How to fix phosphorus deficiency in cannabis plants

With an increasing number of states legalizing marijuana, strains like Berner’s Cookies and White Nightmare have grown in popularity. While that’s fantastic news, cultivating marijuana involves serious work and knowledge to produce a premium product, including understanding the essential nutrients growing plants require. One of the most important macronutrients is phosphorus, which is essential for healthy growth.

Phosphorus is a mineral required for strong plants that play a pivotal role as strains go through all the major stages. Because of this, you have to keep your eye on nutrient deficiencies, especially phosphorus. If your plants suffer from a phosphorus deficiency, it can have a devastating effect to the point where your marijuana becomes unusable and undoes all your hard work.

At Growers Choice, we have some helpful tips to help you understand the signs of phosphorus deficiency, how to fix it, and how to ensure you end up with healthy marijuana plants. 

Multiple green leaves growing out of soil outside
Important information  about avoiding nutrient imbalances
Source: Roman Synkevych unsplash

Benefits of Phosphorus in Cannabis Plants

Phosphorus is a key nutrient, known as a macronutrient, involved in every step of the growth of every living organism, including plants. Along with nitrogen and potassium, phosphorus is one of the most important nutrients healthy cannabis plants need to thrive. Together, these three nutrients are referred to as the NPK ratio, working to fuel your plants into the most amazing marijuana plants.

The mineral is found naturally in the organic matter and minerals of soil. As such, phosphorus enters through the plant roots, including the hairs, tips, and cell membranes to feed cannabis plants. As a mobile nutrient, phosphorus works its way through the plant to handle a wide range of metabolic and chemical processes.

Roles of Phosphorus

By now, you have probably noticed that the nutrient works overtime to take your strains like White Fire Alien from mere seeds to strains bursting with euphoria and relaxation. To support proper plant growth, phosphorus is involved in some of the most important processes that your marijuana goes through.

  • Root development

In the beginning stages, phosphorus is essential in root growth to aid in helping strains grow into strong and healthy plants. After all, the roots absorb water and other nutrients in the soil that are carried to every part of the developing cannabis plant.

  • Metabolic processes

Additionally, phosphorus is a key component in various metabolic processes in the marijuana plant. As a key mineral, it helps to transform energy from the sun into food to feed the plant. 

Consequently, keeping phosphorus at optimal levels is a key component of photosynthesis. Phosphorus plays an important role in photosynthesis by helping the leaves absorb direct sunlight or energy from lights in the grow room during the light cycle. During the dark cycle, all the collected energy is turned into the all-important energy source, glucose, which is sugar. An additional important aspect of photosynthesis is to produce the beautiful green color that marijuana is often associated with.

Phosphorus is essential in DNA/RNA synthesis since the mineral physically links these molecules. DNA in cannabis houses the genetic instructions that promote cannabis reproduction and development. RNA decodes the DNA to continuously produce compounds throughout the growth stages of the plant. 

  • Bud production

Adequate phosphorus levels are also vital for bud development, which is at the very essence of growing marijuana since they are the part of the plant that you can smoke and enjoy. When you have the right amount of phosphorus, it allows your buds to grow large to give that extra potency and flavor, thanks to increased amounts of terpenes.

Importance of Phosphorus Throughout the Growth Cycle

  • Germination Phase

Immediately after the germination stage, plentiful nutrients in the seed fuel the growing cannabis plant. Additionally, most of the soils you purchase will have more than enough of the essential mineral to last through the first three weeks. It’s important to note that if you add too many nutrients too early on, your weed can actually be impacted negatively, including poor root development and subpar plant health.

  • Vegetation Phase

During the vegetative stage, your strains like Tangerine Kush will require more cannabis-forward nutrients, which on average, is approximately 120 parts per million (ppm) of phosphorus. However, since strain varies, do some research to determine the exact measurement to add. Always go by recommended doses to provide a proper nutrient balance.

