Importance of phosphorus for root development and flower production

Phosphorus is essential for plant growth and flower development, but using the right amount at the right time makes all the difference. Phosphorus deficiency can cause serious issues in cannabis plants and phosphorus excess can cause damage too.

In this guide, we’ll dive into the role of phosphorus first and then address the multiple causes of nutrient imbalance, deficiency and toxicity in cannabis plants, helping you keep your grow on track.

Phosphorus helps the seed germinate and supports strong root systems, helping the plant absorb more water and nutrients.

During the vegetative stage, the roots store the most phosphorus.

When there’s plenty of phosphorus, your plants grow taller. Shorter spaces between the branches, called internodes, often lead to better-quality buds. To achieve this, avoiding excessive applications of phosphorus during the vegetative stage and early flowering is a good idea. This helps your plant focus on producing dense, high-quality flowers instead of stretching too much.

During the flowering stage, cannabis plants store much more phosphorus in the flowers than in any other part of the plant.

Recent research suggests that cannabis plants rely on phosphorus uptake to supply the flowers during the early stages of flowering. As the plant’s rapid growth slows down after the start of flowering, the plant reduces phosphorus uptake and starts moving stored phosphorus from other parts of the plant to support flower development.

So keeping balanced the phosphorus applications for all stages of growth is key.

Phosphorus deficiency in cannabis

In this section, we’ll explore the causes, symptoms, and solutions for phosphorus deficiency, so you can keep your plants thriving.

When a cannabis plant lacks phosphorus, its growth slows, and it may struggle to produce strong roots or healthy buds, resulting in reduced yields and quality. 

Recognizing the early signs of phosphorus deficiency—such as dark, dull foliage, purpling of stems, or brown spots on older leaves—can help growers address the problem before it affects the entire plant.

Signs of phosphorus deficiency

• Dark, dull, or purplish leaves: This is an initial symptom, often appearing first on older, lower leaves. The purplish color can also appear in stems and petioles (the stem of the leaf).
• Brown leaf edges and blotches: As the deficiency progresses, brown spots and patches may appear on the leaves, eventually leading to necrosis.
• Lower leaves curl downward: This is a more advanced symptom of phosphorus deficiency.
• Slowed growth: Phosphorus deficiency can lead to stunted growth, particularly noticeable during the flowering phase. Plants may exhibit less branching and reduced height.
• Delayed flowering: A lack of phosphorus can delay the onset of flowering.
• Small, underdeveloped buds: Phosphorus is crucial for flower formation, and a deficiency will result in smaller, less potent buds.
• Olive green spots on leaves (in the flowering stage): This symptom, specifically observed in flowering plants, appears as olive green to yellow discoloration on the leaves at the base of each branch.

Phosphorus deficiency often becomes noticeable later in a cannabis plant’s life cycle, especially when the plant starts to use up energy reserves for developing its flowers.

It’s important to note that some of these symptoms, like purple leaves and stems, can also be caused by factors other than phosphorus deficiency, such as low temperatures or genetics. Therefore, it’s crucial to consider the overall health of the plant and growing conditions when diagnosing a deficiency.

Phosphorus deficiencies in the vegetative stage start as stunted and slow growth of the whole plant and then show on the older leaves as olive green spots and purpling leaves and stems. 

Phosphorus deficiency is particularly damaging during the flowering stage as it directly impacts flower development. Early detection and correction are vital to prevent significant yield reduction and poor flower quality. If left untreated, it can result in significantly lower yields and poor flower quality.

Phosphorus deficiency causes

Phosphorus deficiencies often appear during flowering when cannabis plants need more of this essential nutrient. While it’s a common issue, understanding the causes can help you address it effectively.

pH problems

A pH above 7 in the root zone or nutrient solution can make phosphorus unavailable for the plant to absorb, even if you are fertilizing correctly. High pH locks phosphorus in the soil, making it unavailable to the plant. When growing in organic-based substrates like peat and coco, remember that the pH of the water and fertilizer solution is not the main cause of pH swings. You can’t control the substrate’s pH only by applying a nutrient solution with a certain pH.

