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What is Phosphorus Deficiency and How to Fix It?

Added 10 March 2022

Is your cannabis plant growing slower than expected? Are the leaves turning brown or blue? If yes, your plant is suffering from phosphorus deficiency. 

If you don’t fix this problem, it can become a major problem, cascading into several other problems for your plant. And in the worst cases, it can halt your plant's growth and lower your yields.

But don’t worry, phosphorus deficiency in cannabis is slow, and you still have time to fix the issue.

You can spot the problem quite easily if you take a good look at your plants. Do you see a problem that starts from the lower tiers spreading to the higher ones? It could be a phosphorus deficiency. 

This article will explain what you need to know about phosphorus deficiency — what it is, what causes it, and how to fix it. Read on to know more.

Preventing Nutrient Deficiencies

nutrient deficiencies

Before we venture into the details of phosphorus, know that it’s best to prevent a nutrient deficiency in the first place. The good news is that you can avoid most problems by maintaining the plants' environment by controlling ventilation, humidity, temperature, and light. 

Also, maintain a consistent schedule to water and feed the plants with nutrients and stick to it. If something is already working for you, don’t change anything drastically. For example, if a nutrient solution from a particular brand is working well, stick to it and make changes only when necessary. 

Last but not least, always check test the pH before watering the plants, even when you get the hang of things. 

Signs of Phosphorus Deficiency

phosphorus deficiency

Phosphorus deficiency is easy to spot if you have a keen eye. In the image above from grower xbabybitchx, the lower leaves are clearly turning bluish-purple — one sign that it's a phosphorus deficiency. Remember, this illness travels from the bottom tiers to the higher ones. So, the first symptoms may start showing close to the medium and then move upwards. This is because phosphorus is a mobile nutrient.

What’s a mobile nutrient? Mobile nutrients travel from one part of the plant to another. For example, a nutrient can travel from the lower leaves to the top. Therefore, when a deficiency occurs, you see it first on the lower and older parts of the plant spreading to the top. 

Here is a list of common phosphorus deficiency symptoms:

  • Leaves turning blue-green
  • Lower leaves turning brown or yellow
  • Stem turning purple or reddish
  • Brown or blue blotches on the leaves
  • Affected leaves curling in strange ways
  • Leaves turning thick and stiff
  • Lower leaves are beginning to die off
  • Stunted plant growth

Typically, phosphorus deficiency occurs when the plant enters the flowering stage — the plant loves feasting on phosphorus during this stage. But that may not be the case with your plant. 

Some growers also mistake reddish-purple stems for a symptom of phosphorus deficiency, but often, it is not. Some strains naturally grow reddish-purple stems. Only take this sign seriously if other symptoms of phosphorus deficiency accompany it.

Importance of Phosphorus 

Purple leaves

Image credit - Skill_Murray

Phosphorus is one of the three essential nutrients your plant needs — it is the P in the NPK formula, the other two being nitrogen and potassium. 

And it is one of the vital components that helps the production of adenosine triphosphate (ATP) in the plant, which is crucial for ion transfer between the cell membranes. So, it aids the plant with new tissue growth, cell division, and amino acid transport throughout the plant

Phosphorus is also an essential nutrient in various growth stages of the plant. For example, it aids root development during the vegetative stage, and it kicks off bud development and the reproductive cycle during the flowering stage.

Essentially, without phosphorus, your plant cannot execute basic functions like growth, reproduction, and photosynthesis. You can imagine the problems phosphorus deficiency can cause in your plant!

Causes of Phosphorus Deficiency 

lower purple leaves

Generally, most soil mediums and cannabis-friendly nutrients contain adequate phosphorus for the plant. Still, the plant fails to absorb it via the roots due to some reasons, causing phosphorus deficiency. 

Reasons for phosphorus deficiency include the following:

  • Overwatering causes a lack of air pockets in the medium, affecting phosphorus uptake.
  • Low temperatures or sudden temperature changes below 55°F to 60°F(12°C - 15°C).
  • Incorrect pH levels cause phosphorus lockout in the medium due to high acidity.
  • Excess iron and zinc in the medium also cause phosphorus lockout at the root zone.
  • Improper nutrient solutions

Solutions for Phosphorus Deficiency

Since phosphorus deficiency is usually caused due to issues within the root zone, the solutions are quite easy. Follow the solutions listed below to fix your plant’s phosphorus deficiency.

Fix the pH Range 

P deficiency

Nearly all cannabis-friendly nutrients and mediums are rich in phosphorus. So, if you are experiencing phosphorus deficiency, the chances are that the pH of the medium is incorrect. 

