Do you have plants that look yellow despite fertilizing them? Do you have legumes that look nitrogen deficient? Does your vegetable garden or pasture just seem to fail to thrive? If so, your plants might have a sulfur deficiency. Today we are going to take a closer look at sulfur deficiency in plants, discuss how to recognize it, and how to remedy it.
In case you are unaware, plants have requirements for various mineral nutrients just like humans and animals. They are broken down into groups of macronutrients—those required in large quantities, and micronutrients—those required in very small doses. If you have ever bought fertilizer, then you know that Nitrogen (N), Phosphorus (P), and Potassium (K) are present in almost every fertilizer. They are denoted by the numbers on the fertilizer bag eg. 10-10-10. Fertilizers are based on these nutrients because they are the 3 primary nutrients that are often deficient in soils.
Sulfur (S) is a nutrient that falls into the category of a secondary macronutrient.
Plants still need fairly large quantities of sulfur, but their need is not as great as it is for NPK. Sulfur is not typically present in fertilizer unless it is a ‘complete fertilizer’. This is because historically, and still in many areas today sulfur is not commonly deficient.
However, times are changing, and sulfur deficiency is showing up much more frequently. Many farmers who have never needed to add sulfur to their fields now need to do so yearly. They are starting to see deficiency symptoms such as crops that are failing to perform as expected or are failing to respond to nitrogen fertilization.
Part of the reason we are seeing more sulfur deficiency is likely that our air is cleaner. We are burning less coal, so there is less acid rain falling. As backwards as it seems, in some ways the air pollution was actually doing our plants, and our farmers, a service. It was keeping the soil enriched with sulfur.
Another reason we might be seeing more sulfur deficiencies showing up is due to the depletion of organic material. This is caused by our reliance on chemical fertilizers resulting in our failure to complete the nutrient cycle. For years and years we have been treating manure as a nuisance waste product. We have been failing to return it back to the field where the feed was grown. Instead, it should be treated as a valuable source of plant nutrition.
The majority of sulfur present in the soil is bound up in organic matter. Loose sulfur is mobile in the soil, and can be washed away by rain. So, if we take from the soil without returning the organic matter we remove, we eventually deplete nutrients like sulfur by depleting the organic matter.
How to recognize sulfur deficiency
If you know what a nitrogen deficiency looks like, then you know what a sulfur deficiency looks like. Unfortunately, sulfur deficiency looks virtually identical to a nitrogen deficiency. This means that sometimes it can go unrecognized for years. Often it takes a soil test or some outside-the-box thinking to realize that you have a sulfur deficiency.
Sulfur deficiency is characterized by pale green or yellow plants. These plants are also often smaller than normal and produce small, narrow leaves. Sometimes on plants like corn you will see clear interveinal striping on young leaves while the older leaves remain green. This yellowing of plants is almost exactly the same as seen with nitrogen deficiency, but there is one difference. Sulfur deficiency shows up on new leaves first whereas nitrogen deficiency shows up on old leaves first.
There are a couple additional indicators you should watch for that point in the direction of a sulfur deficiency. Firstly, if you are growing a vegetable garden and you have stunted, yellow, nitrogen deficient looking bean or pea plants, there is a very good chance it is actually a sulfur deficiency. Legumes such as beans make their own nitrogen and are rarely deficient in that particular nutrient.
Secondly, if you have applied fertilizer to plants that look nitrogen deficient and they did not green up and put on a burst of growth, there is likely another deficiency at work. Also, keep in mind that if you have sandy, low organic matter soil, your soil is going to be prone to not only macronutrient deficiencies but also micronutrient deficiencies. Sometimes there can be more than one deficiency present, and curing one can make a second one visible.
How to treat sulfur deficiency
As we mentioned above, the majority of the sulfur in soil is tied up in organic matter. Thus increasing the organic matter in your soil will improve sulfur levels. Manure is a good source of slow release sulfur, and we would encourage you to add manure to your garden soil if you have access to it. It is not only rich in sulfur, but full of all the other macro and micronutrients that plants require. Other sources of organic material such as tree leaves, straw, or untreated lawn clippings can also be excellent additions of slow release nutrients.
If you do not have access to organic matter, or if your sulfur deficiency is so severe that organic matter alone is not fixing the problem, you can add sulfur through the addition of elemental sulfur or through the use of ammonium sulfate fertilizer. Ammonium sulfate is a sulfur rich nitrogen source. Replacing part of your standard nitrogen fertilizer with ammonium sulfate is often all your plants will need to meet their sulfur needs.
There is a third sulfur option often seen at garden centers, aluminum sulfate. We don’t recommend using this soil amendment particularly if you will need to add sulfur to your soil annually. The aluminum can build up in the soil, and that is not a good thing. Aluminum can be toxic to plants, so it is best to avoid adding too much aluminum sulfate to your garden.
Beware–Sulfur is used to lower soil pH
This means that if you add a lot of elemental sulfur or ammonium sulfate to your garden to meet your sulfur needs, you will be lowering your soil pH. You should be periodically getting soil tests done and using lime to amend your soil pH as needed to keep it in an appropriate range. The ideal range varies by crop, but 6.5 is a good place to aim for to satisfy many plants. You can learn more about soil testing and pH in our earlier article on this topic.
You should also not add large quantities of sulfur to your soil all at once. Not only do you risk lowering the pH too far, but sulfur is mobile in the soil. Unless you have soil rich in clay, which has a high nutrient holding capacity, it is best to add only what your plants need for the year. Otherwise the extra fertilizer will be wasted because it will leach to below where the plant roots can access it.
We hope that if you have a seemingly incurable nitrogen deficiency, or a garden full of stunted yellow plants that adding sulfur will help your plants thrive!
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