Soil Sulfur: How It Works

Soil sulfur may be invisible to our eyes, but in our minds, sulfur is a yellow, powdery substance that we most associate with poor-smelling things. Onions and garlic, for instance, both have a certain smell that can be attributed to sulfur. But sulfur rarely gets the attention it should in soil and plant health. Personally, I find it to be the most interesting nutrient we as gardeners can investigate. 

Sulfur itself is a critical plant nutrient that has been written off as something toxic to worry about in the soil science field. While in previous decades not worrying about sulfur may have been fine, things have changed. This is due to more efficacious environmental laws in the US and the world. The downside: that, combined with changes in fertilizer, led to more widespread sulfur deficiencies. 

This article dives into the importance of sulfur in plants and soil. Diagnosing deficiencies, amending sulfur, and learning how sulfur cycles and its importance in plants will also be covered.  

Sulfur’s Role in Plants

Sulfur is supplied to the plant in the sulfate (SO₄^2-) form, taken in by the roots via soil solution. Sometimes sulfur dioxide (SO₂) is taken in from the surrounding air through leaf stomata. However, the quantity taken up in its gaseous form is limited. 

Sulfur has a critical role in amino acid formation. Combined with nitrogen, sulfur forms many amino acid chains, which are the building blocks of proteins. Sulfur is also critical in the process of photosynthesis. Since photosynthesis is a crucial process where a plant synthesizes its own food, ensuring sufficient sulfur is present is critical. 

Sulfur is an essential nutrient in the biological nitrogen fixation process carried out by bacteria in relationship with plants and soil. This is why leguminous crops will need more sulfur compared to other crops. Finally, sulfur is an important part of the production of the pungent oils found in crops like garlic. 

Sulfur in the Soil

Similar to its role in plants, proteins have a dominant role in relation to sulfur in micro and macro organisms found in soil. Since sulfate is not a positively charged ion, it is not involved with cation exchange, where different ions bond to one another to create a functioning system of nutrient transmission to plants.

It is instead involved in anion exchange, where certain ions remove themselves for the same purpose. But some sulfur is still leached out of the soil profile in this process. Outside of these things, the major role of sulfur is for lowering soil pH. 

Soil pH

Sulfur-containing compounds are commonly used in agriculture to lower soil pH and thus make them more acidic. It’s important to note and understand that a pH test measures the number of free hydrogen ions in a solution. The difference between each pH delineation is significant, even if we’re talking about a jump from a pH of 5 to 6. As many of us know, soil pH can be the difference between a hospitable and inhospitable environment for a plant. 

The reason sulfur acidifies soil is due to its reactions in the soil. Specifically, elemental sulfur creates a reaction that yields sulfate and free hydrogen atoms, which increase the soil acidity. Most sulfur-containing compounds affect the soil pH. Elemental sulfur is the most commonly used to lower pH, which gardeners add to soil in the form of compost or a sulfur-specific amendment. 

An important thing to consider with elemental sulfur is that when it lowers pH, there is a significant biological effect on soil microbes. It is not a chemical reaction, like liming the soil, where carbonate works to increase soil pH. This means that it may take a bit longer to lower the pH than it would with lime, but sulfur will get the job done. 

Since this is a biological reaction, applying an increased amount of elemental sulfur is not the best idea. Doing this could lower pH to a level where aluminum toxicity is probable. I would recommend only applying elemental sulfur as directed for your soil texture type and climate. Sulfuric acid is also used to lower soil pH more rapidly. However, I do not recommend this since it can go poorly for non-professionals. 

Ultimately, as with everything involving soil and plants, using a soil test can tell you the pH level of your soil with certainty. In a normal circumstance, the pH will not be highly acidic or basic on the pH scale. 

In some rare scenarios, iron sulfide may be found at the soil surface. If you find this black substance in your soil, know this is of great concern. Most of the time, you won’t find iron sulfide in your garden. But if you rarely see it, cover up this acidic substance.   

Sulfur Requirements

Because it is a macronutrient, plants need sulfur in higher quantities than they would micronutrients. When we think about sulfur’s role in proteins and amino acids, we see how it’s critical for plants. As it is with all nutrients, different species of plants require varying amounts and combinations. 

Sulfur percentages in dry matter can range from .2-.5%. But determining what a plant needs based on the percentage of a nutrient in its tissues is not the best way to measure how much sulfur to add to the soil. The best way to determine this is through measuring the ratio of nitrogen to sulfur. 

A 12:1 (N:S) or lower ratio is typically present in leaf tissues. When the ratio of nitrogen becomes too great, then the plant will start to build up sugars which attract varying pests and diseases. Therefore, the balance of sulfur to nitrogen is important for plant safety.  

Spotting a Sulfur Deficiency

Sulfur deficiencies are often misdiagnosed as nitrogen deficiencies. This is because both help a plant retain its green color. In either of these deficiencies, the main symptom is yellowing, chlorotic leaves. However, details are key when diagnosing issues that occur with plants. 

Yellowing leaves, associated with sulfur deficiencies, show up on newer leaves. On the other hand, nitrogen-deficient plants show this symptom on older leaves. This is because nitrogen is easily translocated in plants and can fluidly move from one location to another. Sulfur, on the other hand, does not move through plant tissues as easily.   

