How To Identify And Treat Phosphorus Deficiency In Cannabis
Published : Sep 22, 2017
Categories : Cannabis cultivation
Phosphorus (P) belongs to a group of three important minerals that plants need for healthy growth. Your cannabis plant requires phosphorus in all growing stages. Although almost any cannabis nutrient will contain an abundance of phosphorus, deficiencies are possible. Learn about phosphorus deficiency and how to solve the issue.
Phosphorus (P), together with nitrogen (N) and potassium (K), is one of 3 major plant nutrients. Cannabis requires phosphorus throughout all phases of growth, during the vegetative phase and even more so during flowering. A phosphorus deficiency happens when your plant does not receive the required amount it needs for healthy growth.
WHAT ARE THE SIGNS OF A PHOSPHORUS DEFICIENCY?
A phosphorus deficiency in cannabis will normally first show on the large, older leaves at the bottom of your plant. The symptoms manifest in leaves that turn dark green or yellow and in many cases, get brownish or blueish patches. At some point, as the phosphorus deficiency progresses, the leaves will thicken and become stiff, crumbly and dry, with the dark patches having a shiny appearance. Depending on the strain, a phosphorus deficiency can also cause the stem to turn a reddish or purple colour.
PHOSPHORUS: A MOBILE PLANT NUTRIENT
Phosphorus, like nitrogen, potassium and magnesium, belongs to the group of mobile plant nutrients. These nutrients are called mobile because once inside plants, they can be transported to regions where the plant needs them most. Since cannabis will take these nutrients from the older leaves at the bottom first and move them to new growth at the top, the older leaves will show the first signs of a deficiency. As the phosphorus deficiency progresses, the symptoms will then climb up the plant.
Cannabis needs phosphorus in all growing stages, but it is particularly important for bud production during bloom. Phosphorus also plays an important role in photosynthesis and general growth.
Not all cannabis strains have the same nutrient requirements. This is no different with phosphorus. This means that during the growing season, even when you use the same amount of nutrients for all your plants, particular strains may show signs of a deficiency while others may go unaffected.
PHOSPHORUS DEFICIENCY DESPITE FEEDING YOUR CANNABIS PLANTS?
Pretty much all commercially available cannabis nutrients, especially those formulated for flowering, contain more than sufficient amounts of phosphorus. Should you feed your plants according to the recommended dosage for your nutrient and happen to spot the signs of a phosphorus deficiency, it is rather unlikely that the nutrient itself is the cause of it.
In such a case, it is more likely a problem with the pH level of your water, nutrient solution, and/or growing medium. If the pH level is off, the plant is unable to uptake phosphorus even if it is present.
WHAT TO DO ABOUT CANNABIS PHOSPHORUS DEFICIENCY?
PH PROBLEMS CAN (LITERALLY!) BE AT THE ROOT
Growing problems with cannabis due to a nutrient deficiency can arise even if you are feeding your plants correctly. Most of the time, an incorrect pH level can be the underlying cause for a deficiency. The reason for this is that cannabis grows ideally in a relatively tight pH window to take in nutrients. For phosphorus, the ideal pH at the root of your plants should be 6.2-7.0 when you grow in soil and 5.5-6.2 when you grow hydroponically.
Because of this, the first thing that you should do if you suspect a phosphorus deficiency is to check and then correct the pH level at the root zone of your plants.
FLUSH YOUR PLANTS TO RESTORE A HEALTHY ROOT ZONE
An incorrect pH at the root zone of your plant can often be the result of a build-up of minerals and salts in your soil. To get rid of the abundance of salts and to restore a healthy root zone and proper pH level, you want to flush your plants.
Flushing means that you water your plant with a generous amount of plain, pH-ed water only. The idea here is to “flush out” these accumulated salts. Once you’ve flushed the plants, feed them a regular recommended dose of your nutrients at the proper pH level.
If you grow hydroponically, you would flush your hydro system accordingly and may even want to prepare a new nutrient solution for your reservoir.
Improper watering of your plants, in particular overwatering, can be another cause for a phosphorus deficiency. Water your cannabis plants properly! Rather than watering often and in smaller quantities, it is better to water less frequently, but to saturate the entire soil contents of the container. In addition to that, you should always allow the soil to dry out between watering.
