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Phosphorus Deficiency

Identifying and Treating Phosphorus Deficiency in Cannabis Plants

Phosphorus is a vital nutrient that cannabis plants use primarily in the flowering phase. It improves the plant’s general health as well as stem and root growth, and it allows plants to convert light into nourishment through the process of photosynthesis.

While phosphorus deficiencies are rare in plants that have been well cared for, when they do occur, they can be quite damaging to a cannabis crop. Numerous factors may lead to a phosphorus deficiency, such as under- and overwatering, improper pH, saturated soil, and cool temperatures. In this guide, growers will learn how to diagnose and treat a phosphorus deficiency in a cannabis crop.

Why is Phosphorus So Important?

All flowering plants (including cannabis) need phosphorus to survive. Plants need approximately 30 different nutrients for optimal health, and most of these are produced by microorganisms in the soil. Though it’s mainly used during flowering, it’s also essential during the vegetative stage for root development and photosynthesis. Once your plants start to flower, phosphorus reduces its activity at the roots and is instead used for bud production. If a plant has visible signs of a phosphorus deficiency, it’s extracting the nutrient from the leaves, so it can be used to produce flowers or build roots.

The Phosphorus Cycle

Before cannabis plants can absorb phosphorus and other nutrients, you’ll have to wait for nature to do its work. Through certain natural functions, such as photosynthesis, plants are able to absorb and utilize raw materials. Phosphorus is considered a macronutrient because it is a crucial component in cannabis plant metabolism and other processes.

The Symptoms of Phosphorus Deficiency

There are several ways to detect deficiencies in cannabis plants. It’s important not to add phosphorus to the garden unless your plants are exhibiting some of the symptoms we’re about to discuss, but when signs are found, it’s equally crucial to take immediate action.

Wether you choose to grow a single seed, you may be dealing with a deficiency if your plant’s growth is slow and new leaves are small. Older leaves will quickly show signs of insufficient phosphorus, especially those at the bottoms of the plants. Affected leaves turn a dark green, with blue and purple discoloration developing later.

Sometimes, leaf edges may curl downward and turn brown or tan while the leaves at the base of a plant may simply die. When cannabis plants don’t get enough phosphorus, they’ll also have root problems and other health issues. Though petioles and stems may become discolored, growers should remember that certain strains are naturally tinted.

If plants experience a deficiency during flowering, their overall yield will be substantially lower than if they’d had enough of the nutrient. During flowering, it’s easy to detect a phosphorus deficiency by looking at the plant’s affected parts. If the leaves are discolored while the remainder of the plant looks healthy, it’s likely due to low phosphorus levels.

Phosphorus deficiencies are much rarer in plants in low-light environments because photosynthesis is much slower to occur. By comparison, plants in well-lit grow rooms will use up their phosphorus stores much sooner.

Resolving Phosphorus Deficiencies in Cannabis Plants

Most commercially available fertilizers contain extra phosphorus and can easily be dissolved in water. Some growers use phosphorus-rich bat guano, while others use rock phosphate or greensand. However, the latter two options are much slower to release phosphorus.

Other natural sources of this important nutrient are worm castings, blood or bone meal, fish meal, and crab shells. Any fertilizer made for flowering or budding plants will contain supplemental phosphorus, as will most cannabis-specific preparations. Marijuana plants are thirsty for this mineral, simply because they’re often placed under bright lights that accelerate the process of photosynthesis.

If the growing environment is at 50 degrees or below, your cannabis plants won’t absorb that much phosphorus. In cases where colder temps can’t be avoided, soluble phosphorus will help to increase yields. When plants exhibit the symptoms of a deficiency during flowering, it’s crucial to ensure that the leaves closest to the buds stay green so they can provide energy during photosynthesis. These leaves are often the first affected during a deficiency, so early detection and prompt action are essential.

As soon as a deficiency is suspected, check the growth medium’s pH near the plant’s roots. If the pH reading is abnormal, your plant’s phosphorus uptake will be limited. Resolve the issue by restoring the pH balance in and around the roots. For soil-grown plants, the pH should be 6.2-7.0; for hydro systems, it should be lower at 5.5-6.2.