  • Flowering Phase

In the flowering stage, your cannabis plants will begin blooming and will need increased amounts of phosphorus. If you’re using a commercial mix of nutrients, that should suffice. On the other hand, some strains might require more, so do that due diligence! Keep in mind that later on during the flowering phase, your plants need appropriate levels of phosphorus as the calyxes begin to develop.

How to Identify Phosphorus Deficiency in Cannabis

When you’re involved in growing cannabis, certain tell-tale signs let you know that you’re dealing with a phosphorus deficiency. The sooner you notice a problem, the better off you and your plants will be. Some of the most common symptoms of phosphorus deficiency in cannabis plants are:

  • Discoloration

One of the most common phosphorus deficiency symptoms is that cannabis leaves will turn dark in color. While healthy leaves are typically green in color, when your plants are deficient in phosphorus, the leaves will appear to be bluish-green looking. Before long, you may notice purple stems and leaves, and as things progressively get worse, plants may develop black or brown spots.

  • Stunted growth

Cannabis phosphorus deficiency can also lead to your plants experiencing stunted growth. Based on your strain, you should have a ballpark figure of how tall your plant should be, and if your crop is far shorter or smaller, you might have a big problem on your hands. Not only can you end up with a smaller yield but this can lead to fewer buds, which means less marijuana for you to smoke and indulge in.

  • Curled Leaves

In some cases, a phosphorus deficiency in cannabis will show up as curled leaves. They will begin to curl around the edges and the tips, causing them to not look as full and broad as typical marijuana leaves. Also, the leaves might appear to be weak and brittle.

  • Weakened stems and branches

Many marijuana growers will notice that a phosphorus deficiency in weed plants will lead to weaker stems and branches. As a result, buds aren’t properly supported, and in some cases, the stems and branches can snap under the weight of the flowers.

  • Lower potency

A lack of phosphorus can have a big impact on the potency of your marijuana. With a deficiency, the plant will have reduced resin levels, meaning you won’t get the full potency you would otherwise. 

When you notice signs of phosphorus deficiency in your plants, you need to take action as soon as possible. It’s important to understand that it’s an issue with phosphorus and not an issue with another nutrient, such as potassium deficiency. Consequently, a misdiagnosis can prevent you from appropriately handling the situation and getting your plants back on track promptly.

Raindrops fall onto the soil, turning it into mud
Discover how to grow cannabis plants in quality soil
Source: Brazil Topno unsplash

Causes of Cannabis Phosphorus Deficiency

There are many causes of cannabis phosphorus deficiency that can negatively impact what would otherwise be a healthy cannabis plant. Understanding the causes of common nutrient deficiencies can help you in the long run by easily avoiding these issues to ensure you end up with top-notch marijuana. 

  • Waterlogged Soil

Too much rainfall leads to your plants losing needed phosphorus. That happens because waterlogged soil causes the mineral to be washed away, preventing phosphorus uptake. 

  • Highly acidic or compact soil

Acidic soil prevents your plants from absorbing nutrients even when phosphorus is plentiful. Also, soil that is too compact limits how much phosphorus your cannabis receives, so ensure that soil is loose enough to allow the nutrients to enter your plants.

  • Poor soil health

Poor soil health is a major culprit, especially soil that has deficient nutrient levels. Based on your strain, ensure your soil has the right amounts of nutrients and proper ratios for healthy growth. 

  • Poor Feeding Regimen

Improperly feeding your cannabis plants can negatively impact phosphorus levels, leading to a deficiency. In some cases, you could be over or under-feeding. There’s also a chance growers are using low-quality, synthetic feeds that can prevent phosphorus from being absorbed.

  • Lack of Microbial Life

When your plants are in soil that lacks microbes like mycorrhizal fungi, that can reduce the chance of phosphorus being absorbed by cannabis plants. The typical culprits are when growers begin with low-quality soil lacking in microbes or utilize substandard feed.

  • pH Imbalance

A pH balance that’s too low or too high is often the source of phosphorus deficiency. That’s why growers must keep an eye on pH levels and utilize various methods to keep it in the proper range.