A good way to check whether the soil is dry is to lift the container to check its weight. It is always much better when cannabis plants start to slightly wilt due to thirst, than it is for them to be inundated with water. In the first case, your thirsty plants will quickly recover. In the latter, consistent overwatering can lead to some serious growing problems.
CHECK YOUR TEMPERATURES
How well your plant can absorb phosphorus may also depend on the temperature. When temperatures drop, especially if they go below 15°C, a phosphorus deficiency is more likely to occur. So make sure that your plants are growing at the optimal temperature.
ADDING PHOSPHORUS WHEN THERE IS A DEFICIENCY
Once you have excluded other possible causes of a phosphorus deficiency, such as an incorrect pH level, overwatering, or cold temperatures, you can easily provide phosphorus.
Commercial potting mixes that are pre-fertilised normally contain sufficient amounts of phosphorus. Almost all pre-formulated cannabis nutrients will contain good amounts of it, especially flowering nutrients. Liquid, mineral flowering nutrients with readily available phosphorus provide a fast and easy way to give phosphorus to your plants.
Cannabis plants uptake sizeable amounts of phosphorus when they grow, which means that it is rather difficult to provide them with too much. Because your plants use plenty of this nutrient, it is unlikely that a phosphorus deficiency would appear before any other nutrient deficiencies.
There are, however, some situations where your plants may require particularly high amounts of phosphorus, such as when you grow under very high-powered lights or when your plants are flowering in direct sunlight. If this is the case, you can gradually increase the dosage of your flowering nutrient to provide the plants with the amount of phosphorus that they need.
CAN YOU GIVE TOO MUCH PHOSPHORUS?
With cannabis, it is normally rather difficult to give them too much phosphorus as compared to other nutrients. On the other hand, giving (way) too much is not exactly healthy either. Although the odds of this are slim, especially if you use pre-formulated cannabis nutrients, too much phosphorus will likely first show in other growing problems and nutrient deficiencies.
Once you take care of the phosphorus deficiency, your plants should recover within several days. As with other deficiencies where mobile nutrients are involved, the plant will not “repair” the already discoloured or crumbled older leaves at the bottom, but will show improving health in new growth at the top.
Learn all about cannabis phosphorus deficiencies. Phosphorus is essential to cannabis bud production, so it's important to correctly spot deficiences, fast.
Problem: A cannabis phosphorus deficiency generally appears on leaves from the lower/older parts of the plant. The lower leaves may turn dark green or yellow, and start getting spots or big splotches that look brown, bronze or even a little blue. The leaves may thicken and curl, and the affected leaves feel stiff. Sometimes the stems of the plant turn bright red or purple, but not always.
Sometimes accompanied by a Calcium deficiency, as Phosphorus and Calcium interact with each other in the plant.
A cannabis phosphorus deficiency usually appears with some or all of the following symptoms:
- tends to affect the lower and older leaves of the plant
- sometimes a phosphorus deficiency is accompanied by bright red stems (though not always), though if you have red stems but no other symptoms, it’s typically not something to worry about
- leaves darken (turning a dark green, blue or grayish color) and may appear shiny
- leaves may start turning yellow in places if the phosphorus deficiency is left untreated, or if the deficiency is combined with other nutrients deficiencies and/or pH problems. However, yellow leaves is typically not associated with the beginning of a phosphorus deficiency.
- leaves get bronze, purple or brown spots and splotches
- leaves thicken and may feel dry or stiff
- stems sometimes turn bright red or purple, but not always
- sometimes accompanied by a Calcium deficiency, as Phosphorus and Calcium interact with each other inside the plant
- this deficiency is more common after buds start forming, when the plant is using a lot of Phosphorus
Phosphorus deficiencies in the vegetative stage usually appear at the bottom of the plant on some of the oldest leaves, and will progressively climb up the plant if left unchecked.
