Sulphur And Cannabis Plants

Sulphur is an essential secondary nutrient for cannabis plants. Although they don’t need much of it compared to nutrients like nitrogen, a deficiency or excess will cause serious growing troubles. Considering that, in this article, we’ll take a look at the roles sulphur plays in the growth of your cannabis. We’ll also teach you how to spot the symptoms of sulphur deficiency and toxicity, and how to treat and prevent them in turn.

What Is Sulphur?

Sulphur, along with magnesium (Mg) and calcium (Ca), is considered a secondary cannabis nutrient. Like the primary nutrients nitrogen (N), phosphorus (P), and potassium (K), secondary nutrients are classified as macronutrients since plants need them in relatively large quantities. Both animals and plants need sulphur in relative abundance. Even if cannabis doesn't require as much of it when compared to other elements, sulphur has many important functions. It aids in the production of chlorophyll, is a component of proteins and amino acids, boosts plant resistance, and more!

In nature, sulphur is available in its pure elemental form as a yellow mineral, but it’s also found in sulphates and sulphites. The elemental form of sulphur, however, isn’t water-soluble, and plants cannot absorb it through their roots. In the soil, sulphur is usually found as sulphate, which is the only form of sulphur that can be utilised by plants. Elemental sulphur has to be metabolised into sulphate by microorganisms, and that can take a long time. Elemental sulphur, therefore, is utilised as a long-term fertiliser, while the sulphate (SO₄) form of sulphur (which is found in commercial cannabis nutrients) is available to plants right away.

The Sulphur Cycle

Like some other elements, sulphur cyclically replenishes itself within the ecosystem. It begins with weathering rocks releasing stored sulphur into the air, where it is then converted into sulphate (SO₄). Sulphate is then taken up by plants and microorganisms, which convert it into organic forms that are eaten by animals. As organisms die and decompose, some of the sulphur is again released as sulphate, which is taken up by microorganisms.

Sulphur is also released from volcanic eruptions and evaporating water. It returns back to earth with rainfall, and, through rivers and streams, finds its way into the oceans. The element is then moved to land by the geologic uplifting of ocean sediment.

Why Do Cannabis Plants Need Sulphur?

The element plays many different roles in the growth of your cannabis plant. These include:

• Promotion of root growth.

• Encouragement of chlorophyll production and distribution.

• Important component of proteins and amino acids.

• Enhancement of plant metabolism.

• Promotion of overall growth and plant health.

• Support of the water and nutrient absorption process.

Many of these functions are critical for the vegetative growing stage of cannabis, but sulphur is also needed in the seedling and flowering stages as well.

Cannabis, Sulphur, and pH

When growing cannabis, sulphur is available upwards from a pH value of 5.5, although optimal absorption happens at 6.0 pH. This applies to both soil and hydroponics. If the pH of the growing medium drops below 5.5, signs of sulphur deficiency will occur.

Early Stage Needs

If sulphur isn’t available, or cannot be accessed, during vegetative growth, plants will lack vigour and vitality, growing fewer branches as a result. Severe growth problems are also likely to occur, as the element is important for photosynthesis and other critical functions.

Flowering Needs

During flowering, a lack of the mineral will result in a greatly diminished final product. Bud growth will suffer and poor yields will be the result. If there is a serious sulphur deficiency during the flowering stage, the flowers themselves might even die.

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Sulphur in Chemical vs Organic Fertiliser

In commercial cannabis fertilisers, sulphur is typically found in its sulphate form readily available to plants. However, the organic cannabis farmer has plenty of options available to provide the mineral to their plants. Some of the best natural sources of sulphur are:

• Epsom salt, which contains both magnesium and sulphur.

• Mineral-rich soils found in nature, particularly volcanic soils.

• Gypsum (calcium sulphate), which is rich in sulphur, calcium, and other elements.

• Potassium sulphate, which provides both potassium and sulphur.

• Animal manures and organic matter, which are rich in organic sulphur.

Sulphur Deficiency

Sulphur is considered a partially mobile nutrient, meaning it moves very slowly within the plant. Any signs of deficiencies, then, typically occur on middle leaves, but symptoms can also show on older or new growth.

What Causes Sulphur Deficiency?

• Incorrect pH level: pH level dropping below 5.5 will result in a “lockout” of sulphur. This means the plant is unable to access the element even if it is present.

• Poor soil: Poor-quality soil will usually contain insufficient amounts of sulphur.

