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A woman looks at a bottle wondering what is NPK?
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What is NPK? Understanding the Numbers on Fertilizer for Cannabis

Dec 1, 2018 - Michelle Janikian

Photography by Georgia Love for Herb

A woman looks at a bottle wondering what is NPK?

Photography by Georgia Love for Herb

Ever wonder what the three numbers on the front of plant fertilizer and cannabis nutrients stand for? That is the N-P-K ratio, or the ratio of the three main macronutrients that all plants need to grow: Nitrogen (N), Phosphorus (P), and Potassium (K).

Different plant feeds have different ratios of N-P-K because different types of plants need different ratios, and some plants need more of certain nutrients during certain stages of growth. Cannabis is no exception. In fact, it’s extra crucial that your cannabis plants have the correct ratios of NPK for each of its unique life cycle stages: seedlings, vegetative growth, and flowering.

Nitrogen

Nitrogen (N) is a crucial element of chlorophyll, the plant molecule responsible for absorbing light and turning it into energy (aka photosynthesis). Because of nitrogen’s crucial role, it is essential for stem and leaf development and growth. Therefore, cannabis plants need more nitrogen during their vegetative growth stage and during early flowering or budding. When growth is slow or your plant turns a bit yellow, it’s a telltale sign your plant is running low on nitrogen.

Phosphorus

Phosphorus (P) also aids in the photosynthesis process by helping to create sugars and starches. Therefore, phosphorus’ main role is to promote root growth as well as fruit and flower development. Because of this, cannabis plants need more phosphorus during their flowering stage. When any plant is struggling to produce fruit or flowers, it can be a sign that it’s lacking optimal phosphorus levels.

Potassium

Potassium’s (K) role is to help plants with their overall health and functions. Potassium helps plants to grow strong and to boost resistance to diseases and pests. It stimulates growth and improves water efficiency because it’s involved in moving sugars, nutrients, and water through the plant. When leaf tips are curling and dry or veins of leaves are yellowing, plants could have a potassium deficiency.

Best NPK Ratio for Weed

Because the cannabis plant has distinct stages of growth, it needs different ratios of NPK for each. For example, seedlings and clones are going to need different levels of NPK than plants which are flowering. However, like most things in the world of cannabis cultivation, there is some debate about the best ratios for each stage.

To simplify what you need and when we’ve rounded up the best NPK ratios for each stage of the cannabis plant’s life cycle.

Best NPK Ratio for Seedlings and Clones

Whether you’re starting from seeds or clones, your baby cannabis plants need their own specific ratio of NPK. There are brands that sell mild formulas for clones, often to promote root growth, like Roots Excelurator. Some growers swear by other products like cloning gel, sometimes known as root gel, to help clones start growing roots, but that’s if you’re making the clones yourself. Once, they start developing roots, they’ll require a different yet still-mild solution.

If your clone has started to develop roots when you buy it, a mild NPK ratio of 1-2-2 should do the trick, according to cannabis college Oaksterdam University.

Best NPK Ratio for Vegetative Stage

What is NPK Understanding the Numbers on Fertilizer for Cannabis What is NPK? Understanding the Numbers on Fertilizer for Cannabis
Photography by Georgia Love for Herb

There are different brands of cannabis nutrients out there that are specifically designed to promote growth during weed’s vegetative stage. The only problem is, many of these brands have different ratios of NPK. For instance, General Hydroponics brand “FloraGro” has a NPK ratio of 2-1-6 for vegging, while Botanicare brand’s “Pure Blend Pro” vegging formula “Grow” has a ratio of 3-2-4.

When all’s lost and you don’t know where to turn, you can always trust Oaksterdam; their approximate NPK ratio for vegging is 3-1-2.

Best NPK Ratio for Flowering

Like with vegging, different brands of nutrients use different ratios of NPK. In General Hydroponics’ classic “Flora Series” line, they have two different formulas used for flowering. The first, called “FloraMicro,” is used during late vegetative growth and early blooming and has a NPK ratio of 5-0-1. Then, for the bulk of the flowering cycle, General Hydroponics recommends home growers use their “FloraBloom” formula which has a ratio of 0-5-4.

If you’re trying to create the ratio on your own, stick to Oaksterdam’s advice and use a flowering ratio of 1-4-5 N-P-K.

Best NPK Ratio for Late Flowering

Late flowering, also known as ripening, is the last two to three weeks of flowering before cannabis is ready to harvest. During this time, cannabis plants need less than the full flowering NPK ratio, but still enough to provide the plant with all the stuff it needs to finish growing those big, resinous buds. Oaksterdam University suggests using an NPK ratio of 0-2-0 during this stage.

Use a different NPK ratio for a particular stage in the plant’s life cycle? Let us know in a comment below.


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