Phosphorus deficiency in cannabis makes the healthy, green leaves blotchy, stiff, and curled, causing understandable anxiety for the grower.

Marijuana needs quite a few minerals to thrive. Cannabis deficiencies are the bane of weed gardeners, especially when you’re trying to tell one apart from the others.

We’re here to break down those nasty insufficiencies, focusing on the signs, symptoms, and solutions to help you restore each herb to a state of prosperity. Today, we’re tackling the ins and outs of phosphorus deficiency in cannabis plants.


The importance of phosphorus 

Phosphorus is among the essential elements responsible for marijuana health. This mobile nutrient supports numerous energy transfer processes. It also helps the weed plant develop ATP, an organic molecule necessary for photosynthesis

Your cannabis crops need this nutrient throughout their life cycle. It:

  • Stimulates root development
  • Strengthens the stems
  • Supports flower and seed production
  • Boosts disease resistance


Phosphorus deficiency cannabis


Given the significance of this element, you can already tell why the sight of your cannabis leaves turning purple is bad news for your garden.


What are the signs of phosphorus deficiency in cannabis plants?

Deficiencies are tricky. Many of them share visible symptoms, and if you mistake one for the other, more trouble might ensue as a result. 

The elimination method is the most straightforward way to tell them apart. Here are the signs of a phosphorus deficiency in weed to help you gauge whether it’s what you’re dealing with.


Discolored leaves

Much like nitrogen deficiency in cannabis, phosphorus scarcity affects the older, lower growth first. New growth might keep its vivid green color, but the older leaves:

  • Change into a dark green, blue, or gray
  • Become thick and shiny
  • Display brown, purple, or bronze blotches
  • Sometimes develop yellow spots

Note: Yellowing by itself isn’t a symptom of phosphorus deficiency in cannabis, but it might accompany it.


Purple or red stems

Bright red stems on fan leaves could also appear. They’re not something to worry about by themselves, but they point towards a cannabis phosphorus deficiency if accompanied by those leaf symptoms.

You might see shades of violet, too. Purple stems and yellow leaves suggest a magnesium deficiency rather than phosphorus, though.


Dry or stiff leaves

Those purple weed leaves often thicken and turn dry. They develop a shiny coating on the surface and feel stiff to the touch.


Abnormal growth

Phosphorus maintains growth and development through photosynthesis. If there’s an insufficiency, you could see slower, stunted growth and new leaves too small for a mature marijuana plant


Issues with calcium

Phosphorus deficiency in cannabis sometimes brings calcium issues along for the ride

The two minerals interact, combining to create and support structural development. It’s a good idea to check for signs of both while figuring out your attack plan.


Phosphorus deficiency cannabis


What are the causes of phosphorus deficiency in cannabis?

There are several potential culprits for the fact you’re seeing your previously healthy leaves turning purple and curling. Let’s run through the most frequent ones.


pH imbalances

A pH imbalance is easier to discover and treat than other causes. It should be the first item on your ‘to investigate’ list after noticing red pot leaves and other red flags. 

Measure the levels near the roots. pH is likely the reason behind the insufficiency if you get:

  • Under 6.2 or over 7.0 in soil
  • Under 5.5 or over 6.2 in hydro


Excess iron or zinc

Too much phosphorus can lead to iron and zinc lockout and vice versa. Check for a nutrient buildup in your growing medium to see whether this might be the source of your troubles.


Cool temperatures

Do you know how sometimes, growers use colder temperatures to make their crops develop into purple beauties? That’s okay to some extent, but an environment continually under 50 degrees Fahrenheit prevents cannabis from absorbing phosphorus.


What is the impact of phosphorus deficiency?

The most troublesome effects of a P deficiency in cannabis happen during vegging and flowering, the periods in which your crops need as much phosphorus as they can get. Let’s examine the potential consequences.


