Phosphorus And Cannabis Plants

Phosphorus And Growing Weed

Steven Voser
Steven Voser
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Phosphorus is one of the three primary nutrients cannabis needs for healthy growth. Find out the functions of phosphorus and how you can spot, treat, and prevent phosphorus deficiency. 

Phosphorus is one of the three primary cannabis nutrients needed in all stages of growth. In this guide, we cover the functions of phosphorus and let you in on how to identify, treat, and prevent phosphorus deficiency and toxicity.

What Is Phosphorus?

What Is Phosphorus?

Together with nitrogen (N) and potassium (K), phosphorus (P) is one of the essential nutrients cannabis plants need for healthy growth. It is classed as a primary macronutrient due to its important roles in various plant functions. Your cannabis plant depends on the availability of phosphorus during all phases of its development, where it requires the element in relatively large amounts.

Phosphorus is essential for photosynthesis, plant metabolism, and nutrient uptake. It is particularly important for new growth and early stage root development, but also for bud formation and yield. A lack of this mineral (or an excess) will lead to your plants getting sick.

Commercially available cannabis nutrients, especially those formulated for flowering, usually contain more than sufficient amounts of phosphorus. But phosphorus deficiencies or toxicities are possible, most often due to pH issues, poor soil quality, improper nutrients, or certain environmental factors.

The Phosphorus Cycle

The Phosphorus Cycle

Phosphorus replenishes itself in a biogeochemical cycle in the ecosystem known as the phosphorus cycle: Plants that have taken up phosphorus from the ground are eaten by animals. Through animal droppings, the element is replenished in the subsoil. Microbes process the element and make it available for plants once again, thus closing the phosphorus cycle.

The element is also washed out from the land and carried into the seas due to erosion and rainfall. This is why marine life, such as fish and algae, is particularly rich in phosphorus. The element returns to the land through seabird or fish droppings, and in some areas also through natural geological processes when land emerges from the sea.

Why Do Cannabis Plants Need Phosphorus?

Why Do Cannabis Plants Need Phosphorus?

Phosphorus plays an important role in essential biochemical actions such as synthesising proteins and carbohydrates. Here is a rundown of some of its crucial functions:

  • Promotes the growth of roots.

  • Strengthens stems.

  • Increases resistance against diseases and pests.

  • Increases frost resistance.

  • Promotes optimal use of water in the substrate.

  • Increases bud quality and yield.

  • Benefits seed germination.

  • By boosting root growth, phosphorus can help reduce the duration of the vegetative phase.

  • Plays an essential role in plant metabolism, where it aids in the production of carbohydrates.

Phosphorus Needs During Early Stage

Right after germination, cannabis can live off the nutrients in the seed. Likewise, most commercial soils will already have sufficient amounts of phosphorus. Additional feeding will not be needed up until about week 3, when the plant has reached a height of approximately 15cm. Otherwise, providing extra nutrients too early in the plant’s life can negatively affect root growth and overall health of the seedling.

Phosphorus Needs During Vegetative Stage

When the plant has entered the vegetative growing stage, from about week 4 on, it will require an adequate feed rich in nitrogen, phosphorus, and potassium. Recommended nutrient levels for vegging are 200 ppm of nitrogen (N), 120 ppm of phosphorus, and 200–250 ppm of potassium. Exact feed rates, however, can vary depending on the strain, container size, and environmental factors. When using commercial cannabis nutrients, it is best to stick with the recommended doses.

Phosphorus Needs During Pre-Flowering Stage

During early bloom (pre-flowering), cannabis will require additional nutrients due to excessive growth. Increase calcium and magnesium levels and dial in about 200–250 ppm of N-P-K (nitrogen, phosphorus, potassium). At this point, the grower should start feeding more potassium than nitrogen in preparation for the upcoming flowering stage.

Phosphorus Needs During Flowering Stage

During bloom, when vegetative growth comes to a halt, cannabis will require increased levels of phosphorus and potassium while needing only little nitrogen. When using commercial cannabis nutrients, flowering nutrients are formulated to accommodate these requirements. At about week 5 into flower (late bloom), shoot for about 125 ppm of nitrogen and provide adequate phosphorus once calyxes form.

Tip: Commercial cannabis nutrients will typically have the optimal nutrient ratios for each stage of the cannabis life cycle. They often come in two variants, formulated either for vegetative growth or for flowering. For vegetative growth, NPK levels of 3-1-2 are industry standard. For flowering, the recommended NPK of 1-4-5 reflects the plant's need for less nitrogen and more phosphorus (P) and potassium (K).

Phosphorus in Chemical vs Organic Fertiliser

Phosphorus in Chemical vs Organic Fertiliser

If one is growing organically and doesn’t want to utilise chemical cannabis nutrients, there are plenty of natural materials that are rich in phosphorus. Many of these can either be added to the soil directly to improve phosphorus levels, or they can be added to the compost pile.

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  • Compost: A great source of phosphorus and many other nutrients, especially when containing banana peels and other fruit and vegetable scraps.

