Sulfur (S) is the 16th element of the periodic table, the 5th most common element on Earth, and the 10th most abundant element in the universe by mass. It is widely found in nature as a constituent of both organic and inorganic compounds. It is a reactive non-metal with a strong odor and very peculiar qualities. Many chemicals we perceive as unpleasant to our sense of smell owe that quality to sulfur. Although it is pretty reactive, it is not that rare to find it in its elemental form (S8). Elemental sulfur is a bright yellow crystal or powder at room temperature. When exposed to heat (>392 °F), it turns into a dark red liquid and burns with a blue flame—quite a color show.
Depending on whom you ask what sulfur is, you’ll probably get a variety of answers. Someone working in environmental protection probably views this element as one of the most harmful pollutants in the environment. This attitude is somewhat justified, considering that the high concentration of sulfur oxides in the atmosphere is known to cause harmful acid rains. On the other hand, if you ask someone from the rubber industry or someone working with agriculture input materials, they will probably talk about sulfur’s great utility and importance in rubber production and plant nutrition. Undoubtedly, sulfur plays an important role in our environment and industrial activities, but it is also one of the essential elements for all living organisms. Although it is familiar to us for more than two millennia, we are still learning about its importance and different functions in living cells.
A bit of history
Sulfur has a long history of use in medicine, agriculture, warfare, and interestingly – the production of fireworks. Archaeologists found the earliest records of sulfur use in Ancient Egyptian medical scripts dating back to 1550 BCE. The Egyptians used it as a treatment for blepharitis, i.e., granulated eyelids. The Ancient Greeks were the first to mention the pesticide effect of sulfur and its practical use as a fumigant to protect plants from pests and fungal diseases. Their descendants, the Byzantines, used this element for different purposes. According to 7th-century scripts, they discovered its high reactivity and explosive qualities and created the famously known Greek fire – a flammable substance used in warfare for burning various structures, primarily ships. According to the texts, Byzantines created Greek fire by mixing sulfur with saltpeter, naphtha, and pitch resin.
Somewhere between the 7th and 9th century AD, a Chinese alchemist concocted a similar mixture and made the first gunpowder. Having more pacifistic ideas than Byzantines, the Chinese first used this explosive powder to make fireworks for their lavish celebrations. However, not long after seeing the explosive light displays in the sky, someone came up with a “better” idea and changed warfare forever. We do not know exactly who was the first to use gunpowder as a weapon, but the first written records of such practice come from 10th century China. The grayish-black powdery mass was used for making early bombs, fire arrows, and fire lances. Knowledge of the gunpowder formula spread west with Mongol conquests in the 13th century and worldwide during the colonial era.
Alchemy & chemistry
Sulfur was in high demand in Europe and Asia during the Middle Ages and the Early Modern Period, but not just as a component of gunpowder. It was a popular disinfectant for contaminated areas during the multiple plague pandemics. Alchemy was also extremely popular at the time, and sulfur was an essential part of every alchemist’s toolbox. The smelly yellow powder was believed to have some quite extraordinary qualities. It was prescribed to passive and melancholic people, assuming it will help them become jollier and more active. This is likely because it was a very established idea that sulfur was the element that represents the soul, and as such, it can heal it.
In the 18th and 19th centuries, scientific breakthroughs debunked those esoteric ideas, sending alchemy (almost) into oblivion. However, that doesn’t mean the interest in sulfur went down with it, nor that its use decreased. Alchemy got gradually replaced by chemistry, and the development of new analytical techniques enabled a more detailed, systematic study of the elements. The new knowledge and exponential development of technology revealed many previously unknown facts about this element. We came to know that alchemists were right in a way; sulfur really is essential for life. Dedicated researchers discovered that sulfur is an essential part of many organic and inorganic compounds crucial for various biochemical and physiological processes in all living organisms. Today we’ll investigate why sulfur is an essential element for plant development and discuss its role in crop cultivation.