Sulfur Cycle

Let’s dive into the ways sulfur enters and exits the soil. It is important to understand the sulfur cycle, and how environmental regulations that have benefited us have also impacted soils. 

Atmosphere

Until recently, sulfur deposition – a process where sulfur sediment in the air settles on the soil surface – was the primary way sulfur entered soils. As years went on and we burned coal and fossil fuels, they released sulfur dioxide into the air. This compound was combined with rainwater, and when it rained, the sulfur turned to sulfate in the soil. 

As institutions worldwide have moved to enact more air pollution measures, there has been a significant reduction in sulfur dioxide released to the atmosphere. These regulations have ultimately benefited us as humans, but they have limited the ways sulfur previously entered the soil.

Minerals

Many sulfate-containing minerals are weathered and broken down through varying natural processes. These minerals include gypsum, epsomite, and several others. The weathering process takes time to break down minerals, but when it is complete, sulfur is released into the atmosphere and becomes sulfate after interacting with oxygen. 

Then, in the process of sulfur deposition we discussed in the last section, sulfur settles on the earth and penetrates the ground through irrigation water and rain.  

Organic Matter

Sulfur enters soils most commonly through organic matter. This is where 95% of sulfur absorption occurs in plants. Sulfates are released to plant roots as organic matter is broken down by soil microbes. As long as soil microbes are active, this continual and important process of releasing sulfate occurs. 

Organic matter content varies depending on the type of soils in question. Soils with greater clay content have a greater capacity to hold organic matter compared to sandy soils. 

Other Sources

Irrigation water provides some of the sulfur supplied to plants. This is another way sulfate is added to soils in a way plants can access it. However, this is not a significant way it is added to soils. In the past, nitrogen fertilizer and other fertilizers contained sulfate as well. 

Unfortunately, sulfur has taken a back seat, and levels are much less in commercial fertilizers, due to decreased levels of atmospheric sulfur. In reality, NPK should change to NPS because of sulfur’s impact on plant health and its increasing demand.  

Ways Sulfur Is Removed

Considering sulfur is an anion, or a negatively charged ion, it doesn’t cling to soil particles and tends to leach out of the soil profile. While it doesn’t have the leaching potential that the anion nitrate does, it still leaches out at a high rate. But it does not have the same role in pollution that phosphorus and nitrogen have. 

The most significant way sulfur is removed from soils is through plant matter removal. Removing plant matter and harvesting crops takes sulfur out of soils. Gardeners can avoid this by leaving as much organic matter as possible intact in the field. Using a chop-and-drop method with cover crops is one way to add sulfur to soils. There are also sulfur-based soil amendment products that do the same.  

Sulfur Amendments 

There are so many amendment products that improve sulfur levels in soil, there’s not enough time for them in this piece. Here, I will only list a few of the most important and common ones.The easiest and most accessible of these products is compost. Including a well-rounded compost with a good carbon-to-sulfur ratio can add sulfur while making it available to your plants. 

Gypsum is another common soil amendment, available in many different products. I really like gypsum as a soil amendment, as it does not have a strong impact on pH but supplies sulfur to plant roots. The only downside is it takes a while to break down compared to other similar products. 

Elemental sulfur, most often in the form of sulfuric acid, is an obvious choice. Especially in sandy soils that leach sulfur and have increased pH, products that contain amendment kill two birds with one stone, lowering soil pH and providing sulfuric compounds to your plants. 

Epsom salt, the stuff you use for personal care, can work well as a sulfur fertilizer. It is known chemically as magnesium sulfate, and a search for it is sure to be fruitful. However, remember to test your soil before you fill it with Epsom salt, as additions of more could damage soils that already have adequate magnesium present. 

Frequently Asked Questions

Q: What does sulfur do to soil?

A: Generally, sulfur-containing compounds are used to reduce soil alkalinity. 

Q: How do you get sulfur in soil?

A: There are several ways to increase sulfur in soil. Increasing the SOM (soil organic matter)  increases sulfur. Providing organic matter also improves microbiome health and increases soil fertility and carbon sequestration. There are also plenty of soil amendments that provide soil with sulfur.

Q: What happens if there is too much sulfur in soil?

A: Too much elemental sulfur causes acidity, which can leach toxic metals into the surrounding air and water. Aluminum toxicity occurs when the soil pH falls below 4.5. Provide liming agents to improve pH. Exercise patience, as adjusting soil pH changes the chemistry of soil. 

Q: How much sulfur should I add to my soil?

A: The amount of sulfur needed depends on several things. Understanding your soil type and doing a soil test is key before you add amendments. 

Q: Does my soil need sulfur?

A: Do a soil test to determine if sulfur should be added. In a container garden, the need for sulfur is less likely than it is in a raised bed or large farm.   

Q: Is sulfur a good fertilizer?

A: It’s an excellent fertilizer. Providing a fertilizer that includes sulfate is good for the garden. Gypsum, Epsom salt, and compost all add sulfur amongst other nutrients to soil.