Fix the Phosphorus Deficiency in your Marijuana Plants!

Your marijuana plant’s growth is slow.

The leaves are turning blue or brown.

This could be an issue with phosphorous deficiency going on within your marijuana plants.

It’s best to try and fix this soon, before it becomes a major problem.

You still may have some time as phosphorus moves from the bottom of the plant to the top.

If you’re not sure if you’re deficient in phosphorus with your plants, this post will clear that up for you.

You will learn everything from what to look for and how to fix it fast.

Let’s get to it,

Signs of Phosphorus Deficiency

  • Leaves turn to a blue-green color
  • Lower leaves are affected first (mobile nutrient)
  • Lower leaves turn yellow and brown
  • Stem can turn a purple color
  • Leaves have blotches of brown or blue
  • Slow growth
  • Affected leaves start to curl is strange shapes
  • Lower leaves begin to die

Why is Phosphorus Important to Marijuana?

Phosphorus is a mobile nutrient within marijuana and all other plants, and it is a primary nutrient for the plant.

Which helps with cell division, new tissue growth helps transport amino acids throughout the plant. According to the University of California.

As well as the overall health and energy of the cannabis plant. 

Phosphorus molecule

Phosphorous is key in the development of flowers on your plants and help with bud formation.

It also aids in the germination stage but do not overdo it.

Phosphorus is the letter P in the N-P-K formula.

You probably noticed this on nutrients when looking online or at the grow stores.

N stands for Nitrogen

P stands for Phosphorus

K stands for Potassium

Vegetative Stage

During the vegetative phase of growth, your plants do require Phosphorus.

It works alongside nitrogen to help with root development and growth.

Also, phosphorus aids with the growth of cannabis seedlings.

During this phase of growth for your marijuana plants, they will require more nitrogen than phosphorus or potassium.

There are many nutrient lines out there with proper ratios of N_P_K for growth

Notice the different stages on this marijuana leaf.

Flowering Stage

In the flowering stage, this is where you want to turn up the phosphorous and potassium and lower the nitrogen for your plants.

Most nutrients lines out there accommodate this in their blooming fertilizers.

Phosphorus is essential for starting flower development to grow fat dense buds.

If you’re into breeding or want to get started, phosphorus is key for the reproductive cycle of your cannabis plants.

Signs of twisted curled leaves, working its way from the bottom.

Causes of Phosphorus Deficiency

There are a few reasons why phosphorus deficiency occurs in your cannabis plants, and they are the following:

  • Over watering, soil has no air pockets left this will affect uptake of phosphorus
  • Low-temperature drops in the grow room to 55- 60 degrees Fahrenheit or 12- 15 degrees Celsius
  • High PH level above 7
  • Low PH levels

You can also run into Phosphorus Lockout.

Phosphorus Lockout happens when the PH is below a certain level.

Causing the medium to become acidic for the roots which stop’s the uptake of phosphorus by your plants.

Here are some PH values to watch for if they are below these values:

When growing in soil: PH levels of 4.0-5.5

Hydro and Soil Less Mediums: PH levels of 4.0-5.0    

How to Fix Phosphorus Deficiency

Here are some of the recommended ways to fix phosphorus deficiency that is affecting your marijuana:


Optimal PH range


Correct the PH level in water/ nutrients.

The optimal ranges should be in the following markers:

Soil, 6.0 – 6.5 pH range. Hydro or coco coir, 5.6 – 6.2 pH range


Flush plants


Use a PK booster


Try some natural solutions


Use high phosphorus organic nutrients

phosphorus deficiency_marijuana_plant
Browning of leaves

PK Boosters for Marijuana:

These are some of the top PK boosters on the market right now that have some good reviews.

Botanicare Hydroplex Bloom  is an organic blooming formula with a nice combination of phosphorus and potassium. Perfect for flower stage. To take your buds to the next level. 0-10-6 values. 

Canna PK 13/14 A popular organic choice among a lot of growers with elevated levels of phosphorus and potassium. This could solve all your problems. 0-13-14 combination.

Humboldt Nutrients Ginormous is the go to PK booster if you need the extra P-K. This will enhance your bloom and give your excellent results it can also give your plants the right combination needed to recover. The levels are 0-18-16.

Natural Ways

Here are some natural ways you can use to recover from the lack of phosphorus, you just add some of the organic fertilizers to your grow medium as directed.

Bone meal is high in phosphorus and can help if you’re low just add to the top of soil or coco. A little goes a long way. It stimulates root growth and the development of buds.

Bat Guano used by many growers in all stages of growth, this is usually just a phosphor mixture. Many products out there ranging from a soil additive or liquid form.

Dr Earth Fish Bone Meal has a little bit of nitrogen so it can be used in the vegetative cycle with high phosphorus and no potassium if your plants are struggling. Can be added to the top of all grow mediums. Like soil or coco.

Organic Nutrients

Here are some organic nutrients that are high in phosphorus:

Big Bloom by fox farms is made with worm casting and bat guano and is a mild solution and can be used in any phase of growth. 0.01-0.3-0.7

Super phosphate is all phosphorus with high amounts this will perk anything up. With a 0-45-0 combination.

