In cannabis plants, potassium is a macro nutrient. Without it, a marijuana plant’s leaves are likely to yellow, brown, show burnt tips. These symptoms are of course a sign of potassium deficiency. Leaves will then eventually fall off the plant. When this happens in too many leaves, the entire plant could be at risk of dying. 


    Why do marijuana plants need potassium?

    The marijuana plant needs potassium for several reasons. Firstly, it helps with the photosynthesis process. This is important for the plant’s health and development and the plants vibrant green colour.

    potassium deficiency in weed

    Potassium also helps during drought periods, as it regulates the movement of water throughout the plant. With sufficient potassium, the plant will be well prepared for periods of drought.

    Potassium helps increase the strength and resistance of the plant’s root system. This helps build strong roots that won’t suffer from things like root rot and protect the plant from cold, harsh weather. This makes it especially important for outdoor plants that are exposed to the natural elements.

    For all of these reasons it is important that growers can identify potassium deficiency symptoms early. So that plants can grow to optimum health and provide the yields growers are looking for.

    Marijuana plants need the most potassium during the flowering stage of growth. During the vegetative stage, plants will still need potassium but they will require more phosphorous and nitrogen.

    Even so, it is important growers look for potassium deficiency in cannabis even during the vegetative stage. The signs at this time must be carefully studied. As deficiency symptoms could also represent and look like other types of marijuana deficiencies.

    During the flowering stage a plant’s potassium levels will need to be increased. This is most often done by simply switching to a blooming fertilizer, that will do this automatically for the grower. However, if a cannabis plant is suffering from a potassium deficiency, it’s most likely going to occur during the flowering stage.

    What are the signs of a potassium deficiency in a marijuana plant?

    There are many signs of potassium deficiency in plants that growers can watch for. Most of these are marijuana leaf problems. The leaves will typically turn pale, rusty-coloured and even look as though they are burnt.

    Potassium deficiency marijuana diagram

    Other symptoms include:

    • More branches throughout the stem.
    • Smaller flowers, and a diminished yield of flowers.
    • Stems that are weak, scrawny, and sometimes brittle.

    A potassium deficiency can look a lot like nutrient burn,  it is sometimes difficult to determine what is wrong with the plant. However, a deficiency in potassium will show yellowing leaves and burnt appearance may creep towards the center of the leaf. Even though the veins of the leaf will remain green and healthy. Nutrient burn on the other hand, typically only affects the edges of the leaf.

    Potassium deficiency can happen for a number of different reasons. The first may be that there is too much salt that has built up in the growing medium. This is particularly true when the growing medium used is soil or coco coir. Although it is less likely to happen in hydroponic setups, it can still occur on occasion.

    Potassium lockout is another large cause of potassium deficiency in plants. This is when the ph level of the grow medium is too low. Therefore, the PH is too acidic. When this happens, the soil will actually lock out certain nutrients, such as potassium.

    To avoid this:

    • Plants grown in soil should have a ph level higher than 4.0 to 5.5
    • Plants grown in hydroponics setups and other soilless mediums should have ph levels of 4.0 to 4.5.

    How to fix potassium deficiency in a marijuana plant.

    PH levels must first be accurate when trying to prevent a potassium deficiency.

    Those growing organic cannabis can use organic nutrients to keep their plants chemical-free. With any grow it’s important to ensure that the PH levels of N-P-L are balanced, for both flowering and vegetative cycles.

    In addition to these nutrients, a PK (phosphorous and potassium) booster can be used. So can a strictly K (potassium) booster. This will incorporate more nutrients into the plant, which will make up for any deficiency a plant is experiencing.

    Organic Potassium options

    Wood ash or kelp meal are also great alternatives for those growing organic marijuana. Particularly kelp meal, will be absorbed during feeding schedules.

    kelp meal organic nutrients for potassium deficiency in cannabis

    Before treating a potassium deficiency, make sure it is definitely not light burn or nutrient burn as potassium deficiency has such similar symptoms.

    After the plant has been treated for a potassium deficiency, the problem should clear up within a few days. However, the leaves that have already been affected will likely not recover.

    For this reason, they will need to be removed. After these steps have been taken, it’s important to continue watching the plant to ensure that the deficiency does not spread to other leaves and parts of the plant.


    A potassium deficiency in a marijuana plant can be fairly serious. If left untreated, it can spread to the entire plant and eventually kill it. It’s for this reason that growers need to know how to identify the problem for what it is, and not mistake it for something else such as nutrient burn. Only then can the problem be properly treated.