Why is sulfur important to plants?
From the perspective of plant physiology, sulfur is a secondary macronutrient, along with calcium (Ca) and magnesium (Mg). Although classified as such, sulfur is needed by plants in quantities similar to phosphorus. It makes up between 0.1 and 0.5% of a plant’s dry mass, and it is primarily present in organic forms – in amino acids cysteine and methionine, thiols, and sulfolipids. These molecules play a vital role in many fundamental physiological and biochemical processes within plants. Plants also contain some inorganic forms of sulfur, but these compounds are less represented compared to organic forms. Typically, younger parts of the plant are more abundant in organic forms of sulfur. However, as they age, their chemical content changes, and the concentration of inorganic forms increases.
Considering that a large portion of the total sulfur content in plants (about 70%) is within their amino acids, the highest concentration of this element is in their protein-rich parts, like leaves and seeds. The roots contain the lowest concentration of this element.
Sulfur is a major constituent of various essential compounds in plants:
- amino acids (methionine, cysteine, cystine)
- enzymes with -SH group
- secondary metabolites (alkaloids, glucosides, etc.)
- acetyl-CoA – one of the most important molecules in cellular metabolism
- lipoid acid – necessary for Krebs cycle
- thiamine (vitamin B) and biotin (vitamin H)
- tripeptide glutathione – an antioxidant that detoxifies reactive oxygen species (free radicals, peroxides) in chloroplasts, mitochondria, and other organelles
- sulfolipids in plant membranes
- phytochelatins – compounds that bind to heavy metals and act as detoxicants in plants
This element is involved in various important processes within the plant, including nitrogen metabolism, production of secondary plant metabolites and pharmaceutical compounds, and plant resistance mechanisms against pests and diseases.
Sulfur in the soil
Similar to nitrogen and carbon, the movement of sulfur in nature is cyclical. The cycle can be understood as a conversion of inorganic forms of sulfur into organic forms and vice versa. It begins with the mineralization of organic sulfur. During this process, sulfur is converted into elemental or inorganic forms (hydrogen sulfide, various sulfide minerals). Inorganic sulfur, including mostly hydrogen sulfide, is then oxidized by various microorganisms to create sulfate ions. The released sulfate ions are utilized by plants and specific microorganisms and reduced to sulfides due to their biochemical activity. The sulfides are later incorporated into organic compounds that will be mineralized, and the cycle begins again.
The activity of plants and certain microorganisms is crucial for all nutrient cycles, not just when it comes to sulfur. This is one of the reasons why soils with rich biodiversity produce healthy, vigorous plants.
How do plants absorb sulfur?
Plants take up inorganic sulfur – sulfate ions and sulfur dioxide. Their root system absorbs this element in the form of sulfate ions (SO4 2-), while the leaves take up sulfur oxide (SO2) from the air. Sulfate ions absorbed by the roots are transported through xylem tissues, moving upwards through the plant to get to the young growth. Although sulfur is highly mobile in the xylem, its mobility is significantly lower in the phloem. This indicates a slow downward movement, explaining why the roots have the lowest concentration of this element.
Plants get most of the sulfur they need from the soil, but they can also absorb it from the atmosphere. They absorb atmospheric sulfur through stomata on their leaves. After the plant absorbs them, SO2 molecules slowly convert to sulfate ions in its cell wall or membranes. The ions are the ready-to-use form of sulfur, and the plants transport them further and use them in their metabolic processes.
Recent studies have shown that a lack of available sulfur in the soil has become a notable problem in many countries over the last couple of decades. This physiological disorder can be very damaging, as it affects both the quality and the quantity of yields. When plants do not receive enough available sulfur, their growth slows down, resistance to biotic stress drops, and they become more susceptible to disease.