  • Root Damage and Stress

When roots are damaged or stressed, plants inevitably become impacted, including a phosphorus deficiency. Ensuring roots are intact and healthy can mean the difference between high-quality weed and marijuana that’s not worth harvesting.

How to Prevent Phosphorus Deficiency

Growers can keep this common deficiency at bay with a few straightforward fixes to ensure adequate phosphorus levels. Improving your soil biology can involve adding mycorrhizal fungi, which aids with nutrient uptake. Mycorrhizal fungi and other microbiomes in the soil affect the plant’s roots to help with both bother water and nutrient absorption.

Ensure your plants are in the proper pH range, which is typically between 5.5 and 6.2. When the pH is too low or too high, the roots can’t properly absorb phosphorus. One effective solution is to use pH water to get the soil to just the right range, or you can use a specialized pH solution.

Avoiding synthetic fertilizers is a must, and instead, focus on compost and manures as natural sources of phosphorus. Some people also like to use items like eggshells and ash to increase phosphorus deficiency. But, keep in mind that it can take a long time for eggshells to naturally break down, so this isn’t always the best option.

A close-up of a thick crop of green marijuana leaves
Here are vital tips for growing marijuana outdoors
Source: Jeff W unsplash

How to Correct Marijuana Phosphorus Deficiency

Now, here’s the good part: we’re here to tell you how to treat phosphorus deficiency in plants once the levels get too low. Ensure strains like Jack Frost are grown in the right temperature range since fluctuations can negatively impact phosphorus levels and cause them to drop too low. You can also adjust nutrient levels by turning to items like ionic nutrient solutions or foliar sprays for an added boost. 

Also, consider flushing your growing medium when you are growing in a hydroponic medium. Flushing out or even changing the reservoir can get rid of built-up salts or other nutrients that can cause a loss of phosphorus. Using water with the appropriate pH level regularly can easily correct phosphorus deficiencies in cannabis plants.

Analyzing the substrate where your plants are growing is crucial since if the soil’s pH is off, it can lead to a phosphorus deficiency. Take a close look at not only the pH levels of the substrate but also the overall composition and nutrient levels.


Regardless of which marijuana strain you choose to grow,  you want to make sure nutrient levels are at the appropriate level, especially phosphorus since this macronutrient supports healthy growth. The crucial nutrient affects every stage of your plant, making a phosphorus shortage an extremely serious issue. 

Remember that early detection is key to preventing disastrous results. Thankfully, low phosphorus levels are easily reversible when you follow these simple tips. In most cases, you can clear up the issue in as little as a week. Better yet, your plants shouldn’t be permanently damaged if you take care of the deficiency as soon as possible, and you’ll get the marijuana you deserve.


  • What causes purple stems in weed plants?

A phosphorus deficiency, a major temperature change, or issues with indoor lighting can cause weed plants to develop purple stems and must be dealt with quickly to ensure cannabis plants are healthy.

  • What does phosphorus deficiency look like?

A phosphorus deficiency looks like discolored leaves that are bluish-green looking, dark spots, smaller than average plants, leaves that curl, and weak-appearing stems and branches. 

  • How to treat phosphorus deficiency in plants?

You can treat a phosphorus deficiency by maintaining the optimal temperature range for your strain, adjusting nutrient levels, flushing out your medium, ensuring the right pH levels, using nutrient-rich soil, and more.

  • What are the common signs of marijuana phosphorus deficiency?

Some of the most common signs of phosphorous deficiency are shorter-than-expected plants, dark brown or black spots, discolored leaves, curled leaves, and fewer buds.

World Wide Shipping

We ship and deliver world wide via USPS and various couriers.

Payment Options

We offer a wide range of secure and anonymous online payment options.

Customer Support

We care about you, our customer. Please contact us with any questions or concerns.

Loyalty Program

Find out more about the benefits of being a loyal and regular customer.