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What’s The Best NPK Ratio For Growing Cannabis?

Cannabis plants require many nutrients, but nitrogen, phosphorous, and potassium are the three most fundamental elements in a plant’s diet.

Known by their chemical symbols N, P, and K, these three nutrients are essential for healthy growth. As cannabis plants move from the vegetative to the flowering stage, the ideal NPK ratio shifts. Meaning growers need to adapt their feeds and fertilisers.

The Importance of NPK for Cannabis

Most fertilisers and plant feeds contain NPK values on their labels. These appear as three numbers denoting the relative concentration of each component. For instance, a ratio of 2:3:8 indicates that a product contains two parts nitrogen to every three parts phosphorous and eight parts potassium. Cannabis growers should be aware that NPK ratios always appear in this order.

The value for nitrogen coming first and potassium coming last.

Choosing a product with the correct NPK ratio is vital for cannabis growth. During vegetation, plants need lots of nitrogen to produce all their green parts like leaves and stems. Cannabis plants generally benefit from an NPK ratio of 3:1:1 at this point in their lifecycle, ensuring they receive plenty of nitrogen and just a little of the other two nutrients.

Most high-quality soils contain plenty of nitrogen. It may not be necessary to add any of these key nutrients during the first month of growth. When growing in low-quality soils or other growing mediums, though, it’s essential to give your cannabis plants the nutrients they need.

When the flowering stage begins, the NPK balance should switch in favour of phosphorous and potassium. The former helps plants produce large buds, while the latter allows flowers to increase in size. Too much nitrogen can be detrimental at this time, as it prevents flowering and can leave your bud with an unpleasant taste.

Ideally, cannabis growers should gradually alter their NPK values, which means using a ratio of 1:3:3 in early flowering and 0:3:3 in late flowering. Nitrogen should be completely absent from the mix for at least the final three weeks of the bloom period.

How To Apply NPK To Cannabis

Ready-made plant feeds and fertilisers with set NPK ratios can be bought from most garden centres and applied to cannabis plants with minimum fuss. Granular feeds are the most common, but liquid and gaseous products are available too.

Other sources of nutrients include human urine and animal manure, both of which are high in nitrogen and should only be used during the vegetative stage. It’s not wise to pee directly onto plants as there’s a lot of salt in urine. Instead, it’s advisable to dilute your pee in the water, using twenty parts water for every one part urine.

Bone meal and fish meal are excellent sources of phosphorous and are popular among gardeners. These can be scattered on the soil around a plant’s roots during flowering to bring NPK ratios up to the ideal value for cannabis.

Meanwhile, sea kelp and wood ash are both high in potassium and are fantastic tools for adjusting your cannabis plants’ NPK ratios.

Finally, growers should be aware that the pH of their soil affects plants’ ability to absorb these nutrients. Cannabis plants can therefore suffer nutrient lockout even when NPK ratios are ideal. It’s vital to ensure that soil acidity falls within the optimum range of between 6 and 7.

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Ben Taub