Symptoms of sulfur deficiency:
- Stunted growth, most obviously visible on leaves and stems
- The pale green color of vegetative parts
- Even yellowing of leaves
- Leaf edges start to dry and develop necrosis
- Elongated roots
Symptoms of sulfur deficiency resemble the symptoms caused by nitrogen deficiency, but you can differentiate between the two by looking at the age of symptomatic leaves. If young leaves display yellowing first, sulfur deficiency is the cause. Active, young tissues need sulfur the most, so they are the first to react to the lack of this element. On the other hand, if older leaves turn yellow first, you’re probably dealing with nitrogen deficiency.
Why does S deficiency happen?
The current trends in agriculture (intensive crop production, increased use of fertilizers, and pesticides that do not contain sulfur, high-yield varieties) have resulted in increased occurrence of sulfur deficiency. Water easily carries sulfate ions, so they are highly susceptible to leaching. Areas with heavy rainfall and weathered soils with sandy structures often have problems with this physiological disorder.
In indoor growing environments and with plants grown in containers, sulfur deficiency usually appears due to poor soil structure and a lack of good fertilization practice. However, considering the natural sulfur cycle cannot take place indoors (at least not without significant investments and a team of seasoned experts), the growers are the ones that need to make sure there is enough available sulfur in the substrate.
As we mentioned before, sulfur is a very reactive element. Sulfur dioxide is a potent oxidizing agent, and its excess can cause adverse effects on the plant and the soil microbiome. As it reduces photosynthetic activity and increases transpiration, SO2 can severely deplete plant health and affect yield quality. Sulfur also competes with oxygen, which can lead to changes in the structure of proteins and the permeability of cell membranes. Adverse effects of sulfur toxicity also take a toll on various metabolic processes – it inactivates key enzymes, inhibits biosynthesis of amino acids necessary for the production of proteins, and reduces the levels of ATP, the energy molecule.
Symptoms of sulfur toxicity include:
- Necrotic areas on leaves – their color and shape depend on the specific plant species
- Leaves detach from the plant and fall (a symptom known as abscission)
This physiological disorder most commonly occurs in high heat conditions. When exposed to temperatures higher than 86°F, sulfur becomes very reactive, oxidizes quickly, and causes burns on plant tissues. This is why you should always check the thermometer before applying it to prevent unnecessary damage. Also, make sure to avoid exposing the plants to high heat at least 2 weeks after application.
Sulfur as a pesticide
When discussing the importance of sulfur in plant production, we also must mention its role in plant protection. This element is the oldest pesticide in the history of agriculture. We have first descriptions of its fungicidal properties from Ancient Greece, and its use in agriculture for such purposes is still widespread today. It primarily prevents and treats powdery mildews, acting mostly as a preventive fungicide. Sulfur also displays acaricidal activity, affecting two important herbivorous mite families – spider mites and gall mites.
With the discovery of synthetic pesticides in the 1950s, sulfur started to lose its significance in the pesticide market. Synthetic pesticides display great effectiveness in small doses, which made them a popular choice among farmers. However, today it seems that sulfur will regain its importance in plant protection. With the growing threat of microbial resistance to synthetic pesticides and the booming popularity of organic farming, interest in sulfur is getting rekindled.
The return of the sulfur
Many recently developed fungicides have a specific (single-site) mode of action and fail to provide long-term protection. Pathogenic fungi become resistant to these types of compounds sooner or later. Relying on such chemicals and constantly creating new ones to get a step ahead of the microorganism’s resistance mechanisms will eventually generate a super-microbe. Pesticide producers assure their customers this is not a likely scenario if they carefully follow the instructions on the product package. This may seem fine and dandy, but the reality is there are many cases of improper application.
Mistakes in dilution, and applying “just a little” to treat minor problems, creates risks of exposing pathogens to sublethal pesticide concentrations. The ones that successfully survive the encounter with the chemical can develop immunity or mutate. It is enough that only one of these ill applications produces a resistant population to create a serious problem.
