How To Identify And Treat Phosphorus Deficiency In Cannabis

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.

WATER PROPERLY

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.