If you suspect sulfur deficiency in your plants, it is important to find out for sure. There are several possible reasons that a sulfur deficiency can occur in plants.
So, what are the causes of sulfur deficiency in plants? Sulfur deficiency in plants can be caused by a soil pH imbalance or by a lack of sulfur in the soil. Heavy rainfall and plant growth will both remove sulfur from the soil over time.
There are other possible causes, which are somewhat less likely. No matter the cause of sulfur deficiency, there are ways to treat the problem so that you can go back to getting a good harvest.
Causes of Sulfur Deficiency in Plants
When plants suffer problems, the cause is often in the soil. The first place to look when you notice sulfur deficiency is at soil pH and nutrient levels.
Your Soil pH is Imbalanced
Each plant has a preferred pH range, when optimal growth will occur. For most plants, this range is somewhere between 5.5 (somewhat acidic) and 7.0 (neutral).
When soil pH is too far outside of this range (much higher than 7.0 or much lower than 5.5), nutrient deficiencies are much more likely to happen.
For example, sulfur availability for plants drops off rapidly when soil pH goes below 5.0. For more information, check out this chart from Research Gate on the effect of soil pH on nutrient availability.
As you can see from the chart, other nutrient deficiencies are also likely when soil pH goes below 5.0.
The only way to tell for sure that your soil pH is unbalanced is to do a soil test. For more information, check out my article on soil testing.
If you send your soil away to a lab for testing, they can also tell you if there is a sulfur deficiency in your soil.
The Sulfur in Your Garden Soil is Depleted
The soil pH in your garden may be within a normal range. However, there is still the chance that there is not enough sulfur in your soil. This shortage of sulfur in soil can happen for several reasons.
First of all, if you plant crops that use up a lot of sulfur, the soil can become deficient in sulfur. Some examples of plants that are heavy sulfur feeders include:
- Allium vegetables (onions, garlic, leeks, chives)
- Cruciferous vegetables (broccoli, cauliflower, kale, cabbage, Brussels sprouts)
- Certain berries, including raspberries and currants
Sulfur deficiency in soil is more likely if you plant the same crop in the same place every year. Instead, you should use crop rotation, which means planting different crops in an area each year.
Of course, all plants need some sulfur so that they can produce protein and chlorophyll (which makes plants green!). Over time, plants will naturally remove sulfur from the soil.
In addition, some crops these days are bred for higher yields. These plants will take more sulfur out of the soil in order to produce these higher yields.
No matter what you plant, you may eventually need to replace the sulfur in your soil – more on how to do this later.
Heavy Rainfall is Washing Sulfur out of Your Soil
It is possible that heavy rainfall or flooding is removing sulfur from your soil, causing deficiency in the soil and in your plants.
Sulfur (and sulfate, which is the form plants use) is water soluble. This means that sulfur dissolves in water when you mix it in (the same way that salt or sugar dissolve when you stir them into water).
As a result, heavy rainfall can cause sulfur in the soil to dissolve and wash away. This is known as leaching.
Leaching of sulfur is more likely to happen if your soil is sandy, since sandy soil drains more easily and does not hold water.
Leaching is also more likely on hills & slopes, where soil erosion more likely to occur. For more information, check out my article on how to prevent soil erosion.
Your Plant Has a Sulfur Deficiency Due to Shallow Plant Roots
It may sound strange, but if you plant crops that have shallow roots, then sulfur deficiency (and other nutrient deficiencies) can happen.
A plant with shallow roots cannot draw nutrients from deep in the soil. So, if sulfur settles down deeper into the soil due to heavy rain, then shallow-rooted plants will suffer from sulfur deficiency.
This is more likely to happen if you are practicing no-till gardening. The reason is that the nutrients deeper in the soil are not being brought to the surface by regular tilling.
You can solve this problem by planting “green manure” crops that have deep roots (such as alfalfa). These crops will pull nutrients (such as sulfur) from deeper in the soil so that they are available for shallow-rooted crops.
For more information, check out my article on green manure.
Lack of Sulfur in Soil Due to Reduced Pollution
It may seem strange, but reduce pollution from vehicle emissions is also reducing the sulfur in soil. Since sulfur is produced by motor vehicles when they burn gasoline, emissions eventually leave sulfur deposits in the soil.
For more information, see the side by side maps on page 1 of this PDF from Purdue University.
Symptoms of Sulfur Deficiency in Plants
So, how do you tell if your plants have sulfur deficiency? One obvious sign is chlorosis, or yellowing of leaves (and other tissue) on the plant.