What to do:

• Test your soil or water pH regularly. Use a pH meter or test strips.
• Adjust the pH to stay between 5.8 – 6.2 for soil or 5.5 – 6.0 for hydroponics.*
• In organic-based substrates like peat and coco, pH stability in the root zone depends more on microbial activity, substrate buffering, type of nutrients and organic matter breakdown than on the pH of the water or fertilizer solution. 
• To avoid pH swings, support a healthy microbial ecosystem, use fertilizers designed for these substrates, avoid over-fertilization, and consider adding buffering agents like dolomite lime. Regularly check the runoff or substrate pH for a clearer picture of root zone conditions.

Waterlogged or poorly drained growing medium

Phosphorus deficiency can appear if the pot is not well drained or the soil is waterlogged. Waterlogged soils can prevent roots from accessing phosphorus. Roots need oxygen to function, and soggy soil suffocates them.

What to do:

• Improve drainage by adding perlite and vermiculite to your soil mix.
• In peat and soil-based media, water only when the topsoil feels dry to the touch, avoiding overwatering. Don’t mix worm castings on these substrate or do it on a small percentage to avoid compact soil mixes.
• Water until runoff.
• Take out the runoff water after an hour to avoid a waterlogged root zone.

Cold temperatures

Phosphorus uptake slows significantly in cold conditions, especially when soil temperatures drop below 15°C (60°F). This can cause phosphorus deficiency symptoms, even if phosphorus is present in the nutrient solution or growing medium.

What to do:

• If growing outdoors, wait to plant until nighttime temperatures stay above 15°C (60°F).
• Use water at room temperature 20 – 22°C (68 – 72°F) for hydroponics and up to 24°C (75°F) for other growing media. Water temperature above 25°C can make salts more soluble and more nutrients will be leached.
• Use a heating mat or adjust the room temperature for indoor grows to keep the root zone warm.

Nutrient antagonism

Nutrient antagonism happens when certain nutrients like iron (Fe), zinc (Zn), or calcium (Ca) are in excess, interfering with phosphorus uptake. Overloading your soil or growing medium with one nutrient can block others. 

What to do:

• Avoid over-fertilizing with products rich in iron, zinc, or calcium.
• Check the balance of your nutrient mix and stick to the recommended dosages.

Purple stems and leaves

Purpling of stems and leaves is a common symptom of phosphorus deficiency in cannabis, often appearing alongside slow growth and reduced vigor. While some cannabis strains naturally exhibit purple hues due to genetics, a phosphorus deficiency causes a more uniform and dull purpling, particularly on the stems and the edges of older leaves.

Notice that purple leaves and stems may be a sign of phosphorus deficiency but low temperatures, strong grow lights and genetics may also cause it like in the following photo.

Understanding the main causes of phosphorus deficiency allows you to take precise actions and keep your plants thriving. Watch for symptoms like purple or dark green leaves with reddish stems and act quickly to prevent stunted growth or poor flowering.

How to fix Phosphorus deficiency in cannabis

First, read the possible causes of phosphorus deficiency in the section before this one.

If the deficiency is caused by a lack of phosphorus in the growing media, use a water-soluble fertilizer that contains phosphorus to treat it.

Many “bloom formulas” for cannabis have the right amount and ratio of P needed to fix the deficiency.

Affected leaves won’t recover after fertilizing but new leaves should recover within 5-7 days for most commercial formulas.

Some organic compounds may be used too, but they take more time to be available for the plant to absorb, because the microorganisms in the soil need to transform the organic compounds into forms of phosphorus the plant can absorb.

Tip: Always wear a mask and gloves when mixing and handling manures and organic compounds such as bat guano, as they can be a source of some toxic fungi like Histoplasma Capsulatum, that can seriously damage your health.

Phosphorus toxicity cannabis

Phosphorus toxicity signs are rare in cannabis plants but excess phosphorus can cause significant issues.

In this section, we’ll explore the subtle signs of phosphorus toxicity, its impact on your plants, and how to prevent and correct it to maintain a balanced nutrient profile.

Excessive phosphorus levels can lead to nutrient imbalances, particularly locking out essential micronutrients, which are critical for healthy growth. Some commercial fertilizer formulas contain excessive amounts of phosphorus, far more than cannabis plants need for healthy growth. While the plant will continue absorbing phosphorus even when it’s not required, this surplus can disrupt nutrient balance, leading to deficiencies in other essential elements.

