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How To Fix Phosphorus Deficiency In Weed Plants

October 14, 2021
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Cannabis, like all living things, needs feeding to survive. The three main elements are nitrogen, potassium, and phosphorus, along with other secondary nutrients. A phosphorus deficiency in weed can be deadly if left untreated. 

Beginner and advanced weed growers alike can experience a phosphorus deficiency in cannabis plants. We’re going to go over how to identify the issue, the causes, the symptoms, how to fix it, and what a phosphorus burn is. Let’s get started!  

Cannabis phosphorus deficiency
Cannabis phosphorus deficiency

What is phosphorus deficiency?

A phosphorus deficiency in weed plants happens when the roots don’t absorb enough nutrients. Slowly, the plant becomes depleted of the excess supply and starts to show signs of dying. 

The main characteristics of a phosphorus deficiency in cannabis plants are discolored leaves, red or purple stems, stiff leaves, or abnormal growth. 

A phosphorus deficiency in weed plants happens for a variety of reasons. The pH becomes imbalanced, there’s an excess of zinc or iron, or the temperature of your grow room is too cold. 

What does phosphorus do for plants?

What’s so important about phosphorus anyways? It’s a critical nutrient for all types of marijuana plants. 

Phosphorus is vital for developing the DNA and RNA of cannabis plants, using the genetic code to cultivate proteins and create a high-yielding harvest. When there’s a phosphorus deficiency in weed plants, the marijuana crop can’t grow. 

During the vegetative stage, this nute aids in root and stem development and strength. When there’s a phosphorus deficiency in cannabis, the roots become weak and won’t absorb nutrients. 

When the marijuana plant is in the flowering stage, phosphorus helps produce large leaves, flowers, and potent buds. Without the nutrient, your harvest won’t be potent or high-yielding.  

What causes phosphorus deficiency in weed plants?

Phosphorus doesn’t just decide to go on vacation every once in a while. There’s a reason why you have a phosphorus deficiency in your cannabis plants.

A few factors can cause the issue. Let’s look at some possible reasons. 

pH imbalance 

A pH meter is a vital piece of equipment for any grow room. You want the range to be 5.5 to 6.5, which is slightly acidic. This pH range allows the cannabis roots to absorb nutrients and water properly. 

When the pH is off, a phosphorus deficiency in weed plants can occur. When you start to see signs, you need to test the pH levels of your cannabis plants

The pH can be influenced by the water or nutrients you’re using. If you use tap water, there could be other elements or minerals that affect the levels. When the nutrients are too concentrated, it can make the growing medium too acidic. 

Make sure to test the pH of your water and nutrient mixture before adding it to your growing medium. Doing this helps to avoid a phosphorus deficiency in your weed plants. 

pH test
pH test

Extra zinc and iron

Many times when there’s a nutrient deficiency, it’s because there’s an excess of another element. Specifically, with a phosphorus deficiency in cannabis plants, there’s often too much iron or zinc. 

Excess iron produces bronze or brown speckles on leaves, resulting in dying blades. As a weed grower, you have to consistently check the stems and foliage of your plants for signs. 

Cooler grow room temperatures

Marijuana plants like a warm 68°F to 75°F, depending on the stage of growth. When the temperature drops below 68°F, a phosphorus deficiency in your weed plants is more likely. 

The roots are too cold to take in more nutrients, which results in stunted growth and a dying marijuana plant. 

Have a thermometer in your grow room, and make sure the temperature is consistent throughout the space. Sometimes lower parts of the room or an area near a window are significantly cooler than other parts. This chill can result in a phosphorus deficiency in cannabis. 

Cannabis phosphorus deficiency symptoms 

Now you know what causes a marijuana phosphorus deficiency, you’re probably wondering what the symptoms are.  

Although many nutrient deficiencies present themselves similarly, there are some key characteristics to look for with a phosphorus deficiency in weed plants. 

Discoloration of leaves

A healthy marijuana plant has emerald green leaves that are solid in color. When you see any discoloration, it’s usually the first sign of a nutrient problem. 

With a cannabis phosphorus deficiency, the leaves first turn a bluish-grey or have a purplish hue if the crop is in the flowering stage. After this, the leaves start to turn a lime green to yellow color. 

When left untreated, the foliage with signs of a marijuana phosphorus deficiency changes from yellow to white, then brown. The leaves will likely fall off at that point. 

Dry or stiff leaves

A healthy weed leaf is pliable but bounces back when bent and let go. A phosphorus deficiency in cannabis plants makes the leaves dry and stiff. When you touch the foliage, they break easily, feel thicker, and start to curl. 