Sulfur remained relevant in plant protection all this time because of its non-specific mode of action and low toxicity. This element attacks pathogenic fungi at multiple sites in the cell with its reactive oxides. Sulfur dioxide and sulfur trioxide act as strong oxidizing agents that disrupt essential biochemical processes within the fungal cells. Pathogens can’t develop resistance to this all-around attack, which is why sulfur managed to maintain its efficacy for more than two millennia.
Another problem that comes with the application of many currently available pesticides is the threat to human health and non-target organisms. Pesticides can leave residues or degrade into harmful compounds that persist in the environment. These compounds can get washed by the rain and end up in underground waters, potentially polluting freshwater supply and harming aquatic organisms.
All these problems have led to an increasing interest in organic farming and healthy food production, and sulfur is receiving renewed interest.
Fertilizing with sulfur
Plants require sulfur in all growth phases, so they need to have a consistent source of it in the soil. Although they can also absorb sulfur oxide from the atmosphere and convert it to sulfate ions (the ready-to-use form of sulfur), they prefer to take up the available ions from the soil. Microbially active soils with a good amount of organic matter usually have plenty of sulfur for plants to thrive. However, intensive crop production and new cultivars (which are often heavy feeders) have created the need for additional fertilizing.
When it comes to the application of sulfur, we need to ask the three basic questions– how, when, and how much should we apply? Most sulfur fertilizers work best when incorporated into the soil together with other fertilizers ahead of planting. Although some general recommendations say to apply sulfur every year, this practice is often unnecessary, particularly if you used slow-release fertilizers. Instead, it is best to tailor the application rate according to the amount of sulfur already present in the substrate and the abundance of bacteria that can convert it to sulfate ions. If the soil has a healthy microbiome and is rich in organic matter, applying a small amount of sulfur every 1-2 years should provide enough available ions for plants to thrive.
Considering sulfur’s reactivity and ubiquity in nature, there is a wide variety of compounds that can provide the necessary plant-available ions. In accordance with that, we can roughly classify sulfur fertilizers based on their chemical content into four basic types:
- Elemental sulfur
- Fertilizers that contain both elemental sulfur and sulfates
- Liquid sulfur fertilizers
Elemental sulfur can serve as a soil pH amendment for alkaline soils and make them more favorable for plant growth. Not long after discovering sulfur as an essential plant nutrient, farmers started to apply it as fertilizer, especially on soils with poor structure and low water penetration. Elemental sulfur is the most concentrated and cheapest type of sulfur fertilizer.
The fertilizing effect of elemental sulfur largely depends on the size of its particles and the application itself. Let’s start by explaining the influence of particle size. Elemental sulfur first needs to be oxidized by soil bacteria to become available to plants. Smaller particles have a larger surface area to circumference ratio than bigger ones, and as such, they have more space available for microbial processing. Because of that, smaller particles oxidize easier and faster, providing plants with available ions fairly quickly. Commercially available products usually consist of both smaller and larger particles to provide a constant and long-term sulfur supply.
Note: Keep in mind that applying elemental sulfur decreases the pH value of the soil solution. So, if you need to add some to your garden, it’s best to combine it with fertilizers that will mitigate this effect.
Sulfate ions are very reactive, and they bond with various compounds (most commonly metal ions), forming sulfates. However, breaking those bonds is significantly easier than oxidizing elemental sulfur, which is why sulfate fertilizers don’t take as much time to provide the plants with available ions. The most commonly used sulfate fertilizers include ammonium sulfate, potassium sulfate, magnesium sulfate, and potassium magnesium sulfate.
This inorganic salt has a broad array of commercial uses, but the most common one is in the agricultural industry. As it contains 24% sulfur and 21% nitrogen, it provides a rich source of sulfur and a good addition of nitrogen. As such, ammonium sulfate is a common choice for soils poor in organic matter. It is also very useful as a nitrogen fertilizer for plants in flooded soils (e.g., rice) because ammonium isn’t as susceptible to denitrification and loss as nitrates.