Excess phosphorus can also accumulate in the soil or runoff, posing a threat to the environment by contributing to water pollution and algal blooms.

Choosing fertilizers with balanced nutrient ratios and avoiding overfeeding helps prevent these issues while supporting healthier plants and more sustainable growing practices.

Signs of phosphorus excess:

• Yellowing leaves (chlorosis)
• Long internodes and weak stems
• Excess phosphorus can decrease the uptake of iron, zinc, and copper, leading to nutrient deficiencies of these elements. 
• Nutrient lockout: Iron deficiency is the most common one and usually the first to shop up when excess phosphorus is applied
• Metallic/bad taste in flowers
• Burnt flower and leaf tissue
• Excess phosphorus leachate/runoff causes serious damage to the environment and contaminates water courses

Excess phosphorus, especially during the vegetative stage, can cause your cannabis plants to grow excessively tall with elongated internodes, so it’s important to avoid overapplying this nutrient to get compact, strong plants.

Applying composts or manures that contain very high levels of phosphorus can result in nutrient burn or toxicity, especially if there is no runoff or leachate.

Plants can experience phosphorus toxicity if they have drip irrigation systems without leachate/runoff and phosphorus accumulates in the soil mix.

While cannabis phosphorus toxicity signs are less common than P deficiency ones, it’s still important to maintain a proper balance, especially during the flowering stage.

Luxury consumption of phosphorus

When it comes to feeding cannabis plants, understanding the concept of “luxury consumption” is crucial. Luxury consumption occurs when a plant absorbs more of a nutrient than it actually needs for optimal growth. 

Phosphorus (P) is a prime example: recent studies showed that cannabis plants can take up extra phosphorus if it’s available in the soil or nutrient solution, but this excess doesn’t necessarily translate into bigger or better quality yields. This also indicates that the plant may uptake P beyond its immediate needs without showing signs of toxicity.

Until recently, many growers worldwide thought that giving their plants very high levels of phosphorus would translate into bigger yields. Instead, now we know that the surplus P is stored within the plant tissues without enhancing growth or flowering.

For growers, this means that adding extra phosphorus beyond what is necessary is a waste of resources, money and could even disrupt the nutrient balance in your plants.

“These results demonstrate that while P plays a large role in plant growth and development, a fertility rate of 11.25 to 15 mg·L–1 P recommended (Cockson et al., 2020) will provide optimal growth. Thus, growers can optimize yield and limit economic inputs without supplying the plant with a greater P fertility rate that would promote luxury consumption.” (Veazie – Cockson, 2021)

Read this paper cited below for more information.

Antagonistic luxury of phosphorus

Excess phosphorus can lead to “antagonistic luxury,” a phenomenon where high levels of phosphorus interfere with the uptake of other essential nutrients, particularly iron. This interference happens because phosphorus competes with these nutrients, making it more difficult for your plants to absorb them, even if they are present in sufficient amounts in your growing medium.

Maintaining the right nutrient balance and avoiding luxury consumption is key to healthy and productive plants.

Many growers overuse phosphorus unknowingly, especially during the flowering stage, often without realizing it since toxicity symptoms aren’t immediately noticeable.

When phosphorus accumulation becomes too high, you typically won’t see direct signs of phosphorus toxicity on the leaves in cannabis plants.

However, what you may observe instead is an iron deficiency, as the excess phosphorus interferes with iron uptake and availability, causing symptoms like yellowing between the leaf veins on the new top leaves.

Iron deficiency yellowing pattern moves from the petiole (stem of the leaf) out to the tip. Iron deficiency shows first on the top new leaves. 

On the contrary, when there’s a nitrogen deficiency the first yellowing leaves are the lower leaves and the yellowing pattern goes from the tip inwards the leaf.

How to fix Phosphorus toxicity in Cannabis

Avoiding over-fertilization with phosphorus is essential for several reasons. 

First, maintaining a proper nutrient balance helps maximize yields by ensuring plants get exactly what they need to thrive.

It also saves money by reducing unnecessary spending on excess nutrients.

Additionally, limiting fertilizer use helps protect the environment by reducing groundwater contamination risk, and supporting healthier surrounding ecosystems.