• Improper use of nutrients: Using the wrong concentration or dose of nutrients may result in plants not getting enough sulphur.

How to Recognise Sulphur Deficiency

The symptoms of a sulphur deficiency (e.g. leaves turning yellow) can mimic those of a nitrogen (N) deficiency, especially if both deficiencies occur at the same time. For that reason, a sulphur deficiency can, at times, be difficult to diagnose. However, sulphur deficiency is far less common than nitrogen deficiency.

How to Distinguish Sulphur Deficiency From Nitrogen Deficiency

• Leaves turn yellow from the inside outwards. Only in the later stages will the entire leaf become discoloured.

• Sulfur deficiency affects more than the old leaves at the bottom, while nitrogen deficiency is typically restricted to that area.

Signs/Symptoms

Early stages:

• Slight chlorosis (leaves turning yellow) on the mid-section of leaves.
• Poor growth, with fewer branches.
• Yellow leaf veins.
• “Burnt” leaf tips that dry out and bend downwards.
• Woody stems.

Later stages:

• More pronounced yellowing, eventually taking over the entire leaf.
• Poor bud formation.

Tip: Feeding cannabis nutrients may lead to minerals (“salts”) accumulating at the root zone over time. When this happens, the pH at the roots gets thrown off (becomes too acidic), leading to nutrient lockout and deficiencies.

Many times, nutrient-related issues, at least when growing cannabis, are not due to improper nutrient administration by the grower. Most of the time, deficiencies are due to pH issues, such as those that arise from overfeeding. Moreover, nutrient deficiencies rarely come one at a time. Chances are, if it happens at all, your cannabis plant will suffer multiple nutrient deficiencies at once. If you suspect pH problems, your first action should be flushing your plants with plain pH-balanced water. This will wash out the accumulated salts and restore optimal pH at the root zone.

How to Treat Sulphur Deficiency

There are two ways to address a sulphur deficiency, depending on the underlying cause:

Insufficient sulphur levels? Provide sulphur in the form of an appropriate cannabis fertiliser or by amending the soil with sulphur.

Deficiency due to lockout/pH issues? Restore correct pH (6.0+) to address the deficiency.

• In Soil

  If low nutrient doses are the reason for the deficiency, just increase the dose for a couple of feeds. The plant should show improvement after a week. As a quick first aid, you can also dissolve 1 teaspoon of Epsom salt into 1 litre of warm water. Use this solution to spray the underside of leaves.

• In Hydro/Coco

  If underfeeding is the reason for the deficiency, gradually increase feed/EC by 0.2 points every few days. Observe your plants' health and stop increasing when they are visibly improving. Use a foliar spray as mentioned above to provide sulphur to your plants directly. If pH level drops below 5.5, correct it to the appropriate range and provide nutrients at the proper dose.

Sulphur Toxicity

Sulphur toxicity (excess) is rarely seen in cannabis, but this doesn’t mean it can’t happen. However, it is not easy to diagnose because a sulphur excess will usually also lock out nitrogen and calcium, showing related symptoms in turn. In case of sulphur toxicity, the first thing to do is flush the medium to get rid of accumulated nutrients in the soil. Along with the flush, you should also stop feeding for a week so the plant can consume the remaining nutrients. Afterwards, reintroduce a correct feeding regimen with the proper amount of nutrients.

How to Recognise Sulphur Toxicity

The most common signs and symptoms of sulphur toxicity are:

• Leaves turning a dark green or brown colour.
• “Burnt” tips and/or dry and brown leaf margins.
• Signs of other deficiencies, e.g. Ca/Mg.
• Acidic pH below 5.0.

How to Treat Sulphur Toxicity

Treating sulphur toxicity involves taking the substrate into consideration. Here’s how to treat toxicity in soil vs hydro/coco.

• In Soil

  Flush your medium with pH-balanced water. When growing in soil, use approximately 2–3x the capacity of your pot for flushing. Afterwards, don’t give nutrients for a week to allow the plant to take up the remaining nutrients.

• In Hydro/Coco

  In hydroponic settings, you should flush and reduce the EC in your reservoir. If pH is low, correct it or change out the water. If you’re using coco as a medium, flush it out and refeed your plants at a lower nutrient level. Things should improve in a week or two after that.

Luke Sholl

Luke Sholl has been writing about cannabis, the wellness potential of cannabinoids, and the positive influence of nature for over a decade. Working with several cannabinoid-centric publications, he publishes a variety of digital content, supported by strong technical knowledge and thorough research.

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