  • Phosphate rock: This is a phosphorus-rich rock ground into fine particles, releasing phosphorus slowly over the course of many years. Not suited as a quick fix, but excellent as a soil amendment.

  • Coffee grounds: Coffee grounds act as a slow-release fertiliser and compost ingredient. They are rich in phosphorus and contain many other valuable minerals, including potassium, magnesium, and copper.

  • Bone meal: Steamed bone meal and fish bone meal are an excellent source of mineral phosphorus.

  • Bat guano: Another excellent source of phosphorus for container plants including cannabis.

  • Manures: Animal manures such as chicken manure or pig manure make good sources of natural phosphorus. Pig manure, however, may contain parasites and pathogens and must be hot-composted before use.

Phosphorus Deficiency

Phosphorus Deficiency

As most commercial nutrients usually contain sufficient amounts of phosphorus, a deficiency is often not due to a lack of this nutrient, but because of pH issues (“nutrient lockout”) or environmental problems.

What Causes Phosphorus Deficiency?

  • Incorrect pH levels: When growing in soil, phosphorus needs a pH level of 6.2–7.0 (in hydro: 5.5–6.2 pH). If the pH level is out of this range, phosphorus is not available to the plant.

  • Poor soil: Poor soil or soil from unknown sources may not contain adequate levels of phosphorus.

  • Cool temperatures: When environmental temperatures drop below 15°C or there are large temperature fluctuations, phosphorus deficiency can show.

  • Root problems from overwatering, stagnant moisture due to poor drainage, or soil being too compact can lead to a phosphorus deficiency.

How to Recognise Phosphorus Deficiency

How to Recognise Phosphorus Deficiency

If plants suffer a phosphorus deficiency, they will usually show signs in lower, older leaves. This is because phosphorus is a (moderately) mobile nutrient within the plant, meaning the plant can relocate it from older to newer growth when it is scarce.

  • Symptoms mostly show on older, lower leaves at the bottom.

  • Leaves turn a dark green, grey, or blue colour.

  • Stems turn bright red. Note: Red stems are not always a sign of phosphorus deficiency.

  • Can also lead to calcium deficiency, with related symptoms.

Later stages:

  • Leaves turn yellow. Note: Can also be a sign of other deficiencies.

  • Leaves have brown, bronze, or purple spots.

  • Leaves thicken and feel stiff and brittle.

Tip: An excess of other nutrients can also lead to symptoms of a phosphorus deficiency, as this prevents the uptake of phosphorus. A common cause is nutrient salts accumulating over time and throwing off pH at the root zone.

If you suspect a build-up of nutrient salts is at the root of a phosphorus deficiency, flush your plants with pH-balanced water to correct pH level at the root zone. After flushing, feed nutrients at the proper levels. When growing in hydro, flush out your system and/or change the water in your tank to get pH back to an optimal level.

How to Treat Phosphorus Deficiency

How to Treat Phosphorus Deficiency

How you treat phosphorus deficiency will depend on the underlying cause and your growing medium/method. If you can exclude other potential causes, the reason for it is most likely pH-related.

  • In Soil

    If low levels of nutrients are the reason for the deficiency, increase the amount that you provide for a couple of feeds. When growing organically, add bat guano, bone meal, or compost to increase soil quality and phosphorus levels. Make sure pH levels for your water/nutrient solution is 6.2–7.0.

  • In Hydro/Coco

    If underfeeding is the suspected reason, increase feed/EC, but do so gradually, increasing EC by 0.2 points every few days. Monitor your plant’s health and stop increasing when you observe the health of your plants improving. Keep a pH of 5.5–6.2. If the pH level is too low (too acidic), change the water in your tank and flush the plant with plain water at the optimal pH. Afterwards, provide nutrients in the proper amount.

Phosphorus Toxicity

Phosphorus Toxicity

Phosphorus toxicity (excess) isn’t easy to diagnose as it will usually also cause a lockout of other nutrients, such as calcium or magnesium, with related symptoms.

How to Recognise Phosphorus Toxicity

The most common signs and symptoms of phosphorus toxicity are:

  • Leaf tips show “burn” (similar to nutrient burn or light burn).

  • Lower leaves start curling and spotting.

  • Leaves at the top have yellow veins.

  • May resemble cal/mag deficiencies.

How to Treat Phosphorus Toxicity

Treating an excess of phosphorus mostly involves safely flushing the medium and starting anew with feeding. Here’s how it breaks down per substrate.

  • In Soil

    Flush your medium with pH-balanced water. In soil, it is best to flush with a large amount of water, approx. 2–3 times the capacity of the pot. Stop feeding for one week and allow the plant to use up the remaining phosphorus stored in its tissues.

  • In Hydro/Coco

    In hydro, flush and reduce the EC in your res. Change water in the res if needed (i.e. if pH is too low). In coco, flush, then refeed at a lower EC level.

Steven Voser
Steven Voser
Steven Voser is an independent cannabis journalist with over 6 years of experience writing about all things weed; how to grow it, how best to enjoy it, and the booming industry and murky legal landscape surrounding it.