Phosphorus Deficiency in Cannabis Plants

How to Spot and Correct Phosphorus Deficiency in Cannabis Plants


Growing cannabis can be really difficult. Every once in a while, your plants turn on you and start showing symptoms of being unhealthy even though you’ve followed nutrient instructions to a T.

Cannabis plants need specific macro and micronutrients in the right ratios to stay healthy and even though you think you are doing everything right, nutrient deficiencies can show up and destroy entire harvests.

Phosphorus (P) is one of those macronutrients that plays a crucial role in the plant’s life cycle and in this guide we will provide an in-depth analysis of phosphorus deficiency in cannabis plants. You’ll learn how to identify early symptoms and correct the deficiency before it’s too late.

  • Symptoms tend to start at the bottom of the plant on older growth
  • Usually happens during flower
  • Darkening of leaves with blotches of dark discoloration
  • Stems in red and purple shades
  • Leaves start to yellow with brown spots
  • Curling and twisting leaves
  • Overall stunted growth
  • Reduced vigor and leaves feel stiff
  • Poor root development
  • Small flowers and poor yield
  • More below 👇
Phosphorus Deficiency in Cannabis Plants

(P) Phosphorus: One of the essential macronutrients that cannabis plants need to grow healthy, big, and fast. Found in soil, roots and stems.

Macronutrient: Absorbed in large quantities from the growing media or the nutrient solution. Macronutrients are the best-known and recognized constituents of plant food and their potency is identified as N-P-K ratings, printed on all commercially available plant food containers.

What do cannabis plants use phosphorus for?

Phosphorus is, without a doubt, one of the most essential components found in soil and needed for all plant growth. Most growers associate Phosphorus mainly with the flowering phase because it is crucial during flower production (and seed production) and is important if you want to grow big colas and get high yields. But it is needed for much more than just flowering and it used by the plant throughout the entire growth cycle.

It is necessary from early clone stages to flowering and harvest. It plays a crucial role across several key plant growth areas, including energy production, photosynthesis, disease resistance, cannabis metabolism, reproduction, seed formation, respiration, and genetics. In fact, both DNA and RNA are linked together by bonds that are made from Phosphorus.

  • Photosynthesis – Phosphorus is a key component of adenosine triphosphate (ATP), the primary energy source for cellular activities including photosynthesis.
  • Root development – Adequate phosphorus levels promote strong root growth, enabling the plant to efficiently absorb water and nutrients from the substrate.
  • Flowering & seed production – As discussed, phosphorus is needed for flower and seed production and it impacts the yield and quality of the final product.
  • Cell division & growth – Phosphorus is essential for DNA and RNA synthesis, facilitating rapid growth of cannabis plants.

Phosphorus has actually become a limited resource worldwide. It mainly exists in mineral form, and commercial agriculture has created an increasing demand for this macronutrient, which some scientists predict will have become completely unavailable by the end of the century.

We urge all growers to understand the importance of taking care of the planet’s resources and to commit to an eco-friendly grow.

How do you identify a cannabis phosphorus deficiency?

Identifying a phosphorus deficiency can be a little tricky, especially if you are a novice grower. You need to know the early symptoms and not get them mixed up with any other deficiency or issue.

The main symptoms of phosphorus deficiency include the following;

  • Darkening of leaves – A phosphorus-deficient plant will exhibit dark green leaves, particularly on the lower and older parts of the plant. Yellowing leaves and brown spots (and in some cases dead spots) will start to appear and you may notice red or purple stems*.
  • Leaf curling & twisting – Leaves may curl downward, twist and become distorted in shape.
  • Stunted growth – Plants suffering low levels of phosphorus have severely stunted growth and reduced vigor.
  • Poor root development: Phosphorus is needed in root development so a deficiency leads to an underdeveloped root system which in turn limits nutrient uptake and overall health.

*Some strain genetics have naturally purple stems and shades so don’t panic if it’s the only symptom you see.

Phosphorus Deficiency

Don’t misdiagnose a phosphorus deficiency symptoms

There are several factors that can cause symptoms that mimic phosphorus deficiency so don’t jump to conclusions at the first sight of a twisted leaf.

  • Nitrogen toxicity & overwatering can also cause leaves to curl downwards.
  • Nitrogen deficiency causes similar yellow leaves that start at the lower parts of the plant.
  • Cold temperatures may slow down phosphorus uptake, causing deficiency-like symptoms.
  • pH imbalances in the growing medium can cause a nutrient lockout, preventing the plant from absorbing phosphorus.
  • Copper deficiency darkens leaves to a purplish-blue coloration

Causes of phosphorus deficiency

Phosphorus is a common nutrient deficiency but it’s quite easy to avoid if you learn the most common causes behind it.

