Nutrients – NPK Basics

what are npk ratios?

NPK which stands for Nitrogen, Phosphorus and Potassium. IF you are wondering why the symbol for potassium is ‘K’, it is because the atomic symbol of Potassium is “K” (from Neo-Latin kalium). When we talk about that NPK ratio, we mean the percentage of each of these elements in a nutrient solution or fertiliser.

For example, if the NKP ratio is 20-10-20, it means there will be 20% nitrogen, 10% phosphorus, and 20% potassium. The numbers relate to Nitrogen, Phosphorus and Potassium and are always in that same order. You will see these numbers on the back of the fertiliser bottle.

For beginners, this can be very confusing at first, with many different companies selling multiple fertilisers and nutrient solutions, each with different numbers and ratios. Once you learn the basics of what your plant needs during each growth stage, this should get easier with practice and trial and error. 

cannabis growing in garden

changing npk ratios per stage of growth

When your seeds are sprouting, you do not need to add any nutrients, as this can overwhelm the plant, as they are not robust enough to take in larger amounts of nutrients.

When you reach the vegetative stage, you will be able to start adding ‘vegetative’ nutrients. Your plants at this stage will need an equal amount of nitrogen, phosphorus, and potassium. Fertilisers high in both nitrogen and potassium, and medium on phosphorus, also work very well. A 7-5-5 ratio usually works well for vegetative growth. Fertilisers will usually be labelled as ‘grow’ or ‘vegetative’ fertilisers.

During the flowering stage, your plant will need less nitrogen, and more potassium and phosphorus. You can keep the nitrogen at a comparable level as the plant enters the flowering stage but as it further develops, you will need to lower this level. More levels of phosphorous will encourage the formation of flowers but it is also a very acidic element. To balance out the acidic value of the phosphorous, you can increase the potassium level. The ratio during the flowering stage can change to around 3-10-10.

Fertilisers may be labelled as ‘bloom’ nutrients, which will help the growth and development of the flowers and buds. Giving too much nitrogen in the flowering stage can lead to an unpleasant taste in the buds, as well as the plant not growing correctly.

Each grower is different and may use slight variations on ratios, so it is important to test it out yourself and see which works for you. Each cannabis strain may absorb the nutrients at different rates, so it is important you read up about the strain you intend to grow.

Tip: Do not buy any slow release nutrients. These may work well in other garden plants, but they could produce a problem when growing cannabis, such as giving too much nitrogen in the flowering stage. 

the water ph level must be correct

Just as important as adding nutrients to your plants is maintaining the water pH level. There is no point in giving your plant nutrients if it cannot absorb those nutrients. If the pH level of your water is too high or too low, the plant will not be able to properly absorb the nutrients, and this can lead to a nutrient deficiency.

It is vital you check the pH level of your water after you have added the nutrient solution to it. If the pH level needs to be changed, you can easily do this by adding either a ‘pH Up’ solution or a ‘pH Down’ solution.

The optimal pH level of your water for cannabis plants is:

Hydroponics: 5.5 to 6.5

Coco Coir: 5.5 to 6.5

Soil: 6.0 to 7.0

You can easily check the pH level of your nutrient solution with a digital pH pen. This will ensure your plants are absorbing the correct level of nutrients and that they are growing to the best they can be. 

ph scale diagram

how much fertiliser do i need?

After about 3 to 4 weeks since germinating your seed, you should start to see the first few leaves come through. When you start to see sets of ‘true’ leaves come through (around 4 at least) you can slowly begin to introduce your fertilisers. Be sure to keep this to a very low amount at first, to get your plant used to it.

The amount you feed your plant will also depend on its size. A larger plant will need more nutrients and water than a smaller plant. Water enough so that the roots are fully saturated from the water/nutrient solution. 


how often to use fetiliser?

This is quite subjective, as it is usually the grower’s preference, from experience, the type of strain you are growing and how large your cannabis plant is. As a rule of thumb, you should give your plants fertiliser around once a week.

It is good to experiment here. A larger plant may need to be fed every few days. If your plant is starting to lack colour, it may be a sign that you are not giving enough fertiliser. You may want to give more regular feedings with a more diluted nutrient solution, or a stronger solution once a week or once every 2 weeks. Each grower will find what works for them.

A week or two before harvesting your cannabis plant, you will probably want to flush it out, also known as ‘flushing’ your plant. This is when you will only water it with plain water and not add any nutrients. It can create a purer and nicer tasting bud.