Deficiency and excess of Phosphorus in Cannabis plants

In the field of agriculture, phosphorus emerges as an essential element that triggers a series of fundamental processes for the correct flowering of plants. From the development of extensive and effective root systems to the activation of vital metabolic processes, in this article, we invite you to explore the importance of phosphorus for your plants.

Discover how this element, present in most flowering fertilizers but also many root stimulators, plays an important role in both the growth and flowering phases of plants, and how its strategic management can mark the difference in the quality and yield of crops.

 Phosphorus is one of the most important elements in the diet of plants, both growing and flowering (Image: Jeff W)
Phosphorus is one of the most important elements in the diet of plants, both growing and flowering (Image: Jeff W)

The phosphorus cycle

In the same way as with other elements, before we can use phosphorus as fertilizer we must wait for Mother Nature to carry out a series of processes to be able to use it as a raw material. Phosphorus is an element defined as a macronutrient since it intervenes directly and in large quantities in the metabolism of all plants, and especially in cannabis.

The phosphorus cycle is known as the biogeochemical cycle and always occurs in an ecosystem within a closed cycle that is constantly repeated on our planet. It is living beings that feed on phosphorus, either through phosphate decomposition processes or through already phosphate rocks.

These phosphates come into contact with plants through the soil. The animals feed on these vegetables, which contain quantities of phosphorus that will be contributed again in the form of excrement to the subsoil, to later be treated by microbial life, adapting the phosphorus again in the form of phosphates, which can be absorbed again by of the plants, thus closing the phosphorus cycle.

On the other hand, a large part of these phosphates present in the substrate are dragged into the sea, so that all marine fauna and flora are impregnated with this mineral. Phosphorus returns to the terrestrial environment through two systems; the first, is through the excrement of seabirds and fish, which have previously fed on the algae from which they have also absorbed phosphorus into their organisms. The second method is from earth movements that emerge outside, that is, tectonic plate movements, although this process lasts thousands of years.

 The phosphorus cycle
The phosphorus cycle

Functions of Phosphorus in marijuana plants

Phosphorus is an essential nutritional element for cannabis plants. It is known that much of the energy that the plant receives from sunlight is later stored, much of it in the form of phosphorus. Subsequently, it is used to carry out different metabolic processes such as photosynthesis, being in turn a component of DNA.

We can say that this macronutrient is very important for the development of plants in their different stages of life, whether we are talking about germination, root growth, growth of the aerial part, cloning, or flowering.

Plants absorb phosphorus in the form of monovalent and bivalent ions, although the most used by the plant is the monovalent ion, which will be more or less available depending on the pH range of the substrate. The higher the pH, the less the plant will absorb this nutrient, causing it to show signs of deficiency.

How to increase the THC level of cannabis plants

Getting the maximum performance from plants is the main objective of every grower, whether we are talking about flower production or cannabinoids and terpenes. Especially in recent years, with the rise in the therapeutic use of THC, getting genetics with a high content of this cannabinoid has become the goal of many breeders, while getting the most out of them is the goal of many growers. Today we are going to tell you some tricks to get the THC content of your flowers even higher, of course within the limits set by the genetics themselves.

From the absorption of this macronutrient, plants can carry out different biochemical actions such as respiration, in addition to synthesizing proteins along with carbohydrates. Its functions translate into:

  • Greater root development
  • Better use of substrate water
  • Stimulation of the general vigor of the marijuana plant
  • Formation of more robust stems
  • Greater resistance to frost, insect attacks, and diseases
  • The duration of the vegetative period is significantly reduced given the vegetative explosion produced by rapid root growth. You can switch to the flowering period early and, consequently, the total time needed to grow cannabis will be reduced.
  • Increase in production and improvement in the quality of buds and seeds
  • Direct influence on the production of carbohydrates, improving the production of sugars and starches, producing better quality fruits

Phosphorus deficiency in marijuana plants

Early stage of Phosphorus deficiency
Early stage of Phosphorus deficiency

Poor assimilation of phosphorus - or the absence of it - will result in a deficiency of this element. Like nitrogen, this nutrient is mobile, which means that it moves easily within the plant's body, moving to those areas with the most activity, such as the youngest leaves.

When this element moves from the oldest leaves to the youngest, the former suffer chlorosis, turning yellow. The oldest and lowest leaves of the plant act as a reserve of nutrients that the plant will use when required.

As the deficiency develops, chlorosis will advance through the plant, affecting all the leaves, and turning yellow. If it is not remedied, the plant will suffer massive defoliation, leaving it without leaves to be able to carry out its vital functions and facing premature death.

Phosphorus deficiency mid-phase
Phosphorus deficiency mid-phase

The lack of this element affects the plant in the following way:

  • Delayed growth of smaller leaves.
  • The stems, petioles, and other parts of the plant change to a purple hue.
  • The tips of the older leaves are claw-shaped along with dark green-blue colors.
  • The most affected leaves create necrosis, acquire a purple/tan color, dry and wrinkle, and finally fall off the plant.
  • The buds are smaller and do not develop as they should.
  • Plants are more vulnerable to any type of infection whether caused by diseases, fungi, or insects.