Mammoth P Bloom Booster is made up of of active microbes specifically designed to enhance the phosphorus in your grow medium.

The bloom booster was developed at the University of Colorado. Mammoth P is said to give you 16 % bigger yields.

I personally have used Mammoth P and the results were amazing. Highly recommended. A little goes along way!

Signs of Excessive Phosphorus

Your marijuana plants can also intake too much phosphorus in different stages of the grow.

Some plants do not take a high phosphorus value when flowering as other cannabis strains out there.

  • New leaves show signs of yellowing from the veins outward
  • Leaf tips are slightly burned
  • Calcium and Magnesium deficiencies occur
  • Zinc and Iron deficiencies occur
  • Lower leaves curl with brown or blue spots
  • Roots start to die
excessive phosphorus

How to correct excess phosphorus

The best ways to remove excessive amounts of Phosphorus in your cannabis plants is to try the following to fix the damage.


Flush your plants; this should be done first thing


Remove dying and infected leaves.


Lower the amount of your nutrients.


Use a low N-P-K values bloomer such as fox farms bloom till plant recovers

Final thoughts

There are plenty of ways to recover from phosphorus deficiency in your marijuana.

In the future remember to watch your PH levels of your growing mediums. Make sure your plants have proper drainage.

Try and control your temperature in the grow room.

Use organic nutrients when growing and you may need to adjust your plant’s nutrition intake.

You should make out alright.

Let me know in the comments if you have/had phosphorus deficiency.

What solutions did you use?

Grow on my friends.

Image credits: Image,  , Image , Image

16 thoughts on “Fix the Phosphorus Deficiency in your Marijuana Plants!”

    • You should see some improvement in a week or two depending on watering. I have also heard of some growers making a weak tea out of the bone meal instead of putting it on the top soil.

  1. I think I have a phosphorus excess. I haven’t been feeding much, at all, but I am using FFOF soil. All went well during veg, but 6 days after flip, signs of excess are showing. Since I haven’t really been feeding them much, I wonder if the FFOF has too much P?

    Also, I just got Root Farms pH down. I had been using cider vinegar. The root farms has phosphoric acid. If I use this, will it contribute to my problem?

    I really have to get this under control.

    • Hi Betty,
      I haven’t used foxfarms ocean forest, so I am not sure on the npK values. So I cannot say if that’s what would be causing a high amount of phosphorus. If its only been 6 days since flip , it could just be the plants getting used to the new nutes your giving and they could be to much with the ocean forest. I would remove old leaves and watch some new growth . Also lower the amount your giving the plants and see if it bounces back in a week. Then slowly increase your bloom nutes again. I don’t think your ph down will effect your phosphorus levels to the extreme. Hope this helps and you plants bounce back.

    • Hey, i know a number of growers that have bails of fox farm soil in the shed because FF messed up their recipe when mixing and they messed it up bad, their recommendation is to buy their organic nutrient line to replace what is missing in the soil….sounds like a strategy to me….. anyways and yes, phosphoric acid will add phosphorus to your nutes. Also, you should still be feeding veg nutes for the first 1-2 weeks after switching to 12/12. In fact, weeks 3-4 of flower is when the plant uses the most nitrogen and phosphorus and after week 6 it starts to taper down and now potassium is the dominant nutrient as it tightens up the buds and helps build potency. I know im amswering an old question but i hope this helps someone

  2. I have a huge problems some plants are in bad shape. Most of the tops of them are purple and the sugar leaves are curled and purple. What can I do about it? Do I need to cut the tops and purple leaves off?

    • Hi Lee,
      Purple could be caused by big temperature fluctuations, this could be when growing outdoors and a cold snap hits. Stress can play a factor. I would use a microscope to take a look at the buds to see if there is any mold or rot. but it may be salvageable.

  3. This is helpful, but how about a portion (i.e. “1/4 teaspoon of phosphorous, 1/2 potassium per quart/gallon”). I could be over fertilizing but how would I know other than wait until the plant shows signs. Could you give more concerte proportions, please/

    • It’s hard to give specific number because all manufactures are different. I ould go with recommended but lower the dose a bit and see how your plants react to it.

  4. Extra phosphorus isn’t harmful to the plant. It simply throws off the balance of cations in the soil solution. Additionally, it can cause a concentration gradient that hinders plant uptake and/or causes water loss. If you’re curious about your soil status, contact your county extension office about a routine soil test including macro and micronutrients, as well as any other measure. They also offer water quality normally. It’s inexpensive and accurate.

  5. Thanks for your article ..

    Got four auto flowering – all same condition / nutes and lights etc but all very different ..

    Most worrisome is the ailing one with Purple/ brown edges allong the serrated edges thin yellow leaves and rusty type spots .. also slow to develop and a stall in bud development

    One of them is perfectly green but not much movement from the popcorn buds

    One has buds that are finally filling out nicely though kind of dry and not so fragrant (also showing yellowing with purple tips and spots)

    And one is full of big juicy sticky buds

    Sounds like too much p/k? ..

    Thanks again,


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