Phosphorus is a basic element, and just like nitrogen and potassium, it is a critical component in almost every growth function in your plant. Phosphorous is the P in NPK and like most nutrients, the plant absorbs it from the growing medium through the roots. There are a few simple rules to make sure your plant gets the right amount of phosphorous:

  • Phosphorous needs to be available in the growing medium in a form that cannabis can consume
  • The plant’s roots need to be healthy enough to uptake available Phosphorous
  • The pH near the roots needs to be in the right range for cannabis to uptake the phosphorous.

Why do cannabis plants need phosphorous?

Cannabis is like all living things and requires phosphorus for normal growth and maturity. Phosphorus is a key ingredient in photosynthesis, respiration, energy storage and movement, cell division and growth and almost everything else the plant does.

How do I diagnose phosphorous deficiency in cannabis?

The easiest way to tell if your plant has healthy levels of phosphorous is to look at the older fan leaves at the bottom of the plant. Look for discoloration, spots and dark blotches that form between the veins. Often the leaves will feel stiff and dry to the touch.

Normal Phosphorous Levels

Lower fan leaves on plants with healthy levels of phosphorous are dark green in color.

Low Phosphorous Levels

Leaves on plants with lower levels of phosphorous start darken with darker blotches and may feel stiff.

Depleted Phosphorous Levels

Leaves on plants with depleted phosphorous develop large blotches and rust spots and may turn yellow or develop a bronze sheen.

What causes blotches on cannabis leaves?

Cannabis leaves in healthy vegetative growth have green leaves due to high levels of green chlorophyll, which is used in photosynthesis to generate energy for the plant’s growth. If there isn’t enough phosphorous, the plant struggles to create enough chlorophyll. To optimize resources, pot plants will reroute phosphorous from older leaves at the bottom in favor of supplying the growth tips.

When vegetating cannabis leaves at the bottom of the plant develop dark blotches, it often indicates a minor phosphorous deficiency

Flowering plants need increased levels of phosphorous in flower production. If it doesn’t have enough, it will begin diverting phosphorous from even the new growth leaves near the buds.

Flowering requires a lot of phosphorous and the plant will divert supplies from nearby leaves to bolster flower production.

Is there phosphorous in the medium?

The easiest way to get phosphorous into the growing medium at the correct level is to use a balanced nutrients set. Using fertilizers and nutrients put all the required elements into the growing medium, so if you are using a nutrient system designed for cannabis, there is probably plenty of phosphorous available.

Are the plant’s roots healthy?

Root health is critical for the uptake of water and nutrients. If a plant is too big for its container, the plant can become root-bound. When this happens, the roots grow inward and can strangle themselves, slowing the plants nutrient uptake. Other root issues, like root-rot, will also impact the plants nutrient uptake.

Is the pH Balanced?

PH is short for ‘potential Hydrogen’ but is most often used to describe the acidity or alkalinity of a substance. This is important when growing cannabis because nutrients are not available for uptake by the plant outside fairly narrow pH ranges. Cannabis plants in soil do best between 6 and 7 while cannabis plants in coco or other soilless mediums do best between 5.5 and 6.5.

If the pH is too high or too low, then the nutrients aren’t available for the plant to uptake. This is called nutrient lockout because no matter how many nutrients you provide, the plant still can’t use them. This is a very common problem for farmers and the best way to combat it is to test the pH at every watering.

Nutrients are available at different pH ranges.

From the chart above, you can see why phosphorous deficiency is one of the first signs that your pH might be low.

How do I correct phosphorous deficiency?

In most cases, you don’t need to worry about a few leaves on the bottom going yellow or getting splotches during vegetative growth. If you are seeing it in new growth during flowering, you should address the problem quickly, since cannabis needs increased amounts of phosphorous during flowering.

If your plant’s roots are healthy and you’re using a balanced nutrient solution, you should check the pH of the soil. In most cases, balancing the pH and performing a feeding should prevent further problems.

How do I check the pH?

There are two methods for checking the pH at the plant’s roots. The most common method is to water the plant until there is a runoff and then check the pH of the runoff water using a hand-held pH meter. If the pH that comes out is higher or lower than what you put in, then you know that it is unbalanced.

The second method uses a soil pH meter. These unit have probes that allow them to test the soil near the roots after a watering.

How do I balance the pH?

To balance the pH, flush with water until the runoff is in an acceptable range. After the pH near the roots is at the right level, follow up with a hearty dose of nutrients and the problem should be solved.

To correct a deficiency, aim for a pH of around 7 in soil, or 6.5 in coco, to get the most phosphorous uptake. If you notice the yellowing continuing, you likely have a root condition. If the pot is too small, transplant into a larger container.

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