The progression of a cannabis phosphorus deficiency
A phosphorus deficiency tends to be more common after plants start making buds in the flowering stage. Cannabis plants tend to love phosphorus in the flowering/budding stage and it is unlikely for a cannabis plant to get too much phosphorus using standard nutrients formulated for a flowering plant like cannabis. Nearly all flowering nutrients will come with an abundance of phosphorus for your plants. So if you’re seeing a cannabis phosphorus deficiency while using standard cannabis nutrients, chances are you actually have a root pH problem (explained below in the solution section)!
Phosphorus (P) is used by your cannabis plant in all phases of growth. It is one of the 3 major nutrients (N-P-K) listed on the front of most nutrient bottles, and phosphorus will be represented by the second number that appears.
When there is a phosphorus deficiency, the lower (oldest) leaves turn dark green. Leaves occasionally get a bluish or bronze tinge, and may thicken or curl downward before exhibiting dark gray, bronze or purplish splotches. Sometimes the stems of the affected leaves will turn bright red or purplish, usually starting from underneath.
It’s common to see a Phosphorus deficiency accompanied by the symptoms of a Calcium deficiency, as those nutrients interact with each other in the plant.
Sometimes you will get a cannabis phosphorus deficiency, and the stems do not appear red or purple at all, or the coloring may not be pronounced.
The leaf below was at the bottom of the plant and turned dark green and shiny, with a bluish tinge. Cannabis phosphorus deficiencies usually appear on the lower/older parts of the plant. The leaf then started showing the spots of a phosphorus deficiency where it was being touched by light (the parts of the leaf working hardest). The leaf began to curl downwards and turn yellow.
Notice that the stems or veins never turned red or purple on this leaf, except for some parts that were actually affected by the phosphorus deficiency.
A common symptom of a cannabis phosphorus deficiency is bright red or purple stems, though it doesn’t appear on all affected plants. Red stems can also be caused by direct light exposure (like a “tan”)
Another example of bright red stems that may be the result of a Phosphorus deficiency, or possibly direct light exposure.
It’s important to remember that some cannabis strains naturally grow with red or purple stems even when all their nutrient needs are being fulfilled, so red or purple stems is not a symptom to worry about on its own.
Do not mistake natural reddish-purple colored stems for a phosphorous deficiency!
When you notice that stems are turning red or purple starting from underneath, it may be a sign of a phosphorus deficiency only if accompanied by other symptoms. If the only symptom shown by your plant is red or purple stems, and you are not seeing any other signs of splotches or unhealthy leaves, the red or purple stems are likely caused by the genetics of your plant. If that’s the case, you have nothing to worry about.
Phosphorus is used heavily by cannabis plants in the flowering phase to produce buds, and is a crucial component of photosynthesis (turning light into energy for the plant).
Some strains of cannabis use much more phosphorus than others, or be more susceptible to a phosphorus deficiency, and you may have many plants in the exact same setup with only some of the plants showing signs of a phosphorus deficiency.
Solution For Cannabis Phosphorus Deficiency
1.) Adjust pH to Correct Range
Your cannabis plant may show signs of a phosphorus deficiency if the pH at the roots is not in the right range. That is because when the pH of your root zone is off, your cannabis cannot properly absorb phosphorus through its roots. Therefore the first step is to ensure that you have the correct pH for your growth medium. Learn more about pH and cannabis.
Phosphorus is best absorbed by cannabis in soil at a root pH of 6.2 – 7.0. Phosphorus is best absorbed by cannabis in hydro at a root pH of 5.5 – 6.2. If you believe you have a cannabis phosphorus deficiency, it’s important to check the pH of your root zone to make sure the deficiency isn’t caused by the pH being too high or too low.
If you suspect your growing cannabis plant has a phosphorus deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes phosphorus. This will remove any nutrient salts that may be affected the uptake of phosphorus and help restore pH to the proper levels.
- In soil, phosphorus is best absorbed by the roots in the 6.2 – 7.0 pH range (in soil, it’s generally recommended to keep the pH between 6.0 – 7.0, but phosphorus specifically tends to be best absorbed above 6.2 and below 7.0)
- In hydro, phosphorus is best absorbed by the roots in the 5.5 – 6.2 pH range (in hydro, it’s generally recommended to keep the pH between 5.5 – 6.5, but phosphorus specifically tends to be best absorbed below 6.2)
2.) Take Good Care of the Roots
Wet, compact soil or overwatering can trigger a phosphorus deficiency to appear even when all other factors are perfect. So make sure you water your plants properly every time to help prevent a phosphorus deficiency.