  • Lack of phosphorus in the growing medium – It may sound obvious but a lack of available phosphorus in the soil or hydroponic solution is the number one reason plants suffer from phosphorus deficiency. Inexperienced growers often underestimate the amount of phosphorus cannabis plants need (especially in the flowering phase) and consequently underfeed them. Follow nutrient instructions closely and use easy-to-use systems like Flower Republic.
  • Improper pH levels – Cannabis plants prefer a pH of 6.0-7.0 in soil and 5.5-6.5 in hydroponic systems. Deviations can affect phosphorus availability to absorb phosphorus. Acidic soil is the most common for outdoor growers.
  • Over-fertilization – Excessive levels of other nutrients, such as zinc and iron, can interfere with phosphorus uptake so revise your entire feeding schedule to see if you have multiple supplements and additives that do the same job.
  • Cool temperatures – Low temperatures can slow down phosphorus uptake resulting in deficiency symptoms.

How do you correct a phosphorus deficiency?

Ok, so you have concluded that you in fact have a phosphorus deficiency on your hands. Luckily, phosphorous deficiencies are not too difficult to fix but you need to act quickly and decisively to address and correct the issue before it ruins your harvest.

  • Monitor temperature – Ensure that the temperature conditions are between 70-85°F (21-29°C) during the day when the lights are on. It can be slightly cooler at night but don’t allow for large temperature swings.
  • Test the substrate – You should always have a pH meter or test kit available so you can determine the pH level of the substrate and adjust it accordingly.
  • Adjust nutrient levels – Increase the phosphorus levels in the growing medium by applying a high-phosphorus fertilizer.
  • Flush the growing medium – As a last resort you may need to flush the growing medium completely with clean pH water to remove excess nutrients and reapply a balanced nutrient solution. Learn how to flush cannabis plants here.

Phosphorus Deficiency During the Vegetative Stage

Phosphorus deficiency during the vegetative stage can have long-lasting effects on the expansion of the root system which in turn affects overall growth and plant health. To address and prevent phosphorus deficiency in this stage, consider these additional steps:

  • Provide a balanced nutrient solution – Ensure your fertilizer or nutrient mix is tailored for the vegetative stage, containing adequate phosphorus levels.
  • Regularly monitor growth & development – Keep an eye on your plants’ growth patterns and adjust nutrient levels as necessary.
  • Implement a preventative maintenance plan – Regularly check pH levels, maintain optimal temperatures, and adhere to proper watering practices to minimize the risk of nutrient deficiencies.

Phosphorus deficiency during the flowering stage

Phosphorus is particularly critical during the flowering phase, as it is essential for bud production and seed development. To address phosphorus deficiency during this stage:

  • Switch to a flowering-specific nutrient solution – Transition to a nutrient solution with higher phosphorus and potassium levels, designed for the flowering stage.
  • Optimize environmental conditions – Maintain consistent temperature and humidity levels to promote healthy flowering and prevent stress-induced nutrient deficiencies.
  • Monitor flower development – Regularly inspect your plants for signs of phosphorus deficiency and adjust nutrient levels as needed.

How can microbes help with Phosphorus deficiencies?

The rhizosphere contains a multitude of microorganisms that help plants grow happy and healthy.

Microbeshelp plants digest Phosphorus and use it more efficiently. Through their ability to modify local pH, microbes have the power to unlock bound nutrients, such as P, transforming them through solubilization and mineralization into a form that plants can intake. 

Bacteria and fungi need phosphate to grow, just like the plants they live in. Don’t forget that these are symbiotic relationships: microbes do get something in return when they work their magic. Your plants’ root give something in return: they release exudates into the soil. These substances include sugars, amino and organic acids that microbes are happy to include in their meal plan.

There are special little fungi called Arbuscular mycorrhizae (AM) that are perfect for cannabis plants struggling in the Phosphorus department. This microorganism forms an alliance with the plants’ roots, therefore increasing their surface. This allows the plant to extend the soil volume it can reach, and so the number of nutrients it can intake. When AM colonizes a plant, it is estimated that the fungus supplies 80% of the phosphate taken by the plant. This is what we mean when we say that it takes a village to ensure your plants grow as strong as they can.

So, microbes are an army that can help your cannabis plant find all the Phosphorus it needs to thrive. And, thankfully, they don’t hurt the environment when they do so. Amplify, our shelf-stable, full-spectrum microbe ferment, is all of this in a bottle. It mobilizes and transforms P so your cannabis can thrive. Interested? Send us a message for more information.

Try our shelf-stable ferment of full-spectrum microbes called Amplify. Combine it with a high nitrogen fertilizer to grow big, healthy cannabis plants that don’t turn yellow while you’re not looking.

Amplify bottles

Learn how to spot and fix cannabis nutrient deficiencies

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