Red or purple stems

The stem of the cannabis plant is like a highway. All the nutrients get absorbed from the roots and sent to all parts of the crop via the stems and branches. You can detect a phosphorus deficiency in your weed plants by looking at the stem’s color. 

Typically, the stems and branches are a nice dark green. A sign of a marijuana phosphorus deficiency is when these areas start to turn red or purple. 

Remember, some cannabis strains have natural red or purple stems. Only pay attention when this is an unusual feature. 

Weed Purple Stem
Weed Purple Stem

Stunted growth 

Another sign of a phosphorus deficiency in weed plants is stunted growth. When a marijuana plant doesn’t have the essential nutrients to grow, it won’t develop or does so at a very slow rate. 

Always research the cannabis strain you’re growing beforehand to understand how tall the crop should grow. Some indica strains only mature to 2 to 3 feet, while sativa plants can reach 10 feet tall. 

When the rate of growth seems low, you could have a phosphorus deficiency in your weed.

How to fix phosphorus deficiency in cannabis plants

Now you know you probably have a phosphorus deficiency in your cannabis plants, so how do you fix it? We’ve got the answers. 

Let’s look at the top ways to fix a marijuana phosphorus deficiency problem. 

Flush your marijuana plant 

The first thing you need to do when you have a cannabis phosphorus deficiency is flush out your marijuana plant. To do this, you need to use purified or distilled water. Make sure there’s good drainage. 

Run water through the growing medium for a few minutes until all of the nutrients are flushed out. 

Restore nutrients with fertilizer

Once your growing material is completely flushed and free of a marijuana phosphorus deficiency, you need to add a balanced fertilizer. 

For best results, use a fertilizer meant for cannabis, like the Homegrown nutrients, to prevent a future phosphorus deficiency in your weed plants. These nutes have the correct blend of nitrogen, potassium, phosphorus, and other essential elements. 

Restore pH levels

If your problem was with the pH level, you need to restore it. To prevent a phosphorus deficiency in your weed plants, make sure you bring it back to a 5.5 to 6.5 level

Test the nutrients before adding them to the growing medium. If the mixture is too acidic, add more clear water to balance it out. 

Increase grow room temperature

Your grow room shouldn’t be lower than 68°F, so the roots can absorb nutrients properly and prevent a phosphorus deficiency in your cannabis plants. 

If heat is escaping your grow room, check to see any cracks in the windows or if your air ventilation is broken. You need to fix those areas to avoid problems in the future. 

When you live in a colder climate where the nights drop well below 68°F, you might need to invest in central heating or a space heater to help prevent a future phosphorus deficiency in your weed plants. 

Temperature and Humidity in Cannabis Grow Tent
Temperature and Humidity in Cannabis Grow Tent

Use guano

Guano is a natural fertilizer made from bat excrement that people take from caves. Although it doesn’t sound great, it’s loaded with nutrients, and it’s an organic option. 

You can purchase this at your local gardening store and add it to your growing medium to avoid a future phosphorus deficiency in your cannabis plants. 

Phosphorus deficiency symptomHow to solve it
Discolored leavesPrune discolored leaves, flush the marijuana plant, and add nutrients.
Dry or stiff leavesCheck the temperature of the grow room and pH levels, adjust accordingly.
Red or purple stemsTrim stems that are red or purple, flush the marijuana plant, and add nutrients.
Abnormal growthCheck the temperature of the grow room and pH levels, adjust accordingly.Flush the marijuana plant, and add nutrients.

What is phosphorus burn in cannabis plants?

A phosphorus burn in cannabis plants happens when there’s an excess of the nutrient in the growing medium. It’s the exact opposite of a phosphorus deficiency in weed. 

A phosphorus burn in cannabis causes a massive nutrient lockout of other essential elements like zinc, iron, magnesium, copper, and calcium. Usually, zinc gets locked out first. 

You can determine a phosphorus burn in your cannabis plants by these symptoms:

  • Yellow veins on upper leaves
  • Tips of leaves look burnt
  • New leaves grow thin fingers
  • Bottom leaves curl and have spots

Final points on a phosphorus deficiency in weed

You should take a phosphorus deficiency in weed plants seriously because it can be lethal. Phosphorus is one of the three essential nutrients for cannabis plants. A deficiency can be caused by a pH imbalance, cold temperatures, or an excess of other elements. 

The signs of a phosphorus deficiency in weed plants are discolored leaves and stems, abnormal growth, and dry or still leaves. You can fix your issue by flushing your marijuana plant, replenishing with fertilizer, checking the pH, regulating the grow room’s temperature, and adding guano. 

Now you know everything about a phosphorus deficiency in weed; look at your cannabis plants. If you haven’t started growing yet, buy high-quality marijuana seeds at Homegrown Cannabis Co. now!

About the author: Derek LaRose

Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.

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