Compared to elemental sulfur, ammonium sulfate acts significantly faster. Therefore, instead of going through microbial oxidation in the soil, it easily degrades to ammonium and sulfate ions, becoming readily available to plants.
Aside from being one of the most important fertilizers in agriculture, ammonium sulfate is also a popular adjuvant for foliar sprays. (Adjuvant is a substance that is added to pesticides to increase their efficacy). It displays high effectiveness, especially when combined with herbicides like glyphosate, glufosinate, and 2,4-D amine.
Also known as arcanite and sulfate of potash, potassium sulfate is a common mineral fertilizer in agriculture. It is a beneficial addition to soils that are deficient in organic matter and somewhat alkaline, as it increases the pH value. It contains between 40-44% K and 17-18% S. Although generally considered a K fertilizer, potassium sulfate can also provide the plants with the necessary sulfate ions. As such, it serves as a source of both nutrients.
Popularly known as Epsom salt, magnesium sulfate is a water-soluble, quick-release mineral fertilizer. Similar to the previous type, magnesium sulfate is primarily an Mg fertilizer. Its anhydrous form contains about 20% Mg and 26% S. In comparison, commercially available Epsom salts often have about 10% Mg and 13% S. This particular type of sulfur (magnesium) fertilizer is popular in hydroponics due to its high water solubility.
Potassium magnesium sulfate
Also known as potash magnesium sulfate and langbeinite, this mineral fertilizer is quite popular among growers. It contains about 22% potassium, 18% magnesium, and 20% sulfur. How quickly it will provide the plants with available ions depends on the size of its particles. The smaller the particles, the faster they will dissolve in water and become available to plants. Applying potassium magnesium sulfate doesn’t affect the pH value of the soil.
Elemental sulfur + Sulfate
If you want to provide your plants with available sulfate ions quickly and give them a long-term source of sulfur, the best choice is to mix elemental sulfur with some sulfate fertilizer. You can easily optimize the ratio of elemental S and sulfates according to the needs of your soil, as well as the type of sulfate applied.
Growers that want swift action often opt for liquid sulfur fertilizer. Although the difference between liquid sulfur and sulfates is relatively small, it is very significant. Instead of sulfate ions, liquid sulfur contains thiosulfate ions. Thiosulfate forms when one of the oxygen atoms within the sulfate ion (SO42) substitutes with an atom of sulfur. Ammonium, potassium, and magnesium thiosulfate are the most common for crop nutrition.
Natural sources of S
Sulfur gets a lot of well-deserved attention in organic farming. Aside from being an efficient acaricide and fungicide, this element improves nitrogen management and fixation in plants, especially legumes. Elemental sulfur is an important item in every organic farmers’ toolbox. Besides elemental sulfur, manure and compost are also good natural sources of this element. We cannot exactly quantify the ratio of S in these types of fertilizers. The quality and the content of the manure/compost are highly variable. They depend on the specific animal/plant material used for their production and microbial content. Due to its balanced nitrogen and sulfur content, manure is the most praised natural source of S among organic farmers.
A proper sulfur diet can significantly help to reduce the leaching of this primary macronutrient. This is a piece of very important information if you strive to use nitrogen fertilizer sustainably in your garden. Sulfur can be an important ally in reducing soil degradation, and cutting grow costs while simultaneously increasing the quality of yields.
If you have any questions about sulfur in plant production, we encourage you to ask in the comments. Happy planting!
- Kovar, J.L. (2011). Nutrient cycling in soils: Sulfur. University of Nebraska – Lincoln.
- Moore, G., McFarlane, J. (2001). Soil Guide: A Handbook for Understanding and Managing Agricultural Soils.
- Paulsen, H.M. (2005). Sulfur in organic farming. Landbauforschung Voelkenrode, Special issue 283.
- Biederbeck, V.O. (1978). Soil organic sulfur and fertility. Developments in Soil Science Vol.8, pp.273-310.