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How To Identify, Treat, And Prevent Phosphorus (P) Deficiency Issues In Cannabis

Phosphorus deficiency cannabis

No matter if you are growing indoors or out, in a hydroponic setup, or just plugging those seeds directly into the ground – nutrient deficiencies are just a fact of life for cannabis cultivators, especially if you are just starting out. A deficiency in one or more of the three main nutrients – Nitrogen (N), Phosphorus (P), and Potassium (K) – can be a real pain in the a** to deal with. And while we all hope and pray that we can get through our first few crop runs without any issues popping up, the truth is that nutrient deficiencies can happen at any time.

Fortunately, if you are able to catch them early on and identify the culprit, many of these deficiencies can be easily remedied and prevented in future grows. That being said, one of the most common issues in cannabis crops is a deficiency in phosphorus.

So hang about as we discuss exactly what to look for, how to go about treating any issues that may arise, and how to prevent any potential problems from popping up down the line.

What Is Phosphorus, And Why Is It Important For Cannabis Crops?

Phosphorus is a naturally occurring element that is vital not just for plant life, but all life on earth. Sitting just below Nitrogen in group 15 of the periodic table, this macronutrient is symbolized by P on the periodic table of elements.

In terms of a cannabis crop, phosphorus is essential for a range of processes –

  • It is vital for root growth and development as well as vigorous stem and foliage growth.
  • It plays an important role in the flowering stage, helping to promote huge yields.
  • It helps to boost the overall resilience of your plants, making them more resistant to stress and disease.
  • Phosphorus is a fundamental part of ATP (adenosine triphosphate) production, which is the main energy source used by your plants.
  • It helps to activate enzymes, boosting photosynthesis and other important metabolic processes.

The Causes of Cannabis Phosphorus Deficiency

Most commonly affecting plants once they have entered full flower production, phosphorus deficiencies can be caused by a couple of different issues. The usual culprit in hydroponic systems is a pH imbalance in the root zone, as phosphorus is much more readily available to hydro-grown cannabis plants when the pH of the root zone is in the 5.5 to 6.5 range. The pH range is less important for organic soil-grown crops, but for the best results you should look to keep the root zone in the 6.0 to 7.0 range.

But it can also come down to a lack of phosphorus in the soil or nutrient solution, or even an oversupply of the stuff. Overfeeding is a pretty standard issue that many novice growers run into, and it’s quite easy to see why. More of those nutrients should equal healthier plants with bigger buds, right?

Nope, not at all. In fact, it’s actually better to have a slightly lighter soil mix or nutrient solution. Overfeeding causes a wide range of problems, and it can even lead to deficiencies in other nutrients – phosphorus included. So be sure to err on the side of caution when you are mixing up your feed water – keep it to around 1/4 of the recommended dose for young plants, and slowly increase that dosage up to 1/2 or 3/4 strength over the vegetative period.

The same goes for the soil growers. You want to make sure that the soil you are using isn’t too rich in nutrients, especially for plants that have just broken the surface. Soil blends that are heavy on nutrients can and will kill cannabis seedlings, so we always suggest that you start off with light, airy soils and then slowly move up to a more nutrient-rich mix once they have gotten their feet under them.

It’s also worth noting that if there is too much calcium present in the soil, it can bind with phosphorus and make it much harder for your plants to absorb.

The fact that a phosphorus deficiency problem can be caused by a range of issues can lead to some confusion but don’t worry as we will get into how to go about working out what the cause is a little further down this article.

Signs And Symptoms Associated With Cannabis Phosphorus Deficiency Issues

Thanks to the fact that phosphorus is a mobile nutrient (meaning the plant can move it around from older to younger leaves), it almost always presents in the old fan leaves to start out. As the plant moves the phosphorus from the bigger fan leaves to the newer growth, they may display a range of symptoms, including:

  • Yellowing between the veins of older foliage.
  • Leaves may curl upwards and become stiff or brittle.
  • Along with the overall yellowing, the leaves may also develop brown or purplish rust spots.
  • The leaves may also thicken slightly.
  • The stems of the leaves may turn a reddish hue, but not always.
  • The vertical growth of the crop may be considerably stunted.
Phosphorus deficiency cannabis symptoms

As we mentioned a little earlier, it is pretty rare to see issues with phosphorus during the vegetative stage. While the plants do need a decent supply of phosphorus during the time when they put on their size, it becomes much more important once the flowering stage begins. You may start to notice an overall decrease in energy and vigor, and the buds themselves won’t be quite as resinous, and the density will be reduced quite significantly.

The general progression of symptoms is as follows –

  1. The older leaves begin to yellow or become a little more pale than usual.
  2. Then the brown/purple blotches begin to form on the older fan leaves.
  3. Next, the stems and petioles (the leaf stems) turn red or purple.

Once the red stems start to show, the deficiency has really taken hold. If you have plants displaying this you will really want to try to get on top of the issue as soon as possible.