When dealing with a pH-related phosphorus deficiency, flush the entire system with pH-balanced water after mixing in nutrients. The flushing process will remove built-up salts that prevent phosphorus absorption and it will balance the system’s pH levels.

Things to Keep in Mind

It’s important to consider that, even once the deficiency has been resolved, already-damaged leaves will not return to normal. Therefore, growers should look for new growth, which will offer a great deal of information on the plant’s current health and condition.

Expect to see healthy new leaves growing within a week of curing a deficiency. It may help to prune away the damaged leaves once the new growth starts coming in. This gives the new leaves enough light and it helps plants focus their energies appropriately.

Whichever course of action you take, do not overfeed your plants with phosphorus. It rarely happens, but when it does occur, it may cause plants to become burned and unable to absorb other nutrients. In extreme cases, phosphorus over-supplementation may give buds an unpleasant chemical taste.

Look for These Signs to Prevent Further Phosphorus Problems

Once you’ve resolved the issues with your plants, look out for the following symptoms and take quick action if any are detected.

Leaf color:

  • Lower and older leaves may turn shades of dark gray, blue, or green
  • Some lower leaves may thicken and begin to form brown, purple, or bronze spots
  • Leaves without spots may turn yellow

Leaf condition:

  • Curling and twisting of leaves
  • Development of mosaic or mottled patterns
  • Dying and burning leaf tips
  • Dropping of old leaves

General plant symptoms:

  • Purple or red stem discoloration starting at the base of the plant
  • Stunted or distorted new growth
  • Slower growth
  • Abnormal growth
  • Skinny buds
  • Root rot
  • Mold and pest infestation
  • Abnormal seed development in pollinated female plants

During the flowering stage, the leaves nearest to the grow lights and the buds are most affected. They may form purple or red spots with surrounding areas becoming pale or yellow. However, the plant’s bottom leaves may still look healthy.

Preventing Phosphorus Deficiency in Cannabis Plants

The above list of symptoms isn’t all-encompassing, and in some cases, not every plant will exhibit each sign. For instance, some stems won’t turn purple or red, even when the plants are seriously deficient in phosphorus. Some strains are naturally colored, which means there’s no cause for concern unless a plant is exhibiting other trouble signs.

In some situations, only a few plants in an entire garden become affected while the rest of the crop looks perfectly healthy. This may occur even in the largest cannabis gardens. Some growers intentionally cause phosphorus deficiencies to make buds grow more colorful. They accomplish this goal by substantially lowering the grow room’s temperature, which inhibits the plant’s phosphorus uptake ability.

Growers should keep in mind that this strategy isn’t equally successful with every cannabis strain and it will inhibit the plant’s overall growth no matter which strain they choose. Leaves without a healthy green color aren’t very efficient at photosynthesis, which is a crucial component in fast plant growth. An easier and safer way to make your buds more colorful is to begin with a cannabis strain that’s already likely to develop such colors. That way, there’s no need to take risky steps to get colorful buds.

If you’ve decided to lower the grow room’s temperature for this purpose, wait until immediately before harvest to do so. This will minimize the effect on overall crop yield. Growers should remember that the reddish or purplish colors that appear on buds aren’t necessarily an indicator of potency. While these buds may look more appealing, it’s not worth the effects on bud production and plant growth. After all, red and purple leaves won’t add much energy to a plant’s natural processes.

Final Thoughts on the Role of Phosphorus in Healthy Cannabis Plant Growth

Though plants need dozens of nutrients to survive and thrive, phosphorus is one of the most important macronutrients a cannabis grower can focus on. It assists in photosynthesis (during the vegetative stage) and it ensures healthy, normal bud formation in the flowering stage.

Phosphorus is a vital component of a successful cannabis grow op, but it’s not the only thing to think of. The best crops start with the best seeds, and plants with robust genetics are less likely to fall victim to a phosphorus deficiency. When starting a cannabis garden, be sure to buy seeds from a reputable seed bank such as With plenty of phosphorus and great seeds from one of the most trusted US cannabis seed companies, you’ll be off to a healthy start.