for your situation. Go into any decent plant shop and you’ll see slow-release fertilisers, organic fertilisers, synthetic or mineral fertilisers. These names can be misleading and appreciating how they interact with your plant is crucial.
Slow Release Fertilisers
Slow release fertilisers are fertilisers that continue to slowly release nutrients into the soil over a period of time. They can last up to 6 months, but they are heavily influenced by their environment (temperature and water), and while widely used outdoors they can be harder to manage on potted plants as you risk over-fertilising. Usually these fertilisers will come in the form of an organic pellet or small ball coated in
a plastic resin or sulphur-based polymers that breaks down over time.
The benefits of slow release fertilisers include:
Slow release fertilisers appeal to people as they don't require a regular routine. But these same benefits can cause a few headaches when using on potted or indoor plants. Higher NPK slow release fertilisers (like those balls I mentioned above) are harder to predict indoors. Temperature and light can vary and the potted plant might not get enough (or too much) fertiliser over a period of time. As the plant is in a pot, the excess fertiliser can build up. For this reason, we always recommend only using organic slow release fertilisers on potted or indoor plants.
With liquid fertilisers we are referring to a liquid-based, organic or mineral, fertiliser. Liquid fertilisers are faster acting as there is no need for any
decomposition or break down and can be absorbed by the plant as they draw water up from the soil. Again, the NPK will depend on what sort of fertiliser you are using.
Benefits of Liquid Fertilisers include:
The big risk with liquid fertilisers is over fertilising. If the directions are not followed and the dilution rate is incorrect, you can risk harming your plant. When used correctly though you can't beat their results.
Nobody likes the term ‘synthetic’, especially when thinking about their plants. With the marketing around terms like ‘organic’ and ‘natural’ people are quick to assume anything synthetic (ie made by humans) must be worse.
But the truth is - your plants do not care where their nutrients come from.
In fact, a plant cannot tell the difference between organically derived potassium or potassium from a synthetic fertiliser. Fertilisers that are made from minerals and have precise ratios and quantities of macro and micro nutrients, so you can have more control on exactly what you're feeding your plant and control when they receive it. They're also the fastest acting fertilisers.They are often more concentrated than organic fertilisers and are diluted by adding to water.
Organic fertilisers are the most mild of fertilisers. They will have some decaying organism which can mean a bit of a smell, and will have a lower N:P:K ratio. Organic fertilisers will rarely have an NPK higher than 6.
When using organic fertilisers, you need to make sure they are providing everything your plant needs in terms of macro and micro nutrients, and supplement if it's missing anything. Always check to see if it is a complete fertiliser and ask if you’re not sure.
Foliar sprays are a great 'fast fix' for plants especially if they have been suffering from a nutrient deficiency. Plants take in nutrients quicker through the leaves than the roots. You can purchase these as ready to go or simply dilute your liquid fertilisers to half or even quarter strength and spray using a mister. They should not be used as an alternative to other fertilisers though as plants would not be able to get enough nitrogen, potassium and phosphorus purely through foliar feeding.
Given the above, the next thing to think about would be where are you plants that require fertilising.
For indoors, both Organic and Synthetic fertilisers have their advantages and we have always been big advocates of a blended approach. While your plants are actively growing (usually they kick off in Spring and keep going all through Summer) we recommend using a high Nitrogen (ie, not organic) liquid fertiliser. It is quickly absorbed by the plant and will reward you with larger foliage and more growth.
When the weather gets cooler and there is less light available, we typically start to see slower (if any) plant growth. You will read a tonne of blogs about cutting fertilising out all together over this period, but a better option is to use an organic slow release plant food. The reason for this is that the risk of over fertilising is higher in Winter as your plants do not require as many nutrients. Using an organic, slow release fertiliser with a low NPK avoids this. Being able to continue to fertilise throughout Winter not only reinvigorates the potting media so it is full of life come Spring, but also provides the plants with much smaller doses of the macro and micro nutrients they need to live their best lives even while in dormancy.
Outside it is a different story. Whereas indoors your plants are relying on you to provide them with their nutrients, outdoors your plants are able to collect some of those nutrients themselves, in the form of decaying organic matter like leaves or dead insects, and through worm castings. The key outdoors is to ensure your soil is full of life - the more microbial life, the quicker organic matter is broken down creating a nutrient-rich bed for your plants and for other beneficial life like worms. As such, using an organic slow release fertiliser outside like our Bio Pellets or even our Soil & Microbe Booster is ideal. Not only do both these products release supplementary nutrients into the soil, but they also add microbial life to help improve the soil structure, which in turn helps with plant health and nutrient uptake. Healthy soil is key to healthy plants.