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Identifying and Fixing Phosphorus Issues in Cannabis Plants

by | Aug 22, 2022 | Cannabis, Getting Started, Nutrients, Troubleshooting

Phosphorus is a key nutrient that plays a crucial role in plant growth. Because phosphorus is so important, it’s important to know how to fix phosphorus toxicity or deficiency in your cannabis plants. Unfortunately, phosphorus toxicity can occur when there are too many phosphates in the soil which leads to the loss of chlorophyll production by leaves and eventual death of plant tissue. On the other hand, phosphorus deficiencies are caused by a lack of phosphate in the soil leading to stunted growth and wilting leaves. Here are some tips on how to prevent phosphorus toxicity or deficiency from happening.

What is phosphorus?

Phosphorus is an essential nutrient for plants. There are three different forms of phosphorus: organic, combined, and elemental. Organic phosphorus can be broken down into phytate P and phosphite P. Combined phosphorus is also known as phosphoric acid H₄PO₄. Elemental phosphorus makes up the rest of the total phosphorus in soils and plants and is the one we will be focusing on.
It naturally occurs when phosphates bond with oxygen and hydrogen, but we can help this process along by adding phosphoric acid to our soils. This is done through fertilizers that contain phosphorus as well as all of the other macronutrients: nitrogen (N) and potassium (K).

What is the phosphorus cycle?

The phosphorus cycle is an essential process for plants. When the soil has excess phosphorous, the phosphorus becomes water-soluble where it becomes bioavailable to aquatic life and helps build riverbanks. Phosphorus in rivers can also attach itself to sediment particles which make their way downstream until they are deposited elsewhere like deltas or beaches. This means that phosphorus can be found in areas of high phosphates, or phosphogypsum. It is also phosphite which helps microbes to reproduce and break down organic matter.

Phosphorous, like all nutrients, first enters the soil as an ion, phosphite, or phosphate. As phosphites are more stable than phosphates in water and soils that have a high pH (alkaline), they accumulate and help increase soil fertility in alkaline soils. Atmospheric conditions can also quickly turn phosphoric acid into phosphite by oxidizing phosphoric acid.

Is phosphorus a macro or micronutrient?

Phosphorus is a macronutrient. A macronutrient has to be ingested in larger quantities than micronutrients, which are needed in smaller amounts. Like phosphoric acid H₄PO₄, it can also move out of the root zone by leaching or through deep drainage if soil conditions are not suitable for plant growth.

Is phosphorus a mobile or immobile nutrient?

Phosphorus is a mobile nutrient. It can move from older, more mature leaves to younger ones, which are the upper leaves, by either being pulled out of the soil or transpiring through its stomata and evaporating into the air.

Why do cannabis plants need phosphorous?

Cannabis needs phosphorus for photosynthesis, root development, seed production, and protein synthesis. Phosphorus is an essential element for plant growth in the form of phosphates (PO₄). It has three main functions: it forms DNA and RNA molecules, transports energy throughout cells via ATP (Adenosine Triphosphate), and helps plants fix nitrogen into amino acids.

Cannabis plants need phosphorus because it is a macronutrient that plants need in order to grow. Phosphorus works with the other macronutrients, nitrogen, and potassium, to create proteins needed for photosynthesis. It also helps activate certain enzymes so your cannabis plant can properly absorb all of its nutrients through the roots.

Phosphorus has many different roles inside the plant including being a structural component for cell walls, membranes, enzymes that regulate metabolism, storage carbohydrates like starch, and sugar polymers used for energy storage in seeds. Phosphorus is also a phospholipid, which means it helps carry messages through the cells of plants and animals alike to create proteins for cell membranes and muscles among other things.

What happens if I have too much phosphorus for my cannabis plant?

If you have too much phosphorus in your soil, it can actually cause nutrient deficiencies of other minerals like calcium or magnesium. Too much phosphorus will eventually kill your cannabis plants so it is important to address the problem when you first notice it.
Phosphorus toxicity in cannabis plants is usually caused by phosphates or phosphite as phosphites are more stable than phosphoric acid and stay longer within the soil solution. Phosphate ions can move out of alkaline soils, so it’s important not to use too much lime if you have a high phosphate issue because this could cause phosphorus toxicity.

The most common toxicity symptoms of phosphorus in cannabis plants are as follows:

  • curling of the lower leaves and they can develop brown spots
  • newer leaves show signs of interveinal chlorosis, which means the cannabis leaf fades in color a bit between the veins
  • new growth has thin blades of leaves
  • less internodal spacing, which means spacing between the nodes
  • zinc, iron, calcium, and magnesium deficiencies start to show on the plant
  • smaller harvest
  • root or root tips turn black
  • leaf tip burn
  • burning of the leaf margins


How do I fix phosphorus toxicity in cannabis plants?

