When you buy fertilizer from a garden center or other store, you most likely will notice its N-P-K ratio; the “P” in this ratio is phosphorus. This alone should tell you how important phosphorus is in any marijuana plant’s healthy survival.

Phosphorus is an essential nutrient that comes into play mainly during your plant’s flowering phase of life. It also helps with healthy root and stem growth, as well as general plant health. It allows your plant to perform photosynthesis or convert light into energy.

It assists at the beginning of your plant’s life during the germination phase and also is useful during your plant’s reproductive phase. Basically, phosphorus is extremely important for much of your plant’s life cycle. Without phosphorus, your plant will suffer.

Tip: make sure to download my free Grow Bible for more information

While a phosphorus deficiency rarely occurs in plants that have been specifically planted and taken care of by a knowledgeable grower, when it does occur it can be devastating for a marijuana crop. There are a number of factors that can cause a lack of phosphorus.

These factors can include an improper pH level, irresponsible watering or over watering, soil that has too much moisture in it, and cold temperatures of lower than 60 degrees Fahrenheit. Temperature is especially  a problem the the change is sudden.

Signs of a phosphorus deficiency

signs of phosphorus deficiency marijuana

There are a number of ways to identify your plant as have a phosphorus deficiency. You should never add extra phosphorus to your plants unless they are clearly exhibiting some of these symptoms; otherwise they are probably perfectly happy, with enough phosphorus to live out their happy, productive lives.

However, as soon as things start looking fishy you should start considering the possibility of a phosphorus deficiency.

You might have a phosphorus deficiency on your hands if your plant’s growth slows down significantly and the only new leaves are very small. The older leaves, however, will be the first to show signs of a lack of phosphorus – especially leaves located near the bottom of the plant. The symptoms will begin there and will move upwards. The affected leaves will turn a darker green, then will show purple and blue colors.

Not sure if your plants have a phosphorus deficiency? Read the article Nutrient deficiencies in marijuana plants for a list with pictures of all deficiencies.

You also might see the edges of the leaves starting to turn tan or brown and they will start turning downward. The leaves nearest to the base will simply turn yellow and die. A plant with a phosphorus deficiency will also have root problems as well as other general health problems.

Download my free marijuana grow guide at this link for more growing tips

Petioles, as well as the plant’s stems, begin turning purple or red. This discoloration usually begins at the base of the plant and works its way upward. It’s important to keep in mind that some marijuana strains are naturally tinted this color, so be aware of what is normal for your particular strain.

If your plant experiences a phosphorus deficiency during the flowering period, its overall yield will most likely be lower than if it had sufficient amounts of the nutrient. During the flowering period, you will be able to notice if it is indeed a phosphorus deficiency simply by the part of the plant affected.

If only the parts getting hit by immediate, direct lightare affected (purple, red, or yellow leaves), while the rest of the plant remains healthy and normal in color, it is very likely a lack of phosphorus.

It is less common for plants in a low-light environment to have a phosphorus deficiency since they perform photosynthesis at a lower rate. In contrast, plants in an intensely lit environment will be more likely to “use up” or go through the phosphorus that they have. This is simply because phosphorus is used in the process of photosynthesis. This means that when photosynthesis speeds up – which usually occurs when there is a large amount of light – then the rate of using phosphorus also increases.

How to fix a phosphorus deficiency

how to fix a phosphorus deficiency marijuana

There are plenty of fertilizers that have an added amount of phosphorus and can be dissolved in water, such as a bloom fertilizer. You can also use guano that has lots of phosphorus and is easy to find. Greensand (wiki) and rock phosphate will also do the trick, but they release phosphorus slowly rather than all at once.

Some other sources of phosphate include bone or blood meal, worm castings or worm tea, soft rock phosphate, fish meal, and crab shell. Any budding or flowering formula is going to carry an extra amount of phosphorus, as well as most nutrient systems that are for plants like marijuana. This is simply because these plants almost can’t get enough phosphorus, and often these plants are placed under strong lights, which increases their rate of using up phosphorus.

Like with most deficiencies, try a product like Marijuana Booster to restore your plant’s nutrient balance. It will also prevent other deficiencies from becoming a problem since it contains all the necessary nutrients for your plant to thrive.

