The Importance of Sulfur in Your Soil
History of Sulfur
Sulfur in soil has a very interesting history. For the longest time, the majority of sulfur actually came from air pollution. Before we started controlling sulfur dioxide about 40 years ago, it would just fall out of the sky. One textbook states that up to 45 pounds per acre of sulfur would be deposited per year. Sulfur deficiency was very rare. Once we began controlling air pollution through cap and trade programs, we quickly started experiencing sulfur deficiencies in different soils.
Sulfur is critical in the creation of aromatic compounds—also termed secondary metabolites—in the onion family. When onions make your eyes burn or when garlic has a strong flavor and aromatic scent, that is sulfur. Researchers have found that most crops prefer a sulfur to nitrogen ratio of about 15:1. But cruciferous plants, like the cabbage plant family, actually prefer 3:1. The amino acids in alliums and the production of cruciferous plants are maximized with a lot of sulfur.
Sulfur in the Soil
Sulfur is an anion. In the soil solution, it is very mobile and therefore leachable. Similar to chloride, sulfur levels on a soil test indicate how much precipitation an area receives, or at least how much water is running through the soil profile. If sulfur is low, that means it's getting leached out of the soil. In those types of soils, the majority of sulfur is actually supplied through organic matter. 90% of sulfur in soil is found in organic matter and as biology mineralizes the organic matter, it releases sulfur for plants. While plants take up about as much sulfur as phosphate, much more of it leaches every year than phosphorus.
Tip #1: If you have a really good crop, start tracking what your sulfur to nitrogen ratio is. Only way to do this is with a soil test.
Sulfur and nitrogen are critical in amino acids and proteins in plants. They are partners in almost every protein in a plant. Optimal sulfur to nitrogen ratios have been found for many crops. Often times when nitrogen is too high, it makes plants much more susceptible to sulfur deficiencies.
Tip #2: If you're in a high precipitation place like the east coast or the west coast, an addition of sulfur every year is a really really good idea.
I prefer gypsum, magnesium sulfate, potassium sulfate, or langbeinite (also known as K-Mag) as good organic sources of sulfur. It depends on what cations you need because sulfate is actually a carrier in most of these products to bring in other catatonic nutrients.
I almost never recommend elemental sulfur. If you think you have a sulfur deficiency, you could foliar apply a powdered elemental sulfur and see what happens. But otherwise, elemental sulfur is usually used to decrease soil pH because when elemental sulfur goes into the soil, it's oxidized by soil biology, creating sulfuric acid, which lowers the pH. It also attaches to cations and over time it is leached out. I personally believe that high soil pH (in the 7.0-7.6 range) is fine, even though optimal pH is in the sixes. When your pH is above 8, your soil is likely calcareous. And above 8.2 you likely have excessive sodium. Many agronomists and researchers have found that you cannot reduce pH very easily in the long-term in a calcareous soil. At 10 tons per acre, elemental sulfur temporarily decreases the pH, but comes right back up after a couple of years.
Did 10,000 lbs of sulfur lower the pH?
Short answer: NO.
There is too much lime in the soil profile to reduce pH of calcareous soil. This is also a loam soil... Plus, who wants to buy 10,000 lbs of sulfur for a few tenths of a pH drop that only last a few years?
If you want to lower the pH, ask yourself "why?" If you do, I suggest acidifying the SOIL SOLUTION with your irrigation water instead of changing the chemistry of the bulk soil. Sulfur burners are great for this.
I don't really ever recommend elemental sulfur because usually I'm applying or recommending gypsum, which brings in plenty of sulfur. On the Logan Labs paste test, which is looking at the soil solution, the textbook definition of sufficiency is 3-5ppm. But I like it to be significantly more than that (for heavy feeders, above 40ppm on a saturated paste test).
Deficiencies of sulfur can physically manifest as the whole plant chlorosis. There's a light pale yellowing on the whole plant. Unlike nitrogen, it doesn't just show up at the bottom. It'll show up on the whole plant.
From my understanding, sulfur toxicity will never happen. What can happen on salt-sensitive crops is when sulfur gets too high is that it becomes the primary culprit in increasing soluble salts, or electrical conductivity. If you look at a soil with high soluble salts, there is likely a lot of sulfates. I've seen sulfur on the Logan Labs paste test upwards of 170 ppm. This is too high in my opinion because it pushes the soluble salts (EC) too high. When that soil dries out, your plants will likely experience osmotic stress. I don't have a specific target. It’s totally crop-specific. I strive for sufficiency first, and then balance with all of the other nutrients and have a healthy level of soluble salts. Usually sulfur will follow suit.
If you'd like to start testing your soil or water, you can get tests here.
If you have any contributions, thoughts, or observations on sulfur, please comment below.