Phosphorus deficiency in cannabis plants can have a huge effect on the final yield of your grow. Though cannabis plants use phosphorus thought out their whole life cycle, they use it most during the flowering stage. This is when you are most likely to see a deficiency in your plants.
In this guide we will cover everything you need to know about phosphorus deficiency in cannabis plants. How to identify it, how it happens, how to fix it, and also how to prevent it.
Cannabis plants use phosphorus for many things, from root formation, to bud growth. It is a macronutrient, which means plants use a lot of it. Because of this, deficiencies in phosphorus have an effect on the growth and overall health of plants and must be fixed quickly.
How Do Cannabis Plants Use Phosphorus
Phosphorus is used to grow the flowers and roots of many plants, not just cannabis. Without it, a plant would not be able to grow. Even from the very first stages of germination, a plant will use phosphorus to make the tap root that pushes out of the seed.
As a mobile nutrient, phosphorus is able to travel around the plant to places where it is needed most. This could be to the leaves at the top of the plant, or in the roots. The first place it will be taken from is the bottom leaves. This is why they show symptoms first.
Phosphorus is used a lot throughout the life cycle of a cannabis plant. If you do not provide them with enough, they will stop growing and potentially die. Keep an eye open for the following symptoms if you think your plants have a phosphorus deficiency.
Symptoms of Phosphorus Deficiency in Cannabis Plants
Without phosphorus your plants will not be able to function properly. They will lack energy, become weak, and stop growing. Phosphorus deficiency is seen most often during the flowering stage. This is because the plant uses a lot of it to grow big buds.
- Leaf stems turn purple
- Growth slows down
- Purple/ brown blotches on lower leaves
- Lower leaves fall off
- Weak droopy plant
The best way to fix a phosphorus deficiency in cannabis plants will differ depending on what medium your plants are growing in. You also need to identify the cause of the deficiency, because it might not be down to a lack of food in the medium!
How Do Cannabis Plants Get Phosphorus Deficiency
Most nutrients used for growing cannabis will have plenty of phosphorus in them to feed your plants. Cannabis plant deficiencies are a little more complicated than simply not having enough phosphorus in the medium. There might be plenty available, but the plants can’t use it.
If the pH of the medium is not within the correct range, cannabis plants can not absorb the right nutrients. For example, phosphorus will stop being absorbed by cannabis plants if the medium is too acidic.
You need to not only make sure there is enough phosphorus available to your plants, you must also make sure the pH is within the correct range too.
How to Fix Phosphorus Deficiency in Cannabis Plants
There are two ways to fix phosphorus deficiency in cannabis plants. Either you need to add more food to the medium, or you need to correct the pH level of the medium. The first thing you should do is check the pH of the medium.
Correct the pH for Cannabis Plants
The correct pH for cannabis plants will differ depending on what medium you’re growing in. To check the pH you need a pH meter. You will then check the run off of your plants after feeding them, or check the reservoir in hydro setups to find out what the pH is.
For a full guide on pH and cannabis plants, click this button
Best pH for Soils
Cannabis plants grown in soil like to sit in a pH between 6.5 and 7.5. Check the pH of soil by collecting the run off water after feeding the plant. Check the pH level of the runoff and it will give you a rough idea of the pH of the soil
Adjust the pH by watering the soil with fresh water, balance within the correct pH range for cannabis plants in soil
Best pH for Hydro
Cannabis plants growing in hydro systems prefer a slight acidic pH level. Between 5.8 and 6.2. Check the pH by using a pH meter in the reservoir. If using soilless mediums like coco, check the run off after feeding your plants.
Adjust the pH by adding pH up or down to the reservoir, or water your plants with feed that is within the correct ph range.
You can adjust the ph of your hydroponic reservoir or the feed for your plants by using pH up or pH down. Usually just a couple of drops is plenty to change the level.
If you have checked the pH of the medium, and everything seems to be in the right range, then there probably isn’t enough phosphorus available at the plant. This is causing the deficiency. In this case, you need to feed the plant more phosphorus.
Feed More Phosphorus
Most nutrients used for growing cannabis will come in two parts. A grow nutrient, to be used during veg, and a bloom nutrient to be used during flower. Because a cannabis plant will need more phosphorus during the flowering stage, bloom nutrients have more phosphorus in them then grow nutrients do.
If you see signs of a phosphorus deficiency during veg, increasing the amount of grow nutrients your plants are getting will usually fix the problem. However it is more common to see these deficiencies during flower, as the demand for phosphorus goes way up. To combat this, growers usually add a PK booster to the feed.
PK Booster For Cannabis Plants
When flowers start to form on cannabis plants, they need a lot more potassium and phosphorus. At this stage you would change from grow nuits, to bloom nuits as they have a higher ratio of potassium and phosphorus.
Sometimes however this still isn’t enough for the plants, and more PK is needed. Because of this, it is a good idea to use a PK booster during flowers too.
These PK boosters are used by many cannabis growers. It is always best to start light and work the feed up gradually. If you need help with the correct dosage, ask for help in our forums for cannabis growers.
Organic Phosphorus for Cannabis Plants
If you are growing outdoors, or growing organically, you should avoid using salt based nutrients. Using them will damage the microbes in the soil, which will affect the way the plant eats. To avoid this, it is better to use organic PK boosters.
Using these nutrients will still change the pH of the water. Make sure you check the pH after adding any food to the water. Adjust it to the correct level if necessary before watering your plants.
Has this Guide Helped You?
Has this guide been helpful to you so far? I hope it has answered all your questions. But if you still need more information about phosphorus deficiency or anything else, just ask for help in our forum for cannabis growers.
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How to Prevent Phosphorus Deficiency
Phosphorus deficiency can be avoided by feeding your plant the right levels of the right food, at the right time. This can be done by monitoring EC.
Monitoring the EC will tell you if your plants are eating too much or not enough. But that’s a whole other guide you can find here:
If you use a decent brand of nutrients, and make sure the pH is within the right range, you should only see a phosphorus deficiency during the flowering stages. At which point you should add a PK booster to the feed to give the plant what it needs.
Cannabis plants use a lot of phosphorus throughout their grow cycle. But they use most of it during the flowering stages. So during the early stages of flowering keep a close eye on the plant. Look for the symptoms mentioned above. If you see any, just add a little PK to the next feed, it will make the plants feel much better.
As always, thanks for reading! I hope this guide has helped. Don’t forget you can come and join us for free, anytime, in our cannabis growers forum. It would be great to see you there!