How to treat phosphorus deficiency

    • We must regulate the pH of the substrate between 5.5 and 6.5 depending on the type of substrate used; In hydroponics, the range will differ between 5.5 and 6.2 depending on the cultivation phase.
    • With excesses of other elements such as zinc or iron, phosphorus will be blocked. To correct the blockage we must carry out a root wash with a stable pH according to the growing phase and then carry out a light irrigation with a balanced fertilizer rich in phosphorus.
    • The fertilizers to use to solve the deficiency can be organic or mineral in nature, the mineral being the most quickly absorbed by the plant.

 Advanced Phosphorus Deficiency
Advanced Phosphorus Deficiency

Excess Phosphorus

Excess phosphorus affects the plant by blocking many other nutritional elements such as calcium, copper, iron, magnesium, and zinc, with zinc being the easiest microelement to block. In this way, you should be attentive to deficiencies in other elements to know the severity of the excess of Phosphorus.

If there is an excess of this element, wash the roots with a minimum of 3 times the capacity of the pot. The washing water can be composed of a mixture of water with enzymes (which act as salt breakers) to facilitate cleaning, with a stable pH value depending on the life stage of the plant to be treated.

These are some of the symptoms of excess phosphorus :

  • Dark or purple coloration: Leaves may show dark or purple coloration, especially on the edges.
  • Delayed growth: Plant growth may be slowed, and new leaves may be smaller than normal.
  • Zinc Deficiency: Excess phosphorus can interfere with zinc absorption, which can result in zinc deficiency symptoms, such as yellowing between the veins of leaves.
  • Chlorosis: Leaves may experience chlorosis, which manifests as generalized yellowing, although the veins remain greener.
  • Problems with mycorrhizae: An excess of phosphorus can interfere with the formation of mycorrhizae, and symbiotic associations between plant roots and beneficial fungi. This can negatively affect nutrient absorption.
  • Phosphorus Accumulation in the Soil: In soils with excess phosphorus, plants can show symptoms of toxicity, even if they are supplied with a normal amount of the element.

It is important to keep in mind that these symptoms can vary depending on the variety you grow and, of course, the conditions in which your plants are found. Careful observation and monitoring of plant health are essential to determine the presence of excess phosphorus and adjust fertilization practices accordingly.

Happy harvest!

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “Deficiency and excess of Phosphorus in Cannabis plants” (4)


Sprdave 2023-09-26
We ware in about the 5/6th week of flower my friend was overwatering them , and with out a ph meter yet all i could do was flush , our problems are with all yellow arse leaves from the tops 1st to the lower canopy, which has some green left but not much mostly bright green yellow colors on all the yellow leaves. I flushe 5 gallon fabric pots with about 4 gallons of water with at least 2 to 3 gallons run off. Think i had lock out. What ya'll think?!. God Bless ya all

Alchimia Staff

Dani Alchimia 2023-09-28
Hi sprdave, Since your plants are still in full bloom, I'd use some NPK-rich nutrient for the next 7-10 days to rebalance the plants. If they've been overwatered, it is not strange that they show deficiencies, since the root zone may have been harmed and can't assimilate nutrients as effectively as it should. All the best!


JosieG 2021-12-09
In the case of young plants, let’s say freshly potted one gallons, would you also then drop your cal mag?

Alchimia Staff

Tim Alchimia 2021-12-13
Hi, thanks for your question. It's very unlikely you'll have problems with Phosphorous in young plants, it's usually much more of an issue during flowering when the plant really demands high levels of this nutrient. Likewise, unless you're growing in hydroponics and/or under LED lights, the plant's need for Calcium and Magnesium shouldn't be too high either. If for some reason you already know that the variety you're growing or the conditions that you're growing in require additional input of CalMag, then I recommend sticking to the recommended dosage. Of course, as the article makes clear, if you have excess P in your nutrient solution or substrate, then this will block the uptake of other nutrients and minerals, including Calcium and Magnesium, so in this case there's little point in adding more until the excess of P has been corrected. I hope that helps. Best wishes and happy growing!


Sky 2019-01-27
I too ran into a mild P excess. FWIW, the strain is Triple Cheese and the P was 65 ppm. Like Richie, I am pushing the P in mid bloom, but maybe I used it at elevated levels for a week too long. I still have huge colas, but many leaves are showing multiple micro deficiencies. Next round I'll top the P out at 60ppm and phase it out a week sooner.


Richie 2018-10-27
I've always said that the outcome of your prize is not due to any one thing but a combination of all of the things you did over the growing cycle. Your due vigilance. And now I alas have fallen victim of trying to take a short cut during the budding stage and using way too much phosphate in the process just to save me time and work. I wish I hadn't. It's too late. The bud is loose and dry. It lacks resin. A lot of it is a deep purple Brown color where it should be green. I would have been better off letting a 5 year old do the job but it's too late.

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