Cannabis nutrient Part four: Potassium and Marijuana

by Alex Robles

Greetings my fellow cultivators, I hope everyone’s garden is doing amazing.  Today I’m going to finish up the macro-nutrients with some information about Potassium. 

Potassium (K) is another vital primary nutrient that a healthy cannabis plant will use throughout its lifecycle and is also essential for photosynthesis (the building of sugars).  It lets the plant metabolize (chemical digestion) those carbohydrates (sugars) to make different proteins and amino acids that are used in different plant processes.  Potassium helps carry water throughout the plant and this allows it to be less stressed during dry periods and improves the plants resistance against frost by working in the plants sap.  It will promote consistent, even and strong growth and when combined with Phosphorus it will also help strengthen the root system.   Potassium is needed during flowering to increase the density, weight and number of buds.


Potassium is known as a mobile element, that’s why the symptoms usually appear at the bottom, older leaves, but the symptoms can also start at the top of the plant.  In the beginning of a Potassium deficiency the leaves will turn white, yellow or brown while the veins will look green. The tips of some of the leaves can also look burnt but they could also look bleached or pale instead of brown.  The brown burnt edges may show up before the yellowing and is sometimes mistaken for nutrient burn.  Nutrient burn will affect the tips of most of leaves, not just the lower older ones and wouldn’t cause yellowing between the veins.  A potassium deficiency could also look like an iron deficiency because of the yellowing except for the burn.  When a plant needs potassium, it might start to stretch in the beginning and you may only notice that symptom if your familiar with the strain.  As the deficiency continues the plant will grow slower and stems will become weak and bend easily like limp celery.  The newer leaves will also be smaller as well as the buds.  The plants ability to fight off disease and pest will also suffer when it doesn’t have enough potassium.


Too much potassium will cause toxicity that can make the plant lock out other nutrients like magnesium, zinc, iron and calcium, which will cause other deficiencies.


To correct a potassium deficiency I would start by checking the pH of my feeding water.   A safe range across most mediums is between 5.5 and 6.3.  When the pH is lower or higher than this, the plant will struggle to take up (absorb) most nutrients.

If balancing the pH doesn’t solve the problem.  The next thing I would do is to make sure I’m feeding the plant a balanced nutrient mixture for its stage of growth.  It could be as simple as double checking my math.

If I feel like I need to add potassium I will add a liquefied kelp fertilizer because it will be high in potassium.   Other good sources of potassium are wood ash, potassium sulfate, potassium silicate.  granite dust and green sand.

Nutrient Lockout can also cause a deficiency and is caused by high levels of a nutrient in the soil that block other nutrients from being absorbed.  One way to correct this is to flush the plant to break up the nutrient buildup that is on the roots and in the soil.  When you flush the plant, you want to water the plant until you see a lot of runoff coming from the bottom of the container.  I like to see the amount of runoff equal to size of the container.   If I’m flushing a 3gal pot, I want to see about 3gal of runoff.   After that I would give it a light feeding, about a 1/8 to a ¼ of the usual amount of nutrient mixture with the usual amount of water.  I’m not going to get into ppm. I feed the plant nutrient so it doesn’t starve as it rehabilitates.  When the soil is dry it’s ready for its next regular feeding, I make sure it has a balanced nutrient mix.

Well ladies and gentlemen that’s all I can tell you about potassium.

Grow, Learn, Teach

Click here for Kelp fertilizer

Click here for Worm castings

Click here for pH up down kit

Click here for Digital pH meter

2 thoughts on “Cannabis nutrient Part four: Potassium and Marijuana

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