NPK seems like a no-brainer as it represents three basic compounds necessary for cannabis growth: nitrogen (N), phosphorous (P) and potassium (K). However, many growers struggle to understand the labels printed on their bag of soil or a bottle of fertiliser. And too much of a good thing is often worse than too little!

NPK numbers are indeed very important, and you should read them religiously whenever you buy a new product. When you check the label, it might say 1-2-1, 10-15-20 or anything else. And this means that there's such and such nitrogen, phosphorous and potassium PERCENTAGE BY WEIGHT in the product you've just bought.

So if label on your EWC bag says 1-1-1, it means there's 1% of nitrogen, 1% of phosphorus and 1% of potassium, and the rest is carbon and organic matter. And if it's a bottle of liquid fertiliser, that says 10-20-15, it means there's 10% nitrogen, 20% phosphorous, 15% potassium, and the rest is water or water with chelates/solvents.

How NPK Affects Cannabis

All three nutrients are quite important for cannabis growth. However, different concentrations are necessary in different stages of growth, and finding a sweet spot takes a little bit of research and practise.

Nitrogen (N) is the most mobile of all nutrients. It's responsible for leaf growth, for turning energy acquired during photosynthesis into amino acids, and also for absorption of all the other nutrients.

In a way it's a commanding nutrient, that has to be available all the time, and plant will always move it to upper growth if there's a need for it.

Phosphorus (P) is mainly utilised by the root system. It spurs its growth and expansion into the soil, but is also used by the plant to divide cells, strengthen the stems, trigger flowering, and produce seeds (shell is mostly composed of phosphorus). It's also very important in sugar production.

However, absorption of phosphorous is directly proportional to absorption of nitrogen, which means both have to stay in perfect balance to provide a healthy growth.

Potassium (K) function on the other hand is probably the most complicated and least understood. We know it enhances stem growth, pest resistance, and is responsible for immunity response to hot and cold temperatures. It also appears to be essential for protein synthesis, and for carbohydrate metabolism.

But we also know its high concentration has adverse effects depending on the actual composition of soil and abiotic conditions. Particularly, it often inhibits calcium uptake.

How Much NPK Is Necessary For Cannabis?

There's no clear-cut answer for it as it depends on many factors such as: CEC of your soil, size of the pot or rather space for plant's roots, temperatures that plants are exposed to, amount of oxygen entering the soil, C:N ratio, sun/light index, particular strain's characteristics (nutrient demands, flowering time).

It also pretty much depends on grower's objectives. For instance, if yield is the most important one, but quality not so much, different route should be taken than when the opposite is desired – we shoot for the highest quality without much caring about the yield.

Very different amounts of nutrients are also used by hydro and soil growers. Both methods have different limits of how much plants can absorb. It makes it farther confusing if you grow with chelated or synthetic nutrients in a sterile medium as opposed to growing in living organic soil.

Due to these factors the answer can only be given after a grower settled on a particular method, and decided what the main aim of his operation is.


Photo credits: 1.) Chem 91 & Dementia grown by brightlight, 2.) Tangie grown by Icemud, 3.) Golden Tiger x Purple Malawi grown by Graytail. All pics courtesy of 420magazine

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