Phosphorus (P) is one of the three main elements that any plant requires to grow and produce flowers. This element is a macro nutrient and it is mobile. Phosphorus can be found in cannabis anywhere from 0.1% to 0.5% of the overall weight.  Phosphorus (P) is a big part of cannabis growth cycle and plants use phosphorus the most at the seedling and flowering stages of its growth. This element is used in such processes as photosynthesis and energy transfer. Phosphorus is also a part of the DNA molecules and plays a big part of the reproduction system. Cannabis plants need large amounts of phosphorus to produce seeds and transfer their genetic information to the next generation. Phosphorus is also used in storing carbohydrates and cell division as well as cell membranes.  The largest amounts of phosphorus can be found in cannabis seeds, growing root tips and new shoots.

Phosphorus amount in different growth stages

Early stage

For the first couple of week’s cannabis plants need a lot of phosphorus because it promotes rapid root growth and increases overall strength. You should choose a fertilizer that contains all the necessary elements but phosphorus can be the biggest ratio of this fertilizer. You should start off very slowly by feeding the plants 1/8 or ¼ of the required amount of fertilizer at the second to third week.  This can be little confusing because I just said that cannabis need a lot of phosphorus at the early stage of its growth but you need to start with only a 1/8 strength nutrient solution. The thing is that little cannabis sprout is very delicate and it can easily be killed by adding too much nutrients so you need to start slow.

If you will use a pre-made potting soil with added nutrients then you need to look for one with a higher Phosphorus ratio but almost any pre-made soils will contain all the necessary nutrients for the first weeks of cannabis growth. If you are using this nutrient rich soil then you don’t need to add any fertilizer in the early stage of cannabis growth and let it absorb all the nutrients it needs from that soil.


At this growth stage cannabis also needs all those elements including Phosphorus but phosphorus is not as crucial as it was in early stage of plants growth.  At this stage cannabis need phosphorus less than Nitrogen or Potassium but too little phosphorus can also stunt your plant. You can use any organic or chemical fertilizer that contains Phosphorus and it will probably be good enough.  You should slowly increase the dose of fertilizer but be careful because autoflower plants are very sensitive.


When cannabis shows its first pre-flowers or actual pistils with hairs you need to change your fertilizer. The choice of what fertilizer to use must be based on the fact that at this stage day neutral cannabis starts flowering and it will require bigger amounts of Phosphorus and Potassium and less Nitrogen, so the fertilizer should be something like 10-20-20 (N-P-K ratio) or anything with less Nitrogen and more Phosphorus and Potassium. Phosphorus is very important for flower production and if ruderalis plant gets too little phosphorus it will develop small flowers and your yield will dramatically decrease. In nature phosphorus plays a big role in seed production but that doesn’t mean that seed-less plants require less. So supply your “baby’s” with adequate Phosphorus doses but don’t overdo it.

Phosphorus in chemical fertilizers

Phosphorus in chemical fertilizers is found in the P2O2 form and it is usually showed in the % of the fertilizer volume. For example if you see 10-20-20 fertilizer you will get 20 % Phosphorus. One thing that no one tells you is that this is not the actual percentage of available Phosphorus. The actual available Phosphorus from a 20 % number is just below 9% because the listed molecule consists of Oxygen and Phosphorus and oxygen is about 56% of that molecule.

Almost any chemical fertilizer with a complete N-P-K nutrient content will be enough to grow your autoflower cannabis plant but if you want to grow your plants to its full potential you will need experience and a lot of knowledge.

Phosphorus in organic materials

Organically grown cannabis tastes better and is more environmentally friendly so here are some organic things that have Phosphorus (P) in them:

  • Bone meal(crushed animal bones) – This is an excellent source of Phosphorus but it is slow released phosphorus and your plant will not be able to absorb it right away.
  • Rock phosphate(mineral) – This is basically a salt that consists almost entirely from phosphorus but the bad thing is that this phosphorus source is very slow released and your plant will not be able to absorb it for a long time, so it must be manufactured some way to make it more available.
  • Bat Guano(bat manure) – This is not as good as previous ones but it consists of 20% or more Phosphorus. It also has a lot of Nitrogen so this can be a good food for young plants.
  • Animal Tankage (Rendered, and dried byproducts of the slaughtered animals) –  This is an excellent source of Phosphorus and Nitrogen too, but Phosphorus percentage is larger so this is a good organic phosphorus source.

Phosphorus deficiency

Phosphorus deficiency appears when cannabis doesn’t get as much of this element as it needs for a perfect growth.  At first Phosphorus deficiency can be mistaken with over-watering or nutrient toxicity symptoms but when it progresses it has distinct symptoms.

If a plant has Phosphorus deficiency then the overall look of the plant is bad and the root mass is small. Plant struggles to grow and produce flowers. When cannabis gets phosphorus deficiency at the flowering stage of growth then the plant will produce weak and fluffy flowers and the harvest yield will be significantly smaller.

First symptoms of phosphorus deficiency are when the older leafs turning bluish / purple with black blotches at the middle of the leaf. There can also be some steam color change to redish-green or purpleish-green. If the deficiency is not treated then the older fan leafs will develop more and more black/brown blotches and will eventually drop off and these symptoms will start affecting younger leafs.

Phosphorus deficiency can be treated by adding any chemical or organic fertilizer with adequate Phosphorus content. The old affected leafs will not be cured but the overall look of the plant will start improving in a couple of days or a week.

Phosphorus toxicity

Toxic amounts of phosphorus have no direct effect of the cannabis plant but this illness shows itself in the form of Iron, potassium, zinc, magnesium, copper deficiencies. Too much Phosphorus will directly influence uptake and stability of all these elements. Phosphorus overdose  will manifest some signs not faster than in a couple of weeks so probably by the time your plant shows these deficiencies your yield and overall look of the plant will be small and bad.

TIP: You can threat Phosphorus overdose by flushing your marijuana plant!


  1. in order to save a little money, I decided to compost my soil with last years crop, and mix it with new organic potting soil. PROBLEM….My plants are going into flower mode right away. Some of the clones i put in store bought organic soil are fine, and growing like you know. Obviously, something in the my compost is causing the problem. Too late to transplant with out killing the plant. Talked to someone, and they said that there is too much phosphorus in soil. I am starting to think that maybe I really screwed up. Any suggestions??

    • This is an interesting problem that I have not come across. TI could be that the soil has too much nutrients, but I cannot be sure about that because I have not experienced such a thing.

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