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Phosphorus deficiency in cannabis

Phosphorus Deficiency in Cannabis

The leaves on your crops are changing color, and you’re trying to figure out why. It can’t be a lack of feeding since you’re providing enough nutrients to harvest the fattest buds, right? What’s the possibility of it being a phosphorus deficiency in your weed plants?

Phosphorus (P) is one of the vital mobile macronutrients cannabis plants need to sustain healthy growth. A lack leads to a few discrepancies in how crops absorb nutrients from their growing medium.

A deficiency in any of the nutrients often causes a ripple effect on how and when the roots absorb them. One or more symptoms show us what the plants lack. Identifying the correct deficiency is vital.

We’ll look at telltale signs of a phosphorus deficiency in cannabis plants and how you can fix it. Then we’ll give you some tips on how to prevent it from occurring.

Let’s go!

What causes phosphorus deficiency in plants?

To understand the causes, let’s get behind the importance of the nutrient to marijuana crops. A phosphorus shortage often occurs with a calcium deficiency because of their interaction within the plants’ cells.

Cannabis crops need large quantities of phosphorus, potassium (K), and nitrogen (N) (macronutrients) to sustain healthy growth. Plants use more P during the flowering stage to produce bountiful yields.

The plants use micronutrients like Iron (Fe), zinc (Zn), sulfur (Si), and calcium (Ca) in smaller doses to perform specific biological processes.

Four weeks into the vegetative stage, your crops need adequate amounts of nitrogen, phosphorus, and potassium.

The presence of phosphorus in marijuana crops helps with: 

  • Photosynthesis and energy transfer
  • Stimulate root development
  • Boosts the strength of the stems
  • Improves bud development
  • Promotes seed production
  • Strengthens resistance against disease

Unbalanced pH levels in the growing medium are one of the primary causes of deficiencies in cannabis plants. If there’s a change in the appearance of your crops, first check that your roots are sufficiently absorbing nutrients.

Weed plants take up adequate amounts of nutes within a small pH range of 6–7, where it’s neutral. If it’s below six, it’s too acidic, and above seven, it’s too alkaline. In both instances, it prohibits sufficient absorption of cannabis nutrients.

A cannabis phosphorus deficiency also occurs when there’s an excess of Fe and Zn in the root zone. Too much iron causes a bronze or brown patch on the surface of the leaves. Check the stems and foliage of your crops regularly for early signs.

phosphorous deficiency in weed plants chart
Phosphorous deficiency in weed plants chart

The roots, much like the growing plants, don’t progress well in colder environments. They prefer warm temperatures of 68–75℉. Anything below 68 encourages a P deficiency as the roots are too cold to absorb any nutes.

While a lack of nutrients causes deficiencies, over-saturating the soil with chemical fertilizers promotes nutrient lockout. Even if there’s plenty of phosphorus available, the roots won’t absorb any into the plant.

Heavy rainfall and cold temperatures cause nutrients to seep out of the soil, leading to a phosphorus deficiency.

Symptoms of phosphorus deficiency in cannabis

When cannabis plants experience a deficiency, the first signs become prominent on their leaves. Symptoms of a phosphorus shortage are pretty distinctive but are often confused with the following:

It’s vital to familiarize yourself with the symptoms of a phosphorus deficiency in cannabis.

Changes in leaves

Older, lower leaves begin showing signs first. They adapt brownish, bronze spots on the surface, or the entire leaf takes on a gray, blue or purple color. The early stage usually shows dark but drained blades where the vein color sticks out over the pale tissue.

phosphorus deficiency is known for purple color of weed leaves
Phosphorus deficiency is known for purple color of weed leaves

As the phosphorus deficiency in the cannabis plants progress, the purple shades appear on the base while the edges turn brown and curl down. The color of the petioles may also begin transitioning to a purplish hue.

large brown spots are appearing on the leaves
Large brown spots are appearing on the leaves

Once the foliage starts turning yellow in certain spots, the deficiency is well into its final stages. Purple, brown, or bronze speckles develop on the surface, and leaf tips die. Older blades eventually start to fall.

yellowing weed leaves
Yellowing weed leaves

The leaf texture may also change when a P shortage is at play in marijuana plants. Shiny or smooth leaves feel stiff and brittle when you touch them.

red and purple spots on weed leaves
Red and purple spots on weed leaves with phosphorous deficiency

During the flowering stage, the P deficiency only shows symptoms on parts of the plant in direct light. You’ll notice cannabis leaves turning red, purple, or yellow in a secluded spot while the rest of the plant is healthy.

at the late stage of phosphorous deficiency leaves tips die
At the late stage of phosphorous deficiency leaves tips die

Changes in plant

A delay in treating the symptoms on time leads to the crops’ bright red or purple stems. They don’t always change color; sometimes, it’s the only symptom you’ll notice. 

purple stems of a weed plant can be a result of genetics as well as phosphorous deficiency
Purple stems of a weed plant can be a result of genetics as well as phosphorous deficiency

In this case, it’s safe to assume it’s possibly not a phosphorus deficiency. Specific cannabis cultivars naturally grow red-tinted stalks.

When other symptoms accompany the scarlet-shaded shoots, it’s a cause for concern. The discoloration usually assembles at the base of the stem.

A progressive phosphorus deficiency in weed slows down the plants’ growth, allowing them to take on an abnormal composure. It affects bud development leaving you with low-quality yields. 

A shortage of phosphorus encourages root rot and makes the plant vulnerable to pests and diseases.