If the surrounding environment is at sub-50-degree temperatures, your plants won’t be able to absorb phosphorus very well. If you can’t avoid colder temperatures, try soluble phosphorus to improve your yield despite the undesirable outside cold.

Download my free marijuana grow guide at this link for more growing tips

If your plant exhibits signs of phosphorus deficiency when it is in its flowering stage, it’s very important to make sure that the leaves nearest to the buds remain green. This is so they can provide as much energy as possible to the buds through the process of photosynthesis. Since these are often the first leaves affected, you can catch the deficiency early on and prevent it from affecting any of the other leaves. If left unchecked, these buds will never reach their full-size potential.

As soon as you suspect a phosphorus deficiency in your plant, you should check the pH level of the soil (or water) near the roots. If the problem was caused by the wrong pH value near your plant’s roots, it simply means that the roots are not able to absorb phosphorus at their normal rate.

You, therefore, will need to restore the balance of the pH level around the roots. If your plant is being grown in a soil system, the pH level should be between 6.2 and 7.0 for it to absorb phosphorus best. In a hydro system, keep the pH level between 5.5 and 6.2.

When you are dealing with a phosphorus deficiency caused by a pH problem, flush your whole system with fresh water that is at the desired pH level after mixing in nutrients (which include phosphorus). This flushing will remove nutrient salts that could be preventing your plant from absorbing phosphorus, and it will bring the pH level back to normal.

If your plant has too much iron(Fe) or zinc(Zn), it will prevent your plant from absorbing phosphorus normally. There could be a buildup of nutrient salts in the soil or water (depending on your chosen system of growth).

If this is the case, then you can simply do the same flushing process that you would do if the pH level is off. Use fresh, clean water that is at your desired pH and is infused with a balanced dose of nutrients, and flush your entire system, whether soil or hydroponic, out with this solution.

It is important to remember that, even once a phosphorus deficiency problem is fixed, the leaves that have already been damaged will never go back to their normal healthy state. You, therefore, should keep your eyes open for the new growth, which will tell you more about your plant’s current state of health.

You will start to see regular, healthy new leaf growth within one week of fixing a phosphorus deficiency. It might be a good idea to prune the damaged leaves once you see the new, healthy ones coming in. This will allow the new leaves to get plenty of light, and will help the plant focus its energy on those leaves rather than the old, damaged ones.

Whatever you do, make sure you don’t give your plant too much phosphorus. It generally doesn’t happen, but when it does it makes your plant get “burned”’ or the buds will be “chemical buds.” It could also harm your plant’s absorption of other nutrients. Don’t add extra phosphorus unless you clearly identify a phosphorus deficiency.

List of marijuana plant sypmtoms

The following symptoms may appear on a plant that has a phosphorus deficiency:

Leaf Color:
– Darkening (dark green, blue, or gray) of the lower, older leaves
– Thickening and spot forming (bronze, purple, or brown in color) on lower leaves
– Yellowing of leaves where spots did not occur

Leaf Symptoms:
– Twisting and curling of leaves
– Mottled or mosaic-patterned leaves
– Burning and dying of leaf tips
– Old leaves falling off

Plant Symptoms:
– Stem discoloration (red or purple) beginning at the base
– Twisted or stunted new growths
– Slower growth in general
– Abnormal growth in general
– Buds not fattening properly
– Root rot
– Other problems: mold, root disease, and pests more likely
– In pollinated females: seeds won’t develop fully

*during flowering phase: leaves nearest to buds and right below lights are affected the most strongly, forming spots of red or purple with the surrounding flesh yellow or pale. Leaves underneath may remain their normal green color

Prevent deficiencies in marijuana plants

prevent phosphorus deficiency marijuana

There are some symptoms of the list above that won’t show up at all, even if your plant has a serious phosphorus deficiency. For instance, sometimes the stems won’t turn red or purple at all, even when there is a phosphorus deficiency. Some strains already have red or purple stems, meaning it is of no concern unless the other symptoms appear as well.

Sometimes only a few plants in your entire grow system will show the symptoms of phosphorus deficiency while the other ones around it look perfectly healthy. This can happen even if many more plants are located within the same system!

When it comes to the red and purple colors that come with phosphorus deficiencies, growers intentionally cause a temporary phosphorus deficiency in order to make the resulting buds be more colorful. It is quite beautiful to see the reds and purples mixed in with the green. They do this by lowering the temperatures a shocking amount, therefore stopping the plant’s ability to absorb phosphorus.

