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Phosphorus Deficiency

Problem: 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. 

Phosphorus is needed for overall health of cannabis plants, but is especially important for healthy roots and bud production during the flowering stage. Phosphorus is also necessary for photosynthesis to happen in plant leaves, which means a phosphorus deficiency will prevent your plant from being about to properly make energy from light.

Cannabis plants tend to love phosphorus and it is unlikely for a cannabis plant to get too much phosphorus using standard nutrients formulated for a flowering plant like cannabis.

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.

This marijuana plant leaves are showing signs of a phosphorus deficiency

A cannabis plant which is deficient in phosphorus in the vegetative stage may have slower growth and will be more susceptible to root problems and general plant maladies. New leaves may be smaller than average or look twisted or stunted.

When there is a phosphorus deficiency, the lower (oldest) leaves will sometimes turn dark green, occasionally with a bluish or bronze tinge, and may thicken or curl downard 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.

This cannabis leaf is showing the final fatal signs of a phosphorus deficiency

A cannabis phosphorus deficiency usually appears with some or all of the following symptoms:

  • lower /older leaves get darker (sometimes turning a dark green, blue or grayish color)
  • lower leaves may thicken and begin to form bronze, purple or brown spots
  • leaves begin to twist and/or curl downward
  • affected leaves eventually turn yellow in places not affected by splotches
  • stems may turn red or purple, starting from underneath, but not always, and some plants naturally have red or purple stems
  • new growth may be twisted or stunted
  • plant may grow more slowly or be more susceptible to problems such as mold, root disease, or pests
  • roots may grow slowly or be prone to root rot
  • in the flowering stage, leaves near buds and directly under the lights seem to be most affected, and will usually display red or purple splotches while the rest of the leaves turn yellow or pale
  • bud growth and final yields may be below average, especially if phosphorus deficiency appears in early to mid-flowering and discolors the majority of the leaves nearest the growing colas
  • pollinated female plants with a phosphorus deficiency may produce underdeveloped seeds
  • phosphorus deficiencies can sometimes appear when the temperature gets too low, because it is harder for the plant to use phosphorus at lower temps

Cannabis phosphorus deficiency in vegetative stage - Brown splotches, dark bluish color, curling and affected parts of the leaf turn yellow.

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. 

Cannabis phosphorus deficiency in vegetative stage -  First lower leaves turn dark, then get brown or bronze spots, stems may turn red or purple starting from underneath, leaves curl and twist downwards and eventually turn yellow.

The leaf below was at the bottom of the plant and turned dark green and shiny, with a bluish tinge. The leaf then started showing the spots of a phosphorus deficiency where it was being touched by light. 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.

Cannabis phosphorus deficiency in vegetative stage -  First lower leaves turn dark, then get brown or bronzy spots, leaves curl downwards and will eventually turn yellow.

A common "symptom" of a cannabis phosphorus deficiency is red or purple stems. 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.

Healthy purple stems on this cannabis plant are caused purely by genetics, not by a phosphorus deficiency

In the flowering stage, a cannabis phosphorus deficiency usually manifests near the buds that are being hit with strong, direct light.

This is because 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). 

Cannabis phosphorus deficiency in flowering - Leaves near buds directly under lights get reddish splotches and turn yellow. The leaves not touching the light will stay green.

A phosphorus deficiency is most common in the flowering stage when plants are under very bright light, and cannabis is constantly using up phosphorus in the process of building buds and making energy from light. A phosphorus deficiency is less likely to appear in lower light conditions.

In the case of a cannabis phosphorus deficiency in the flowering stage, the leaves which are not getting hit by direct light will usually remain green. The leaves directly under the light and nearest to the buds are the first to turn reddish or yellow as they display the signs of a phosphorus deficiency.

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.

Cannabis phosphorus deficiency in flowering stage - red leaves

The following example shows what happens if the Phosphorus deficiency is allowed to continue in the flowering stage. Some strains are especially prone to turning dark purple or reddish when hit with a phosphorus deficiency in the flowering stage, especially "blue" strains.

Cannabis phosphorus deficiency in flowering - Leaves near buds directly under bright light (or sunlight) get purple splotches and turn yellow around the edges.

A big problem with a major phosphorus deficiency like this, especially early in the flowering stage, is the affected leaves are not good at photosynthesis and won't provide nearly as much energy to the plant as they would if they were green. It's important to keep leaves near the buds green and healthy during the majority of the flowering stage to help ensure you get the best yields possible.

