phosphorus deficiency marijuana

Phosphorus Deficiency In Cannabis Plants

Phosphorus is a vital nutrient required by cannabis plants that fulfills many important biological functions. There are numerous different signs of deficiency symptoms to look out for when it comes to this nutrient.


One factor that growers must constantly be aware of when cultivating their plants is the risk of nutrient deficiency. Cannabis plants require an array of important nutrients that carry out vital biological functions throughout the grow cycle. If a certain key nutrient is missing during the vegetative or flowering stages, plants will start to display symptoms of deficiencies. These signs can help growers determine what is missing and therefore, what needs to be added to the soil before it is too late. One of these vital nutrients is phosphorus.


Phosphorus plays an important role in the health of all living organisms. Plants require this nutrient in order to achieve normal growth and to reach maturity. Phosphorus contributes to the DNA of the plant, as well as the RNA, which reads the genetic code in order to build proteins and other structures. Phosphorus is also vital in the creation of ATP, the unit of energy that plants use during photosynthesis. Phosphorus helps to stimulate root development, increase the strength of stems, improve flower formation and seed production, improve the quality of crops, boost resistance against diseases and support development throughout the overall growth cycle.


It is obvious that phosphorus deficiency could do some damage to the health of your plants and their potential yields. There are quite a few different symptoms to look out for that could signal phosphorus deficiency within your crop. First of all, the deficiency will usually start to affect the older leaves that are lower down on plants. These leaves may begin to exhibit a shiny appearance and turn to darker shades of green, blue and grey. Along with this change in colour, leaves will also start to develop purple and brown spots. Leaves will also become very dry and begin to thicken. The stems of the plant may also begin to turn bright red or purple.


One cause of a phosphorus deficiency is the pH of the roots. A root pH between 6.2 and 7 is best in order to maximise absorption of the nutrient. Therefore, it is ideal to strive for a soil pH between this range when deficiency symptoms manifest. This can be achieved by using pH up and down products.

An organic fertiliser containing adequate amounts of each vital nutrient can also be used to ensure that your plants are receiving the proper levels of phosphorus that they require. Additionally, other sources of phosphorus can be added to the soil in order to maximise exposure. Good source of phosphorus include warm casting, fish meal, crab shell and soft rock phosphate. Overwatering and compact soil can also be causes of phosphorous deficiency. Be sure to water your plants correctly to avoid this.

Try to remain calm if you notice a phosphorus deficiency setting in. Panicking and adding far too much phosphorus back into the soil could prevent your plants from uptaking other nutrients and end up doing more harm than good.

Temperature is another detail to pay attention to in order to achieve optimum phosphorus levels. Lower and colder temperatures can make it more difficult for your cannabis crop to absorb adequate levels of the nutrient. Temperatures that drop below 15 degrees Celsius may start to cause negative effects.

If your plants have fallen victim to phosphorus deficiency symptoms, you will notice a recovery stage take place if you take the right steps to restore the health of your plants. The spread of brown spots, red and purple stems and other symptoms will stop effecting new leaves. Don’t worry if old leaves do not recover, as this is normal.

Phosphorus plays many vital roles within cannabis plants and is needed in order to build proteins, provide energy and for normal growth.

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.


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, 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.


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.



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.


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.


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.


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.


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.