Understanding NPK in fertilisers

Understanding fertiliser labels

Ever looked at a fertiliser label and wondered what they mean? Join the club. On a macro level, I believe I understand what it’s about, but it’s the final detail that gets me flummoxed. Particularly when it comes to dosages. By this, I mean why should one fertilizer with an NPK of 4: 3: 8 (call it fertiliser A) get diluted at the rate of 20 ml per 4.5 litres of water at fruit set, while another (fertiliser B) with an NPK of 2.7: 1: 4. gets diluted at 10ml per litre at the first sign of buds?

The instructions for the fertiliser B recommend using it at 5ml per litre for young plants. The recommendation is then to up the dosage to 10ml from the onset of flowering. In other words, double the dosage. By doing that, does that mean the NPK value of the fertiliser being fed to the plant has been doubled?

Logic would dictate to me that if the container of fertilizer B says it has an NPK of 2.7: 1. 4.4, this ratio would have to apply to at the 5ml dosage.  Surely by upping the dose to 10 ml, this would mean the NPK has now been doubled to 5.4: 2: 8.8.  On the face of it, this would make sense, as it ties in with the NPKs of other fertilisers recommended for flowering plants.

If this is correct, we are getting closer to the NPK of fertilizer A which is 4: 3: 8. However, if I convert the dosage of the fertiliser A to a per litre basis, I arrive at 4.44 ml per litre.  This doesn’t remotely compare to the 10 ml per litre dosage for fertiliser B. To me, something doesn’t add up. It I converted the 4.4 ml per litre of fertiliser A to the same dosage of 10 ml for fertiliser B. Would this mean that at this dosage the NPK of fertiliser A would be 2.25 times greater (i.e. 9: 6.75: 18) at 10 ml?  See where I am coming from?

Why is this important?

The right dosage

Now the question needs to be asked. Why is this important? Surely follow the advice on the tin, and you’ll be alright. I wish it was that easy.  For example, when a label says feed young plants 5ml per litre, what does that mean?  To me, this recommendation is not very specific. After all, when exactly does a seedling become a young plant? Is it directly after the cotyledon stage or at some point later?

To attempt to answer that question, let’s look at what we already know. My understanding is that a seedling does not require any fertilisation until it develops its first set of true leaves. After that, the seedling should be potted on and start receiving weak dosage fertiliser. It only needs low dosages, because most potting mixes already contain about six weeks of nutrients. The weak dosage fertiliser is only there to supplement what is already there

Because a seedling gets potted on fairly frequently after it germinates, there is little risk of nutrition depletion in potting mixes until seedlings are potted on into one litre pots.  In my case, the “young plants” as I now regard them will spend between two and three months in these pots.

To me, this is where the game changes. Not only is the plant needing more feeding because it has grown in size, but also because nutrients in the potting mix will also get depleted more quickly. So, where I might have been feeding the seedlings at 2.5 ml per litre of water while they were seedlings, they will now start receiving 5 ml of fertilizer per litre of water. I will continue with this dosage until the plants are hardened off and potted on into their final containers for the season.

That’s all fine, and well, if I only wanted to use one fertilizer without trying out new ones. I know from previous experience that the NPK ratios of the fertiliser I use (Chilli Focus) to bring my seedlings to hardening off stage. But what if I wanted to try another fertilizer? For example, let’s say I wanted to use tomato feed instead of Chilli Focus or seaweed extract. How do I approximate the dosage to achieve similar ratios?

This is the crux of wanting to understand the finer detail, instead of just blindly following recommendations on a label. After all, the recommendations are generalised recommendations, rather than aimed at stages of development in a Chilli plants growth.  After all, a plant’s nutrient needs are different at the various stages of its development. To give plants the correct nutrient at the right time is what it boils down to


To get my head around all this, I decided to do some research. What I learned and have pieced together is interesting. I certainly didn’t understand before going into this that there is  so much more to it than first met the eye. It seems that not all fertilisers are tested in the same way to arrive at their NPK declarations.

For now, I will have to leave there, because I have run into the maximum length for this post. I will discuss this and my other findings in more depth in an upcoming article.


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