Phosphorous is one of the major nutrients marijuana plants need. A lack of phosphorous will affect root growth and slow overall plant growth. If left untreated it can potentially kill the plant. Marijuana plants with a phosphorous deficiency will start to show signs on bottom growth first, as it begins destroying older leaves first. 


    Why do marijuana plants need phosphorous?

    Phosphorous is one of the key components of a marijuana plant’s life. Plant supplements come in the form of a formula, N-P-K. These are the three primary nutrients for the plant. Phosphorous is the “P” within that formula, making it one of the most vital nutrients for marijuana.

    Phosphorous is a mobile nutrient. This means that when it is absorbed by the root, it will travel from leaf to leaf.

    Early phosphorus deficiency in marijuana

    Marijuana plants need phosphorous during both the vegetative and flowering stages of growth. During the vegetative stage, the plant will use phosphorous to develop strong and healthy roots. It also helps by assisting the plant during the photosynthesis process. However, at this stage plants also need more nitrogen than they do phosphorous or potassium.

    Once the plants move into their flowering stage, they rely on phosphorous even more for the production of flowers. Resulting in fatter, denser buds. Phosphorous deficiencies will likely show during this stage if they are lacking. Many nutrient blooming mixes increase the amount of phosphorous in their bloom mix to prevent this.

    When phosphorous deficiencies are first spotted, there are steps that can be taken to correct the problem.

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

    The first signs of a phosphorous deficiency will likely appear as a marijuana leaf problem. Here are some of the signs to look for:

    • Dark green, blue, or gray leaves.
    • Older leaves may begin to turn bright yellow. They will then turn bronze, purple, or brown.
    • The leaves may also look shinier than usual.
    • As the leaves change colours, they may also become very stiff and dry.
    Severe phosphorus deficiency in cannabis

    The stalk of the plant may also sturn bright red or purple, when experiencing a  severe phosphorous deficiency . It’s important to note that many strains will include purple colouring in their genetics. So this colour may not always indicate a nutrient deficiency.

    There are a few different causes of phosphorous deficiencies. But they are all relatively easy to fix.

    One of the most common reasons for this type of deficiency is overwatering. When plants are watered too regularly, it can flush important nutrients out. Too much water also makes soil very compact. With no air pockets, the nutrients can not move around and become absorbed by the soil.

    When temperatures around the plant drop, this can also slow their nutrient intake. Resulting in a deficiency if left for too long. Plants should always be kept in a climate over at least 60 degrees Fahrenheit, or 15 degrees Celsius.

    As marijuana plants rely on phosphorous so heavily, it is rare that simply too much has been given to the plant. It is more likely a problem with the ph level of the grow medium. If these levels are off, it will prevent the plant from absorbing phosphorous from the grow medium.

    How to fix phosphorous deficiency in a marijuana plant.

    The first step to take in correcting a phosphorous deficiency, is test the ph level of your grow medium. If the ph level is too high or too low, as this impacts how nutrients are absorbed.

    PH levels in marijuana

    Growing in soil

    Plants grown in soil should sit in a cannabis ph level between 6.0 and 6.5, although the optimal level for absorbing phosphorous is 6.2

    Growing in hydroponics

    Plants grown in hydro or coco coir should have a ph level of 5.5 to 5.9.

    Fixing your ph levels

    If the levels are off from these numbers, steps need to be taken to correct the ph level. This can often be done by flushing the grow medium with ph-balanced water.

    Even when the ph levels have been tested and are ok, it is best to flush the plant with ph-balanced water. This will help remove any nutrients in the grow medium or on the roots. This also prevents the plant from experiencing nutrient lockout, when supplements are added.

    Once the plants have been flushed, a PK booster can be added. Some of the best PK boosters on the market right now are Botanicare Hydroplex Bloom, Canna PK 13/14 and Humboldt Nutrients Ginormous.

    Growing organic cannabis

    Those growing organic cannabis also have many options. Many of the above products, including Botanicare, are organic and so will not use any chemicals. Growers can also add fish bone meal, eggshells, and bat guano to organic marijuana.

    Keep temperatures higher than 60 degrees Fahrenheit/15 degrees Celsius. This makes it easier for the plant to absorb phosphorous. Plants should also be watered carefully to avoid overwatering. This will not only keep the necessary nutrients in the grow medium, but will also prevent root problems.

    The plant should start to appear healthier within a few days to a week. New leaves should not show any signs of the deficiency within a week. Affected leaves will not recover, so those leaves should be removed.


    Phosphorous is a major nutrient for marijuana plants. When any plant is deficient in this vital nutrient, the plant will start to show symptoms of being sick. Luckily for growers, phosphorous deficiencies can be easily fixed.

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