3.) Provide the Right Temperature
Cooler temperatures lower than 60°F (15°C), as well as large temperature swings, can make it harder for the plant to absorb phosphorus. Cannabis plants are therefore more likely to show signs of a phosphorus deficiency when the temperature drops too low, or if they go through a cold spell.
Cannabis likes a comfortable room temperature (they like about the same temperatures as we do).
4.) Give the Right Nutrients
Most growers have actually already given plenty of phophorus to their cannabis plants since it is found abundantly in quality soil and cannabis-friendly nutrients. However, even if you are giving phosphorus, it’s important to give your cannabis the right ratio of nutrients.
An excess of Fe and Zn may cause the symptoms of a phosphorus deficiency by preventing the plant from being able to absorb phosphorus properly. If you believe there may be a buildup of nutrient salts in your growing medium (or if you are growing in hydro and have not recently flushed or changed your reservoir) you should make sure it’s not an excess of other nutrients that is actually causing the phosphorus deficiency to appear. Flush your plant thoroughly with properly pH’ed water containing a regular dose of cannabis nutrients including phosphorus, or completely change your reservoir if you believe that an excess of nutrient salts may be causing the phosphorus deficiency.
Sources of phosphorus:
If you’ve tried everything else, then you may try adding a higher percentage of phosphorus to your feeding schedule and see if that helps clear up the problem for your plant. Cannabis plants love phosphorus, and therefore it is unlikely that you will give your cannabis too much phosphorus.
Most nutrient systems that are formulated for a plant like cannabis will carry and abundance of phosphorus, especially in budding/flowering formulas, so it is unlikely that you will see signs of a phosphorus deficiency before other nutrient problems when using nutrient systems formulated for cannabis (as long as you keep your root pH in the correct range and prevent the plants from getting cold or being overwatered). If you’ve got very high powered lights, or if your plants are growing in direct sunlight, they may be going through a lot more phosphorus in the flowering stage than average and may need you to provide extra phosphorus to make sure buds get as big as they could be.
Just remember that if there’s no actual phosphorus deficiency currently appearing on your cannabis plant, adding more phosphorus is probbaly not going to help plants grow better or make bigger buds – in fact adding too much phosphorus may actually hurt your plants by preventing the uptake of other nutrients! While it’s difficult to overdose your plants on phosphorus, adding too much compared to other nutrients will often cause other strange & unexpected deficiencies to appear.
5.) Take Good Care of the Roots
Phosphorus deficiencies can show up with the plant is having root problems or if the plant is overwatered, even if the pH is right and the phosphorus is there. Proper watering practices help plants grow healthy and avoid a host of problems!
6.) Watch for Recovery
After going through all the above steps, watch to make sure that the phosphorus deficiency starts to clear up within a few days to a week or so. After a phosphorus deficiency is cleared up, the problem (brown spots, unhealthy lower leaves, red/purple stems, etc) will stop appearing on new leaves, usually within a week.
Please note that leaves which have been damaged by a phosphorus deficiency will probably never recover or turn green, so you want to pay attention to other leaves for signs of recovery.
If you cannot get rid of your phosphorus deficiency, please consult our 7-Step Cure to 99% of Cannabis Growing Problems
- Bronze or brown patches
- Brown or slimy roots
- Brown or yellow leaf tips/edges
- Buds dying
- Buds look odd
- Bugs are visible
- Curling or clawing leaves
- Dark leaves
- Drooping plant
- Holes in leaves
- Mold or powder
- Pink or purple on leaves
- Red stems
- Shiny or smooth leaves
- Spots or markings
- Twisted growth
- Wilting leaves
- Yellow between leaf veins
- Yellow leaves
This page is part of our Plant Doctor series. You can use our tool to filter by symptom and help diagnose your plant.
Phosphorus deficiencies cause dark splotches on leaves and can appear at the bottom of the plant on some of the oldest leaves. Learn to spot & fix the issue.