TIP: Looking to buy seeds? Visit the ILGM marijuana seed shop

This doesn’t work with every strain equally, and no matter what strain you are using, it will slow down the overall growth of the plant and its buds. This is simply because leaves that are not green do not efficiently accomplish photosynthesis, which is essential for speedy growth of any plant.

A better way of making the buds of your plant include shades of purple is to start with a strain that is already prone to reddish and purplish colors. That way, you won’t have to take any extra steps to “make” it turn purple.

If you have decided to use cold temperatures for this purpose anyway, at the very least you should wait until right before harvest time. This will prevent you from decreasing your final yield by too much.

You should also keep in mind that the purplish or reddish color that can appear on the buds through this process does not actually indicate a more potent bud. Green buds and purple buds have the same potency. While the buds and leaves might look pretty with purples and reds mixed in, you should be aware that this will slow down your plant’s growth and bud production. A purple or red leaf will not do much to add energy to your plant’s processes.

Remember that plants with strong genetics have less change of getting sick. Make sure you buy cannabis seeds from a trusted seedbank.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible


The founder of I Love Growing Marijuana, Robert Bergman, is a marijuana growing expert that enjoys sharing his knowledge with the world. He combines years of experience, ranging from small-scale grows to massive operations, with a passion for growing. His articles include tutorials on growing... [read more]


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  1. By La Diva: Day 64-harvesting soon :) – Page 10 ,14 Mar 2013
    [...] this helps her Her older leaves are basically a textbook example of the middle pic on this page: http://www.ilovegrowingmarijuana.com...cy-phosphorus/ Reply With [...]
  2. By juan ledesma ,29 Apr 2014
    Where might I get some root shooter grow cubes
  3. By Erik ,06 Jun 2014
    I have an auto flower blueberry just transplanted to a bigger pot I think I have a cal-mag or phorphous bcuz the yellowing and rust spots in the center of the leaves also they look sad and dropped over plz […]Read More
  4. By Symptom Checker – Identify Marijuana Plant Problems ,24 Jul 2015
    […] Phosphorus deficiency […]
  5. By Stop Nutrient Deficiencies In Marijuana Plants Now! ,18 Aug 2015
    […] To take care of phosphorus deficiencies, try to use a fertilizer which contains a higher ratio of phosphorus than whatever you were using before. Fertilizers specifically developed for the flowering phase work well, as does bat guano. Water-soluble fertilizers […]Read More
  6. By Macro Nutrients For Cannabis Plants – N-P-K – ,24 Aug 2015
    […] second element is Phosphorus or P in the periodic table with an atomic number of 15 and an approximate atomic mass of 31. Its […]
  7. By What Do Marijuana Plants Need? ,23 Oct 2015
    […] used by growers to describe the nutrients that cannabis plants need the most. This list includes phosphorus, nitrogen, potassium, magnesium, calcium, and sulfur. Micronutrients refer to the remaining […]
  8. By Best Soil Mixture For Outdoor Marijuana Plants ,10 Nov 2015
    […] Three foundational nutrients that you will need to focus on are nitrogen (N), potassium (P), and phosphorus (K). The fewer these nutrients, the less weed you will […]
  9. By Tom ,02 Jul 2017
    How do you fight phosphorus deficiencies on outdoor plants.
    1. By latewood_ILGM ,11 Jul 2017
      Tom, More info would be nice. As simple as your question is; I have to say add; Phosphorous.?.?.? Maybe it would be a good idea to join our forum at ILGM. Happy growing
  10. By Joe ,22 Aug 2018
    I have had a few grows under my belt, but with my latest one, im having serious trouble with this darkning of the leaves, please help
    1. By latewood_ILGM ,22 Aug 2018
      Joe, In order to help you; I need you to join our forum at: support.ilovegrowingmarijuana.com where we have many friendly members and experts to help you grow successfully. See you there :)
  11. By Chris ,20 Mar 2019
    Thank you for sharing your knowledge. 🙏Peace and happiness brother!
  12. By Nicolle ,25 Apr 2019
    I yaed foxfarm soil but i believe my problem was over Watering her :( or do You Guys recommend different soil?
  13. By Chris ,25 Aug 2020
    Awesome info but blood meal is not a good source of phosphorus 🙏🏼

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