Because phosphorus is so important to the proper development of buds, a plant with a major phosphorus deficiency early in the flowering stage will not develop buds as big as they could be.

If just the parts of your plants under direct light are showing purple/red/yellow leaves in the flowering stage, and the rest of the plant is mostly green and healthy, this is a big sign that it may be a phosphorus deficiency.

Cannabis phosphorus deficiency in flowering - Leaves that are not near buds or directly under bright light stay green - they usually don't get purple splotches or turn yellow around the edges unless the leaves are under bright light conditions.

Solution: Your cannabis plant may show signs of a phosphorus deficiency if the pH at the roots is too high or too low. 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.

Please note: After a phosphorus deficiency is cleared up, the problem (brown spots, unhealthy lower leaves, red/purple stems, red/purple/yellow coloring on leaves near buds) 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. After you're sure the deficiency has stopped spreading, you may want to remove damaged leaves, especially at the tops of colas, to expose green healthy leaves to the light and aid with bud development for the rest of the flowering stage.

  • 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)

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.

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.

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.

Some growers will try to get their plant to artificially display colors by shocking the plant with very cold temps. The reason this can work is because it disrupts the uptake of phosphorus, and in some plants this will cause the upper leaves under the light (and the adjacent buds) to turn red or purple. This does not work with all strains. The type of reddening or purpling caused by low temps will often result in smaller yields becasue non-green leaves are not good at photosynthesis, and the plant needs phosphorus to produce buds properly.

The leaves of this plant turned purple over night after that plant was exposed to chilly temps.

Cannabis phosphorus deficiency in flowering caused by cold temps at night - Leaves near buds directly under bright light (or sunlight) get purple splotches and turn yellow around the edges.

Cold temps may help the plant purple up, but it will also likely slow plant growth and hinder almost every single aspect of a flower's development. It's generally not recommended for growers to turn down the temps just to achieve colored buds - you may be doing more harm than good! If you are looking for purple buds, you'll get the best results by finding purple genetics that look/smell/taste the way you like. A "true" purple cannabis phenotype with turn purple no matter what you do in the environment.

If you do plan to use cold temps to shock your plant into producing colors, at least to wait until just before harvest so you don't accidentally reduce your yields. It's also important to note that just because the buds have turned red or purple, it has nothing to do with their final potency. Purple buds are not any more potent than green buds.

Leaves near the colas may turn pretty colors after being exposed to dark temps, but it also slows down growth and inhibits proper bud production. You need green healthy leaves for the best bud production in the flowering stage, not purple and yellow leaves that don't have the phosphorus they need to complete photosynthesis and make energy to product buds. A leaf like the following looks neat but will deliver very little energy to your plants.

Cannabis phosphorus deficiency in flowering - Leaves near buds directly under bright light (or sunlight) get purple splotches and turn yellow around the edges.

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:

  • Bat guano (phosphorus is readily available, especially if made into a teat)
  • Bone or blood meal (takes quite a bit of time to break down in soil unless made into a tea first)
  • Worm castings or worm tea
  • Soft Rock Phosphate 
  • Fish meal 
  • Crabshell
  • Most cannabis-friendly "bloom" or "flowering" nutrients contain high levels of phosphorus to aid in flower production, and phosphorus from a liquid nutrient is one of the most readily available forms of phosphorus you can provide to your cannabis plants

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.

If you cannot get rid of your phosphorus deficiency, please consult our 6-Step Cure to 99% of Cannabis Growing Problems


Cannabis Phosphorus Deficiency -
Big Symptom Picture: 
Leaf Color: 
Edges Appear Brown or Burnt
Pale Color Leaves
Yellow Leaves - New Growth
Yellow Leaves - Lower, older leaves
Dark or Purple Leaves
Black or Gray Patches on Leaves
Red or Pink Color on Leaves
Brown or Dark Spots
Mottling / Mosaic Pattern
Leaf Symptoms: 
Upper Leaves / Newer Growth Affected
Lower Leaves / Older Growth Affected
Small Inner Leaves Affected
Leaf Edges Appear Burnt
Leaf Tips Appear Burnt
Leaf Tips Die
Thick Growth Tips
Red Stems
Mottling / Mosaic
Old Leaves Dropping Off
Slow Growth
Twisted Growth
Abnormal Growth
Leaves Curl Under
Plant Symptoms: 
Red or Purple Stems
Weak Stems
Old Leaves Dropping Off
Slow Growth
Twisted Growth
Leaves Curl Under
Too Short
Root Symptoms: 
Slow Growing
Other Symptoms: 
Buds Not Fattening