How to fix a phosphorus deficiency in weed plants

When a phosphorus deficiency in marijuana plants is present, it’s vital to act swiftly to get rid of it. When it festers, it creates more damage, eventually leading to stunted growth and small yields.

The first thing to do is to test the pH in your soil before feeding more nutrients

  • Low levels (too acidic) in soil: Add powdered or liquid dolomite lime to the medium.
  • High levels (too alkaline) in soil: Add small amounts of vinegar or lemon juice into the feeding water.
  • Low levels in hydro: Use an organic pH-up solution.
  • High levels in hydro: Add small amounts of pH-down to the reservoir.

Organic nutrients like bat guano, worm castings, rock phosphates, and fish meal balance the soil’s pH too.

If you’re having trouble regulating the pH in your growing medium, flush the system with tepid water to recalibrate the levels. For maximum absorption of nutrients, the pH in the root zone must be:

  • 6.2–7 in soil
  • 5.5–6.5 in hydroponics

Potential hydrogen levels lower than six and above seven hinder absorption rates and lead to nutrient lockout.

Maintain a healthy root system to ensure your crops get all the standard nutrients in the correct quantities. To do so, use rooting agents to promote healthy growth and protectants to repel pathogens.

When we see dry leaves on our plants, the initial reaction is to water them because we assume they’re dehydrated. Have you checked the saturation of the soil before adding more?

Overwatering (especially with pots that have poor drainage) promotes water logging in the ground, which leads to nutrient lock.

Maintain the right temperature for cannabis in grow rooms. Resolve your marijuana phosphorus deficiency by warming up the environment a bit. Increasing the heat by 5–10℉ is sufficient to warm the roots’ surroundings. Most weed strains absorb nutrients well at 69–79℉.

Provide the correct nutrient proportions at each stage of growth. You may be giving enough phosphorus, but an imbalance of the others prevents absorption. It’s vital to know what nutes to feed and when to do so.

Keep an eye on your crops after applying these fixes. It usually takes up to a week before you see a change in health. Newer growth doesn’t inhibit symptoms, while older leaves most likely won’t survive.

Remember, the longer a phosphorus deficiency is left unattended, the harder it is to fix.

Phosphorus deficiency prevention

Preventing nutrient deficiencies in weed is the same process as fixing them. Provide your crops with the proper nutrients at each growth stage and regulate pH levels.

It’s important to note that the symptoms don’t simultaneously appear on the crops. Sometimes the stems won’t turn purple or red. Perhaps there are one or two plants in a grow room that show signs of a deficiency while the rest flourish.

Sometimes growers deliberately lower temperatures to promote a phosphorus shortage so that the plants produce purple buds. While this is a great way to stimulate a color change, it’s not necessarily healthy for the crops. 

To naturally grow plants from seeds to produce these shades choose purple strains.

Cannabis plants rely on healthy green leaves for photosynthesis to perform their function. When there’s a change in leaf color, it means chlorophyll isn’t present and can’t convert light into energy. The crops need biological processes so they can mature without error. 

During the vegetative stage, a phosphorus deficiency in weed plants stunts growth below and above the soil. Check pH levels after every nutrient feed to ensure that it’s within range for absorption to take place.

If the deficiency is present during the flowering stage, it results in poor bud production and low harvests. Marijuana plants need more phosphorus and potassium during the budding phase.

Increasing photosynthesis and avoiding nutrient lock ensures that the crops receive sufficient lightening. It’s, therefore, vital that the plants receive enough phosphorus as required. During photosynthesis, they use up much more of this nutrient.

Plants in lower light settings use less phosphorus, reducing the chances of a deficiency occurring. Be extra cautious about pH levels, and nutrient feeds if you cultivate cannabis seeds in well-lit grow rooms.

Happy roots for healthy plants

Keep the roots happy of your marijuana plants to prevent brown, bronze, red, or purple spots on weed leaves from occurring. Growers may assume that the crops aren’t receiving enough nutrients, which is why their plants seem unhealthy.

Remember that a calcium deficiency works in tandem with a phosphorus one. Before diagnosing your crops with any nutrient shortage, check the pH levels in your growing medium. If they’re unbalanced, even them out by flushing, upping, or downing their ranges. 

Remember that the right temperature for cannabis is 68–75℉. The roots don’t sufficiently absorb minerals when the environment is too cold.

Provide the proper nutrients at each growth stage. Prevent those red spots on your cannabis leaves and keep your crops phosphorus deficiency free. Even growing from seed, you should follow a maintenance chart to stay on top of your feeding schedule. Always check pH levels post feeds.

Don’t let discolored leaves discourage you from cultivating marijuana. Snip them off once your crops are on the road to recovery. Get some more seeds and keep the flower growing despite deficiencies in your plants.

Source cannabis seeds from us at i49 to ensure your growing journey starts with top genetics. Healthy ones are less likely to produce plants vulnerable to deficiencies like phosphorus.

AUTHORED BY: Douglas Kester Mr. Kester came to i49 with a wealth of experience. He’s worked in the cannabis industry for more than ten years. As a growing expert at i49, Doudlas finds it hard to choose a favorite strain. Instead, he regards each one as unique and full of potential. Douglas finds it rewarding to experiment with specific cultivars and cross-breed to discover a new one. He strongly believes in sharing the benefits of marijuana with as many people as possible to avoid any misconceptions about the herb. Mr. Kester creatively produces information based on what he’s learned and his experience obtained by implementing what he knows. i49 is proud to have Douglas as part of the team.

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