Due to improper pH levels, especially when it is lower than recommended, the medium turns acidic, inhibiting phosphorus uptake

We recommend trying this solution first. To do this, simply flush the medium with pH-balanced water to remove any residual salt buildup within the medium that may be altering the pH. Then, let the plant rest for a while and feed a regular dose of nutrients. 

The ideal pH levels to tackle phosphorus deficiency are as follows:

  • 6.2 to 7 in a soil medium
  • 5.5 to 6.2 in hydroponic or coco coir mediums

Check the Grow Medium

phosphorus deficiency

Similarly, an unhealthy grow medium can also cause phosphorus lockout for the roots. This usually happens when the medium is drenched in water or too compacted, which eliminates the air pockets that are important for phosphorus uptake.

Consider revamping your grow medium or use a bigger pot if you think this is the issue. 

Increase the Grow Room Temperature

bluish purple leaves

Temperature is another essential aspect of proper phosphorus uptake in the soil. Under the right soil temperature, phosphorus gets to diffuse properly, which the roots then absorb. However, if the soil is too cold, phosphorus fails to release from the nutrient solution.

Phosphorus deficiency in cannabis occurs when the grow room temperature goes below 60°F. So, stick to the recommended grow room temperatures for cannabis, ranging between 75 to 80 degrees Fahrenheit. Some plants may thrive even at 86°F, but it depends on genetics. 

Feed the Right Nutrients

P deficiency

Although rare, phosphorus deficiency can still occur due to improper nutrients. Typically, there’s adequate phosphorus in soil and cannabis-friendly nutrient solutions — most times, it is already abundant for your plant to absorb.

However, if your plant is experiencing phosphorus deficiency due to improper nutrients, you need to check your nutrients. 

Phosphorus deficiency often occurs due to excess iron and zinc in the medium or nutrient solution, preventing the roots from absorbing phosphorus properly. To fix this, try choosing other solutions that are accurate and contain adequate amounts of all nutrients. 

Another reason why your plant may be experiencing phosphorus deficiency is if there is an excess salt buildup in the grow medium. For this, you don’t need to change the nutrient solution. Instead, just flush the growing medium with pH-balanced water to clear out the salt buildup.

Once the medium is flushed, start feeding your plant milder nutrient solutions. Build your way up to stronger solutions until you strike the right balance of adequate nutrients but not enough to crease a salt buildup.

Lastly, you can try using phosphorus-rich solutions, too. Here are some of the best phosphorus-rich solutions that can help tackle phosphorus deficiency:

  • Bone meal — add a little to the growing medium to stimulate root growth, bud development, and phosphorus uptake.
  • Bat guano, which you can buy in various forms, from liquid to soil additives.
  • Fishbone meal, which can be used during the vegetative stage. 
  • Worm casting or worm tea.
  • Crab shells.
  • Soft rock phosphate.

If your plant is experiencing phosphorus deficiency during its flowering stage, it is because the plant goes through a lot more phosphorus during this stage. Similarly, intense light can also boost your plant’s phosphorus uptake

In such cases, choose bloom-specific organic solutions. Bloom-specific solutions are rich in phosphorus because plants take up a lot more phosphorus during this stage.

Despite all the efforts to fix your nutrient solution, if your plant is still suffering from phosphorus deficiency, you can just raise the phosphorus levels in the nutrient solution. 

Watch for Recovery

P deficiency

The final step in fixing your plant’s phosphorus deficiency is keeping a close eye on your plant. Be patient, and you’ll get there. Phosphorus deficiency starts at the bottom of the plant and gradually moves up, so you have some time. 

Once you try the solutions mentioned earlier, you should start seeing your plant recovering within a few days to a week. Look for subtle signs like slowing down symptoms, healthier leaves, speedy growth, etc. 

Sometimes, the damaged leaves may not recover or turn green — don’t worry. Instead, prune the damaged leaves and take care of your cannabis plant so that the deficiency does not occur again. Prevention is better than cure, isn’t it?

Summary: What is Phosphorus Deficiency and How to Fix It?

Your cannabis plant needs three macronutrients — nitrogen, phosphorus, and potassium (NPK) — and even if one of those is not reaching your plant correctly, your plant is going to experience a variety of issues. If you don’t fix these issues when they occur, they will eventually stunt your plant’s growth and kill your yields. 

So, if you notice phosphorus deficiency in your plant, fix it as soon as you can before it’s too late. And always watch out for other deficiency symptoms, and give your plant the right nutrients and conditions so it does not happen again.