To fix phosphorus toxicity, you should flush out the excess phosphorus from your soil. As phosphorus will move out of the soil, it can be flushed away through deep watering or by rainfall over time. Flushing a plant requires you to give it a lot of water, usually two to three times the amount of a normal watering, which dilutes phosphorous. You should flush your cannabis plant with fresh, clean PH’ed water.
While you are flushing out phosphorus from your system, be sure to keep an eye on other signs that there might be a phosphorus deficiency. As phosphorus is a mobile nutrient, deficiencies can be caused by factors like transpiration from wind or increased heat from the sun when it’s close to summertime in your area. If you think there might be a phosphorus deficiency after flushing out excess phosphite and/or phosphate ions, add nitrogen-based fertilizers to your soil to help phosphorus become mobile.

How do you prevent phosphorus toxicity in cannabis plants?

Avoid adding phosphates or phosphoric acid into your soil too early after planting your cannabis seeds or seedlings since phosphorus can become immobile in some soils when it reaches a certain pH level. This happens most often with soil that has too much clay, but phosphates and phosphoric acid will also cause this same problem for you if your soil is already at the proper pH levels to begin with as most pre-mixed soils will already have phosphates or phosphoric acid mixed into the soil.

If you need to supplement phosphorus because your plants are deficient, do so at half strength until they start showing signs of needing more. This is also true if you’re using foliar sprays including compost tea or simply adding things like soft rock phosphate or bone meal to your soil. We only say this as a phosphorus excess can occur when trying to fix a phosphorus deficiency.

What happens if my cannabis plant has a phosphorus deficiency?

If your cannabis plant has a phosphorus deficiency, it will show signs in the leaves first. Phosphorus deficiencies are often seen in older leaves, but phosphorus is needed for new growth as well so it will affect your entire plant when you see phosphorus deficiency signs, or signs of any cannabis nutrient deficiencies, on your cannabis plants.

This can be caused by many things including over-fertilizing with phosphorus or potassium, not having enough drainage, or not letting the soil dry out enough between watering.

The most common signs of phosphorus deficiencies are:

  • dark green leaves or bluish, purplish, greyish color leaves
  • stunted growth
  • petioles and stems turn a red or purplish color although this can also be caused by other factors like genetics
  • dark copper color spots on the lower leaves
  • leaves contort and turn more purple or bronze before dropping and dying
  • susceptibility to pests
  • shiny and dark leaves

Calcium deficiencies can sometimes be caused by phosphorus deficiencies

How do I fix a phosphorus deficiency?

If phosphorus is the problem, you can usually correct a phosphorus deficiency by adding more phosphorus to your soil. You’ll want to mix in phosphoric acid with your fertilizer or compost tea so that it gets into the plant through its roots or use dry amendments that have phosphorous.

Phosphite forms of phosphorus are not very good for cannabis plants and should be avoided.

How do I prevent phosphorus deficiency?

The best way to prevent phosphorus deficiencies is by making sure you plant your marijuana seeds or seedlings in soil that has phosphates or phosphoric acid already mixed into the soil. You can use fertilizers with these things added if needed for early growth until phosphorus starts getting produced within the roots of your young cannabis plants.

You can also add phosphorus to your plants in many other ways including compost, which is a good way for phosphorous-deficient soil. Other options include earthworm castings, bone or blood meal, soft rock phosphate, and fish meal. However, phosphite forms of phosphorus can cause some problems for your cannabis plant if you’re not careful with it since they are very toxic to the roots and leaves (but less harmful to seeds).


Phosphorus is a mobile, macronutrient that is needed by the cannabis plant to perform many functions, including photosynthesis and root production. If your plant has phosphorus toxicity, it will often show itself in a variety of ways including curling of the lower leaves and black roots. Flushing the plant with two to three times the normal amount of water should fix the problem.

If your plant is showing a phosphorus deficiency, it will often manifest itself as bluish shiny leaves and dark copper-colored spots on the lower leaves amongst other symptoms. To fix a phosphorus deficiency you simply need to feed your plant more phosphorus either through dry amendments or nutrients in the water.

Phosphorus is an important nutrient for cannabis plants and phosphorus toxicity or deficiency can cause problems in your garden, so make sure to give phosphorus and other essential plant nutrients when needed with the proper amounts of nutrients and know how to take corrective action with phosphorus toxicity or deficiencies if they do occur.

Thank you for reading our article, please see our other articles regarding flushing, PH levels for cannabis plants, calcium deficiency, and magnesium deficiency.

To learn more about growing cannabis at home consider checking out our guides on: Getting Started, Troubleshooting, and Optimizing your Grow or search our Knowledge Base.

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