How To Treat A Cannabis Phosphorus Deficiency, Or Prevent It Happening In The First Place

Ok, so we now know how to correctly diagnose a phosphorus deficiency issue. Now it’s time to get cracking on fixing it, yeah?

Regulate The pH First

The very first thing you should do if you think there is an issue with any nutrient, not just phosphorus, is to check the pH. Now, if you are growing with hydro or coco-coir then this is something that you really should have been doing the entire time – but not everyone realizes just how vitally important this is when they first start out.

So, if you don’t already own a digital pH meter,  then head right here to grab one. They are cheap, easy to use, and usually very accurate – put simply, this is a piece of kit that every single weed grower should have at the ready, no matter the style of cultivation.

Let’s begin by flushing the plants with clean, plain water at the correct pH. Again, if you are growing in soil then you will want this water to be in the 6.0 to 7.0 range, or 5.5 to 6.5 for hydroponic systems (including coco-coir). To flush the plants correctly, it’s important to see at least 30% runoff from each pot. Its also critical to collect some of the runoff and use the pH tester to see if the pH is correct. No matter if it is too high or too low, you will want to continue flushing with water at the correct range. Many new growers think that if the runoff is too high they should use water at a lower-than-optimal pH to help balance things out, but this is a pretty dangerous tactic (and vice-versa). It’s best to just flush multiple times with water in the optimal range until the runoff comes back correct.

Make Sure Your Nutrient Levels Are Correct

Once the pH is back in order, you can move on to adjusting your nutrients. If the phosphorus levels were low before, the best way to fix this is by adding an organic or mineral-based phosphorus fertilizer. This will replenish the levels and should help get your plants back on track. Now, just like with pH, there are testing instruments that can be used to make sure the nutrient solution or feed water you are providing to the crop is in the correct range. These are a little more pricey than the pH meters but are also vital to having a successful grow. Check out the one we use right here.

Phosphorus deficiency weed

If you think that the issues may have been caused by an over-supply of nutrients, then the flushing routine should be done multiple times to make sure that any excess nutrients are removed so that the plants can recover. You can also test the runoff with your TDS (or EC) meter to see if the nutrient levels in the root zone are above the recommended level.

Finally, if the plants still aren’t responding to the adjustments you have made, then it’s time to start looking at other possible causes. Check the environment, as too much heat or too little light can also lead to deficiencies. Check for other pests, diseases, and any fungal growth that could be causing issues.

The Best Synthetic And Organic Phosphorus Sources That Can Be Use To Boost The Phosphorus Levels

In terms of synthetic solutions, there is a huge range of phosphorus-based fertilizers on the market. Some of these are ready-to-use or come in concentrated form, and some even have multiple other nutrients like calcium, magnesium, and iron included. Most flowering-boosting additives will have ample amounts of phosphorus as part of their makeup, but which option is best?

Well, that really depends. For the most part, you can’t go wrong if you stick with an additive from one of the more established brands. Look for options from brands like

That is just a short subsection of trusted brands, with many other choices available. Always use discretion when using these types of additives, and start with 1/2 of the recommended dose.

When it comes to organic-based phosphorus sources, there is a wide range of options available. Powdered bone meal, rock phosphate, and fishbone meal are all great choices for supplementing your soil or hydroponic system with phosphorus. If you’re looking for something more natural, then guano is another good option. Bat guano, in particular, is high in phosphate and has many other beneficial micronutrients.

These organic sources can be added directly to the top layer of the soil, but for the best results you may want to look into making a compost tea. This is where you take a bucket of water and add in your chosen organic phosphorus source before adding a handful of compost. This concoction is then left to steep for a few days, before being strained and used as a foliar feed or added directly to the soil.

No matter which type of phosphorus source you choose, it’s vital to remember not to over-apply. Too much phosphorus can lead to a whole host of problems; from nutrient lock-out to an increase in pests and diseases. So make sure you always start with a lower dose before gradually increasing it if needed.

How Dangerous Is Phosphorus Deficiency For Cannabis Crops?

While a low-level phosphorus deficiency isn’t the biggest problem and is (usually) easily fixed, the issue can quickly become more serious if left unattended. As the plants move into the flowering growth stage, the need for the correct amount of phosphorus increases exponentially, and so does the risk of nutrient deficiency. If your plants are showing signs of phosphorus deficiency, then you should act fast to fix the issue before it starts to affect the yields.

In some cases, phosphorus deficiency can be so severe that it results in plant death. This is why it’s so important to try and identify the issue early and take corrective action.

The Wrap Up

So, there we go. Everything you need to know to not only identify a phosphorus deficiency, but also the step that should be taken to get things back on track, and how to prevent any problems from arising in the first place. 

Remember, phosphorus is a vital nutrient for your cannabis plants. Without access to this important element the plants simply won’t reach their full potential, and yields will suffer. So make sure you pay close attention to phosphorus levels in the soil or hydroponic system and don’t be afraid to supplement with a phosphorus-based fertilizer if needed.

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Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author

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