Chlorosis occurs because sulfur is used to produce chlorophyll, which is the chemical that makes plants green. A lack of sulfur means a lack of chlorophyll, which leads to a lack of green color and yellowing of leaves.
Usually, the upper (younger) leaves on a plant suffering from sulfur deficiency will turn yellow first. This is because sulfur is an immobile nutrient.
As such, the plant cannot easily move sulfur from lower (older) leaves to upper (younger) leaves. So, a sulfur deficiency first appears as chlorosis of upper leaves.
Sulfur is also necessary for protein synthesis in plants, so a lack of sulfur can cause slow or stunted growth, resulting in small and sickly plants.
For more information, check out this article on plant nutrients from the Mississippi State University Extension.
How to Treat Sulfur Deficiency in Plants
The best way to treat sulfur deficiency in plants is to apply sulfur or a compound containing sulfur to the soil.
For most soils, an application of sulfur will be sufficient for 2 to 3 years. However, sandy soil need more frequent application of sulfur, since the sulfur is more likely to leach away in heavy rain.
For more information, check out this article on sulfur requirements from the Michigan State University Extension.
Also, remember that you should always do a soil test before adding anything to your soil. The last thing you want to do is add sulfur to your soil to treat sulfur deficiency when the problem is really caused by something else!
Here are a few ways to add sulfur to your soil.
Add Elemental Sulfur to Your Soil
Elemental sulfur is available at most garden centers, or online. It contains over 90% sulfur by weight (it is not 100% sulfur, since it is mixed with some clay to give it a different texture).
Remember that sulfur lowers soil pH (that is, it makes soil more acidic). If your soil pH is already a bit low, consider one of the other options below to supplement sulfur, since some of them do not lower pH.
Also keep in mind that sulfur must be mineralized into sulfate so that plants can use it. Sulfate is a compound that contains both sulfur and oxygen.
In a process called sulfur oxidation, bacteria convert sulfur from the soil and oxygen from the air into sulfate. These bacteria will need warm, moist soil with air circulation to mineralize sulfur into sulfate. Peak sulfur oxidation occurs in late spring.
Add Epsom Salt to Soil
Epsom salt contains 14% sulfur by weight, and is another good choice for adding sulfur to your soil.
Epsom salt is magnesium sulfate, meaning that it contains 11% magnesium by weight, which is helpful if you also have a magnesium deficiency in your soil.
Epsom salt contains sulfur in the form of sulfate, which is directly usable by plants. So, you do not have to wait for sulfur oxidation to occur.
Adding Epsom salt to your soil will not affect pH, so you don’t have to worry about making your soil too acidic.
However, be careful not to use too much Epsom salt, or else you can end up with a calcium deficiency in your plants. The reason is that too much magnesium can prevent plants from absorbing calcium, even if there is enough calcium in the soil.
Add Gypsum to Soil
Gypsum contains 17% sulfur by weight. Gypsum also contains sulfur in the form of sulfate, so you do not need to wait for sulfur oxidation to occur.
Gypsum is calcium sulfate, meaning that it also contains 23% calcium by weight. This is helpful if your soil also has a calcium deficiency.
Adding gypsum to your soil will not affect soil pH.
Add Ammonium Sulfate to Soil
Ammonium sulfate contains 24% sulfur by weight, and also adds nitrogen to your soil (in the form of ammonium).
Ammonium sulfate contains 21% nitrogen by weight.
This compound also lowers your soil pH, so be careful about adding too much, or your soil can become too acidic.
Add Potassium Magnesium Sulfate (Sul-Po-Mag) to Soil
Potassium magnesium sulfate (Sul-Po-Mag) contains 23% sulfur by weight. It also contains 22% potassium and 11% magnesium by weight, making it a good all-around source of secondary nutrients for your soil.
The best part is that Sul-Po-Mag has no effect on soil pH.
You can also use potassium sulfate, which contains 18% sulfur and 50% potassium by weight, with no magnesium. This also has no effect on soil pH.
Add Manure to Soil
Finally, you can add manure to your soil to replace some sulfur. Remember that manure will not contain as much sulfur as the other methods we mentioned earlier.
Also, you need to make sure that manure is decomposed before using it in your garden. Otherwise, you can burn your plants with too much nitrogen or with salts in animal waste.
For more information, check out my article on manure.
By now, you know about the causes of sulfur deficiency in plants, and what symptoms to look for to identify the problem. You also know what to do to treat sulfur deficiency in plants.
I hope you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about sulfur deficiency in plants, please leave a comment below.