When using phosphoric acid to lower pH in hydroponics, be aware that it adds phosphorus to your nutrient solution. If you don’t account for this, phosphorus toxicity can occur, leading to nutrient imbalances.

Tracking the phosphorus added through pH-down solution adjustments is key to keeping your nutrient solution balanced, ensuring healthy plant growth, and avoiding phosphorus toxicity. Log your nutrient applications on the Grow with Jane app!

Conclusion

KEY: It’s important to diagnose the cause of the deficiency correctly before trying to solve it by “adding more nutrients”, as this can worsen the situation.

In the first photo there’s a plant in the vegetative stage, and in the second photo is the same type of plant in the flowering stage, both grown with 30 mg/L of phosphorus (low P). Look closely at the leaves at the bottom of each branch on the flowering plant. They have an olive-green to yellow color, which is a sign of phosphorus deficiency. These symptoms match what other studies have described as phosphorus deficiency (Cockson et al.2019). (Cockson, Whipker, 2020)

Tip: If your plants seem stunted or are producing small buds, adjust your nutrient mix to include more phosphorus. Regularly inspect your plants for signs of slow growth, and keep a record of their progress to spot patterns or issues.

Figure 2 shows how different amounts of phosphorus affect cannabis growth. Plants that receive the right amount of phosphorus grow big, healthy flowers, while those with too little phosphorus produce much smaller buds and don’t grow as well overall. Adding too much phosphorus doesn’t make the plants grow better and might even cause problems.

Regularly inspecting your cannabis plants for signs of nutrient deficiencies is a crucial habit for any grower. Often, by the time visible symptoms like yellowing leaves, necrotic spots, or stunted growth become obvious, the plant has already been experiencing the deficiency for several weeks. This delay can significantly impact overall health and yield, making early detection critical.

Periodic checks allow you to catch subtle changes early, so you can adjust your nutrient regimen or growing conditions before the deficiency worsens.

Check your fertilizer label and follow the recommended phosphorus levels for cannabis. Avoid overfeeding—too much phosphorus can waste nutrients and reduce growth efficiency. If you see these signs even when applying the recommended doses, consider lowering the doses and see how your plants react.

Recommended NPK for each stage

Some authors recommend different NPK rates for each stage of growth. Here’s a ratio I recommend too in general but take into account that each strain and even individual plants may require slightly different nutrient recipes for optimal growth.

NPK for the vegetative stage: 3-1-2

NPK for the early flowering stage: 1-2-2

NPK for the flowering stage: 1-3-4

In this guide, you can find more information and recommendations about NPK ratios for each stage.

Have you encountered phosphorus issues in your grow? Share your experience in the comments, join our community to connect with other growers, or access personalized guidance through Grower Support in the Grow with Jane app!

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In the next guide of this series, we’ll explore the role of Potassium in cannabis plants.

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References and sources
Cockson, P., Landis, H., Smith, T., Hicks, K., & Whipker, B. E. (2019). Characterization of nutrient disorders of Cannabis sativa. Applied Sciences, 9(20), 4432. https://doi.org/10.3390/app9204432

Cockson, P., Schroeder-Moreno, M., Veazie, P., Barajas, G., Logan, D., Davis, M., & Whipker, B. E. (2020). Impact of phosphorus on Cannabis sativa reproduction, cannabinoids, and terpenes. Applied Sciences, 10(21), 7875. https://doi.org/10.3390/app10217875

Llewellyn, D., Golem, S., Jones, A. M. P., & Zheng, Y. (2023). Foliar symptomology, nutrient content, yield, and secondary metabolite variability of cannabis grown hydroponically with different single-element nutrient deficiencies. Plants, 12(3), 422. https://doi.org/10.3390/plants12030422

Westmoreland, F. M., & Bugbee, B. (2022). Sustainable cannabis nutrition: Elevated root-zone phosphorus significantly increases leachate P and does not improve yield or quality. Frontiers in Plant Science, 13. https://doi.org/10.3389/fpls.2022.1015652

Rosenthal, Ed. Cannabis Grower’s Handbook: The Complete Guide to Marijuana and Hemp Cultivation (English Edition)

Photos: Cited literature, Alicia Muzio (Author) and Unsplash photographers credited.

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