Deficiency and excess of Phosphorus in Cannabis plants

The Phosphorus Cycle:

Before the phosphorus can be used as nutrient for plants, we must wait for Mother Nature to perform its vital functions in order to take advantage of the raw materials that it offers. Phosphorus is an element classified as macro-nutrient since it directly participates - in large amounts - in the metabolism of all plants and especially in cannabis crops.

The phosphorus cycle
The phosphorus cycle

The phosphorus cycle is known as biogeochemical cycle, and is a closed cycle which is repeated constantly in a given ecosystem.

Phosphates come into contact with the plant through the ground. Animals eat these plants containing phosphorus, so it’s going to be provided anew to the subsoil in the form of excrements. Later on, it is processed by the micro-life and made available again for the plants (phosphates), thus closing the phosphorus cycle.

Furthermore, a large part of these phosphates present in the substrate are carried away towards the oceans, so all marine flora and fauna is coated in this mineral. Phosphorus returns to the terrestrial environment by two systems; the first one is through seabird or fish droppings, which have previously fed on algae, which have in turn absorbed phosphorus in their organism. The second way is through both the earth-movings that emerge from the sea or shifting of tectonic plates, although this processes take thousands of years.

The function of phosphorus in cannabis plants:

Red Phosphorus
Red Phosphorus

Phosphorus is an essential nutrient for marijuana plants. It’s known that much of the energy that the plant receives from the sunlight is later stored mostly as phosphorus. Afterwards it’ll be used to perform the different metabolic processes - such as photosynthesis - being at the same time a component of the DNA.

This nutrient is essential for the plant development in its different life stages: seed germination, root growth, growth of the aerial part, cloning, and flowering.

Plants assimilate phosphorus in the form of univalent and bivalent ion, although the most commonly form used by the plant is the univalent ion, which is more or less available depending on the PH level of the substrate. The higher the PH, the lower the uptake of this nutrient by the plant, what will surely lead to a deficiency.

Plants can perform different biochemical actions from the uptake of this macro-nutrient, such as breathing or synthesizing proteins and carbohydrates, what results in:

  • Improved root development
  • Better use of the water in the substrate
  • Stimulates the resistance and overall health of cannabis plants
  • Stronger stems
  • Better resistance to frosts, diseases and pests
  • The duration of the vegetative period is significantly reduced given the explosive growth produced by a fast root growth. The grower can start the bloom stage of plants much earlier, thus reducing the total time required to harvest the cannabis crop.
  • Improved yields and quality of buds and seeds
  • It has a direct role in the production of carbohydrates, improving the production of sugars, starches, etc,  and thus producing best quality fruits.

Phosphours deficiency in cannabis plants

First sypmtoms of phosphorus deficiency
First sypmtoms of phosphorus deficiency

An inadequate phosphorus assimilation can lead to a deficiency of this element. This nutrient is mobile, it moves with ease within the organism of the plant to areas with more activity such as the youngest leaves.

When this element moves from older to younger leaves, the former show chlorosis, turning yellow. The oldest and lowest leaves of the plant perform the function of nutrient reserve which will be used by the plant if necessary.

As the deficiency progresses the plant shows symptoms of chlorosis in more and more leaves, which turn yellow and ultimately fall. If the deficiency remains untreated, the plant suffers a massive defoliation losing all the leaves needed to perform its vital functions, what can lead to premature death.

Phosphours deficiency in mid-stage
Phosphours deficiency in mid-stage



A deficiency of this element affects the plant in the following way:

  • Delayed growth of the smallest leaves
  • The stems, petioles, and other parts of the plant turn  purple
  • The most affected leaves become necrotic, turning purple or bronze, drying up and wrinkling and finally falling down from the plant
  • Buds are smaller and don’t develop as they should
  • Plants are more susceptible to any type of pest or disease caused by fungi, insects, viruses...

Advanced Phosphorus deficiency
Advanced Phosphorus deficiency

How to solve the phosphorus deficiency

  • We must adjust the pH of the substrate to a 5.5-6.5 range, depending on the type of used substrate: in hydroponic systems this range goes from 5.5 to 6.2, depending on the growth stage.
  • An excess of other elements such as zinc or iron will block the uptake of phosphorus, causing a nutrient lockout. To correct this issue we must flush the roots with a stable pH according to the growth phase and then water with a balanced fertiliser rich in phosphorus.
  • The nutrients used to solve the nutrient lockout can be of either organic or mineral composition, being mineral fertilisers those of faster absorption by the plant.

Phosphorus excess

A phosphorus excess affects the plant by causing a massive nutrient lockout of other elements like calcium, copper, iron, magnesium and zinc, being zinc the first micro-element to get blocked. In this way, we must check for deficiencies of other elements to correctly diagnose a phosphorus excess and to know how severe it actually is.

If we detect an excess of this element we should flush the roots with at least three times of water than the capacity of the pot. There are products to break down salts to ease the flushing of the roots, which should be always used with an adjusted pH level depending on the life stage of the plant.

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” (3)


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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