Phosphorus Deficiency on Marijuana

Identifying and Solving the lack of Phosphorus

2 leaves at different stages of phosphorus deficiency

Hi there and welcome (back) to Free the Tree!
In today’s article of our Nutrient Deficiencyย series we decided to finally cover Phosphorus Deficiency on Weed.
We decided to cover this deficiency today since we’ve been struggling with it during this grow.
Our Big Buddha Cheese plants had Potassium deficiencyย at the end of the vegetative stage and now we’re dealing with a lack Phosphorus (on top of the Thrips and all), quite a grow this round has been.
Here’s what we’re going to cover

What do plants use Phosphorus for?

Phosphorus is one of the 3 main elements that all flowering plants need to thrive. For full disclosure, plants need about 28 different nutrients in order to thrive, most of which are produced by micro-organismes present in the soil.

Phosphorus is as vital for the well-being of the plant during the vegetative as it is during the flowering stage.
In the growth cycle, it’s main use is in the photosynthesis process and root development. Once the plant is in her flowering cycle, this nutrient reduces its activity on the root level and is used for the production of flowers.

Meaning that if you have signs of phosphorus deficiency showing on your leaves your plant is extracting it from the leaves in order to re-use it to build roots or to produce the flowers.

Phosphorus during Vegetative and Flowering Stages

Stage of Life Phosphorus (P) Usage in Cannabis
Vegetative Stage
  • Encourages root growth
  • Essential part of the photosynthesis process
  • Involved in the formation of sugars, starches and all oils.
  • Helps with the transformation of solar energy to chemical energy (photosynthesis)
Flowering Stage
  • Encourages flower blooming
  • Essential part of the photosynthesis process
  • Involved in the formation of sugars, starches and all oils.
  • Helps with the transformation of solar energy to chemical energy (photosynthesis)
Organic Nitrogen, Potassium and Phosphorus nutrient mixes

What Is Phosphorus Deficiency?

As the name indicates Phosphorus deficiency is a sign that the plants roots cannot access the Phosphorus available or that there isn’t enough available.
These situation occurs either when your pH levels are off, or that simply there is no more Phosphorus available to the plant.

2 leaves at end of the deficiency

At which pH level is Phosphorus available

As you may know the roots of marijuana can intake nutrients at different pH levels.
All the nutrients are accessible at pH’s between 6 and 7, meaning that it’s best to aim for a pH of 6.5.

Phosphorous itself can be extracted by the plants at a pH ranging from 6.2 to 7.2, meaning that if your watering mix is out of that range, you can put all the Phosphorous in the world the roots won’t be able to extract it from the medium, rendering it useless.

If you want to review the pH levels for each nutrient check out our table available in our piece on watering your marijuana plant

How to spot Phosphorus Deficiency on Cannabis?

As most cases in order to spot Phosphorus deficiency the signs come on the leaves of the weed plant.
I found that the most significant sign for this was the purple/black color that the leaves take once the deficiency is well advanced on the leaf.
Let’s look closer at these aspects

What are the signs of Phosphorus deficiency?

Phosphorus deficiency can be a little tricky to spot at first, but once its spreads the diagnosis is pretty straight forward, there aren’t many things that make the leaves turn a purplish/black.
Before that though it can be a little tricky though, here’s a couple ways to identify it:

  • Brown spots appear on the leaf. This can appear a bit randomly.
    Soon after you’ll notice the sides and/or tip of the leaf dry out and curl.
  • As the deficiency spreads you’ll notice the brown spots take over the whole leaf and shift to a darker color, a purple/black.
    The leaf will keep curling as the plant extracts the phosphorous.
  • Generally the biggest leaves are impacted first.
    It can occur at the bottom, middle or upper levels of the plants, although through our research it seems like it generally spreads from the bottom up.
    In our case it seemed to appear randomly but only impacting the largest leaves of the plant.

Now, here’s the evolution in image, as always you can click on the picture in order to see the full view.

Evolution of Phosphorus Deficiency

First brown spots appearing on the leaf

Early Signs – Brown spots appearing and leaf curling

Signs of phosphorus deficiency spreading

Brown spots spreading and leaf tips and sides curling

Leaf totally purple/black showing that all the phosphorous has been extracted

Leaf had fully turned purple/black – All the Phosphorus has basically been extracted.

2 leaves at end of the deficiency

2 leaves going through the last stages of Phosphorus extraction

How to solve Phosphorus Deficiency on Weed?

Solving this deficiency will depend on why you have it. If the issue is related to p levels, all you’ll have to do is adjust the pH levels and you’ll be fine. If that’s not the case then you’ll want to gradually raise the amount of Phosphors within your watering mix.

It’s important not to spike up the levels too harshly right away, you don’t want to be giving her too much nutrients and get her into a Nutrient Burn situation.

To sum up:

  • When a plant lacks Phosphorus it will mainly have issues with:
    • Photosynthesis
    • Root development during the vegetative stage
    • Flower development during the flowering stage.

Thankfully, instead of not being able to produce flowers or develop its root system, the Weed plant will extract the phosphorus available in the leaves to re-use it where it’s needed.

  • Make sure your pH levels are between 6.2 and 7.2
  • Increase the levels of Phosphorus provided in you watering mix in order to solve the problem.
    You should do this gradually in order to find the right levels needed and avoid Nutrient Burn.
  • Leaves that have been affected will not recover.
  • Keep an eye on the spreading of the symptoms in order to see if you solved the deficiency or not.

Most Common Reasons for Phosphorus Deficiency

The main source for Phosphorus deficiency is pH issues. As we stated earlier, this nutrient can only be absorbed when the pH ranges between 6.2 and 7.2, meaning that if you’re out of that range you can put all the Phosphorous in the world the roots won’t be able to extract it from the soil.

If you’re experiencing this, the first thing you should do is check your pH levels, if you’re good then the 2nd step is to add some extra nutrients at your next watering.
For a quicker intake, you can also spray the leaves with a Phosphorus rich solution. If you do so, make sure you spray your plants about 15minutes before lights out, you don’t want to burn your leaves.

That’s it for this one folks.
Until next time, be safe and grow easy!

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