Vegetative stage

As marijuana enters the most rapid growth phase, it uses phosphorus alongside nitrogen to accelerate that development. An insufficiency might result in:

  • Stunted root growth
  • Slowed photosynthesis process
  • Fewer and smaller leaves

Worst case scenario, the lower leaves die and fall off. If left unchecked, these side effects prevent your weed plant from reaching its full potential


Phosphorus deficiency cannabis


Flowering stage

Phosphorus deficiency in the cannabis flowering stage is the most common and problematic. This nutrient is key to flower development, and a lot of it gets used up with blossoming.

The areas nearest to the buds and right below lights will display the warning signs first. Leaves turning purple during flowering ultimately results in a suboptimal harvest.


How to solve phosphorus deficiency?

Now that you know how to recognize a phosphorus deficiency in cannabis, it’s time to discuss solutions. We’ll first suggest quicker methods for growers preparing to harvest and then move on to more long-term options.

In either case, the purple marijuana leaves should start regaining their natural color after a week or so.


Bloom fertilizers

Supplements are a synthetic yet easy way to resolve a lack of phosphorus. If your marijuana plant is late into development, there’s no time to fix the underlying causes, which makes this option your best bet. 

Cannabis-friendly bloom nutrients come with high phosphorus levels since it aids flower production. These products help your cannabis plant reach the natural end of its life cycle.



Guano is a natural fertilizer made from seabird and bat excrement. It might smell bad, but it’s 100% organic and phosphorus-packed, which makes it an absolute treat for weed. 

It’s pretty accessible, too, and another straightforward way to resolve marijuana phosphorus deficiency in the short term.


Increase the temperature

A simple temperature increase can do wonders for your cannabis plants, especially combined with higher light exposure. That way, the roots can take in more phosphorus and get the leaves emerging green and gorgeous. 

Note: If you can’t increase the temps, possibly because of outdoor cultivation, supplements are your go-to.


Restore the pH levels

Ensuring that the growth medium is well-balanced in terms of pH is vital for proper marijuana development. If you discover an imbalance or a buildup, you’ll need to restore the levels, as you would for potassium deficiency in weed.

You’ll find general guidelines for soil and hydro, but slight tweaks make a world of difference when dealing with an insufficiency. When it comes to optimal phosphorus absorption, these ranges are ideal:

  • Between 6.2 and 7.0 in soil (a bit higher than the average recommended soil pH of 6.0 - 7.0)
  • Between 5.5 and 6.2 in hydro (a bit lower than the average recommended hydro pH of 5.5 - 6.5)


Flush your plants

There’s already a lot of phosphorus in healthy, organic soil. If you’re using a high-quality medium, the real reason for your issues might be a nutrient buildup, not a scarcity. Nothing cures buildup like flushing.

Use clean, pH-neutral water with a regular dose of nutrients (including phosphorus). A flush removes nutrient salts and aids the pH restoration we discussed above.


How to prevent future phosphorus deficiency in cannabis?

You can breathe a bit easier once you start seeing green leaves again. Your fruitful harvest is on its way, but don’t forget everything you learned because you escaped the worst of it once.

The issues you just faced are an invitation to take better general care of your marijuana plants. Here’s how to prevent phosphorus deficiency in marijuana from re-appearing and threatening another yield:

  • Keep the pH levels in check. An imbalance is a surefire way to a wide range of issues.
  • Keep the temperatures stable. Low temperatures and cold spells stress the crops and stop them from reaching their full potential.
  • Provide balanced nutrients. No element can make your cannabis flourish in isolation. These nutrients work together to make your garden thrive.
  • Learn good watering practices. Oversoaking your container could create water pockets around the roots and block phosphorus absorption, even if everything else is perfect.
  • Flush often. Especially if you use nute supplements, regular flushings are your right-hand man for keeping things balanced.

Pay close attention to the health of your herbs and react quickly and proactively if anything seems amiss. That way, you’re more than likely to nip the problem in the bud.


Phosphorus deficiency cannabis


Keep calm and fix the issue

Phosphorus deficiency in cannabis is troublesome but not impossible. Take it easy, don’t panic and add too many nutrient supplements at once, and don’t despair.

Instead, stay calm, try one method at a time, and keep trying until your marijuana garden is healthy and flourishing once again. 

Stay tuned to our blog for more breakdowns of common cultivation pitfalls and other handy guides, and grow with confidence.

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