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NPK Ratios for Cannabis Fertilizers: Understanding the Balance

Ashley Priest

by Ashley Priest

August 3, 2023 08:00 am ET Estimated Read Time: 5 Minutes
NPK Ratios for Cannabis Fertilizers: Understanding the Balance

Ever heard of NPK Ratios? As it turns out, understanding the nitrogen, phosphorus, and potassium levels in your soil can help you grow bountiful cannabis plants.

Growing cannabis can be fun. It can also be a real pain in the rear end. Yes, it grows like a weed but, what many people leave out is it grows like a very moody, temperamental weed. Choose the wrong environment and things get out of hand quickly.

For one, Fresh AND clean air is a must. This means being free from pests like mites and aphids and pathogens like mold and powdery mildew. Using filtered air is highly recommended.

The right amount of light is also essential. Too much light and you will burn your plant’s leaves, stunting their growth, possibly even killing them. Not enough light, and they will not grow right. You’ll end up with frail, weak plants that produce little to no buds.

Perhaps the most important factor to a successful grow, however, is the soil and fertilizer. That’s where NPK ratios come in. You have to nail the PH level of the soil for your cannabis plants to thrive. You’ll want to get that PH level between 6.0-6.8. Getting your fertilizer right presents its own massive challenge. To make the most of your fertilizer you need to understand the ins and outs of what is commonly referred to as NPK.

What Does NPK Mean?

NPK stands for nitrogen (N), phosphorus (P), and potassium (K). These are the three most important nutrients in fertilizer. They play a vital role in plant growth and development. The letters found on a container of fertilizer tell us the ratio of these nutrients in the fertilizer, while the numbers represent the amount of nutrients present in the fertilizer.

As an example, a container of fertilizer with a label that reads 3-1-1 is 3% nitrogen, 1% phosphorus, and 1% potassium. According to Amsterdam Genetics, “Roughly speaking, NPK ratios of 3:1:1 are best for any cannabis strain throughout the growth stage. Once the plants start to flower, nitrogen percentages should be lowered as potassium and phosphorus levels increase. Early flowering calls for a 1:3:2 ratio; late flowering requires 0:3:3 instead.”

  • Nitrogen (N) – Nitrogen promotes the healthy growth of leaves.
  • Phosphorus (P) – Phosphorus promotes root growth and helps develop strong buds.
  • Potassium (K) – Potassium supports the overall function of how plants grow.

During the vegetative phase, NPK ratios of 3:1:1 are recommended.  The nitrogen in the plant’s fertilizer helps to build a strong leaf structure. A strong leaf structure is used in photosynthesis to help the root system grow, leading to flowering, which is where the beautiful buds come from.

When plants reach their early flowering stage, the recommended NPK goes to 1:3:2. Increased phosphorus levels help establish a strong root system, while the potassium increase helps meet the needs of the growing plant.

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As cannabis plants begin to finish approaching the late part of the flowering cycle, the recommended NPK is 0:3:3. Most cannabis plants take an average of 120 days or more from seed to harvest. However, this can change depending on the strain being cultivated.

Before the plants are harvested, it is recommended to flush them. This helps remove nutrient buildup allowing the full flavor profile, aroma, and potency of your plants to come out. Flushing can be done by stopping the use of any nutrients during the last two weeks of growth. You can also use water to flush the nutrients out of the soil and maintain good NPK ratios.

How to Tell if NPK Ratios Are Off

Cannabis plants talk to you when they grow. They tell you what they want and expect you to give it to them. When you ignore them or neglect them, it shows. You don’t need to be a scientist to tell if something is wrong with a cannabis plant. You do need the knowledge to identify and correct issues or at least how to research various problems that might arise. When a nutrient deficiency or proficiency is present, it’ll show in the plant’s leaves.

Understanding how to tell when your NPK ratios are off is essential to the growth of your cannabis plant.

If nitrogen levels are too low plants will show signs like pale or lime green colored leaves, yellowing, curling of leaves, premature falling off of leaves, fewer branches, poor yields, and more. Too much nitrogen, and you can see plants start yellowing, becoming droopy, producing poor bud formation, and becoming eagle clawed.

If phosphorus levels are too high, you’ll see brown leaves and a dying plant triggered by a condition known as nutrient lockout. When this happens, the plant stops receiving nutrients, including the proper amounts of micronutrients like copper, iron, zinc, etc. If those phosphorus levels get too low, lower leaves on the plant will begin to look shiny and display hues of dark blue, grey, or green. You may also see purple veins and curling of leaves.

If potassium levels are too low, you’ll notice burnt leaf edges, curling of leaves, and frail, weak stems. Plants with low potassium levels are at a high risk of becoming infected with pests and pathogens. If plants make it through to harvest, they will produce a very poor yield or poor-quality cannabis. If potassium levels are too high, you’ll see signs like interveinal chlorosis, dark spots, and curling of lower leaves.

Other Ways to Check NPK Ratios

Beyond these visual signs, you can also test the NPK levels of soil at home using some basic science. Oregon Agriculture in the Classroom has a great way to test soil NPK levels at home. You can also purchase analog or digital meters to test soil NPK levels starting at around $15.

When cannabis plants have the right environment and the right NPK ratios, they will let you know. You‘ll see beautiful leaf color, strong stems, gorgeous bud structure, and pest-resistant plants that produce great yields of high-quality potent bud.

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