In the last 30 days, I have had conversations with three different industry experts. One makes fertility recommendations for a competing fertilizer company. The second supervises the maintenance of about 25 athletic fields. The third oversees fertilization of well over 1000 acres of commercial turfgrass. These guys are smart. Educated. Experienced.
The only problem is, they don’t adequately understand soil chemistry. Fair enough. Soil chemistry is a complicated thing. I don’t fully understand it either, but I work at it. Hard. Because it’s important, at least to me. So, what did these guys do or say that prompted this blog? Well, the first guy stated in a fertilizer recommendation that high sulfates were causing media pH to drop. The two turf guys stated that they were using magnesium sulfate to help lower the high pH they were dealing with.
I like these three guys, and I respect them. However, in this case, they are simply factually incorrect. Let’s go back and re-examine pH. Two letters, p and H. They stand for “potential of Hydrogen”. When you measure the pH of a soil or water sample, what you are really measuring is the amount of hydrogen ions in the sample. It is not all that different to measuring the amount of magnesium or zinc in the sample. You are simply measuring the amount of hydrogen ions. As you may remember from sixth grade science, a hydrogen ion is also known as a proton. So, when we test something for pH, we are really testing for the quantity of protons it contains.
Therefore, in order for something to be acidic, it has to react in such a way that hydrogen ions or protons are generated and released into the soil or water. Sulfates do not do that. They do not contain hydrogen, nor do they react in such a way to release it. We measure the acidity of fertilizers by something called “calcium carbonate equivalent”. Calcium carbonate equivalent simply means how much calcium carbonate would be required to neutralize the acidity of 1 ton of a given fertilizer. A pH neutral fertilizer would have a calcium carbonate equivalent number of zero.
So, knowing this about fertilizer acidity, let’s look at several common fertilizer ingredients: sulfate of potash (SOP), ), magnesium sulfate, calcium sulfate and K – mag, also known as sul-po-mag. All of these fertilizer ingredients are sulfates. All of them have the same calcium carbonate equivalent. Zero. Zip. Nothing. They are completely neutral. Not acidic at all. The sulfate ion is simply not acidic. It does not contribute hydrogen ions to the system. It can’t and it doesn’t.
But now, isn’t ammonium sulfate acidic? It certainly is. Ammonium sulfate has a calcium carbonate equivalent of -2200. That means for every ton of ammonium sulfate per acre you put out, you need to put out over a ton of calcium carbonate to neutralize it’s acidity. So, yes, ammonium sulfate is acidic. But not because of the sulfate. Because of the ammonia. The ammonia reacts with water to become nitrate, leaving behind, you guessed it, hydrogen ions or protons. Hydrogen ions or protons are by definition ‘acidity’.
About once a month, I receive an email from a Harrell’s customer concerned about the sulfate levels in their soil tests. They are worried because the number in their soil test is in the high or very high range. They are frequently also concerned that this will make their soil pH drop. Well, if you have read this far, high sulfates cannot and will not lower soil pH. Many Harrell’s fertilizer formulations do contain a fair amount of sulfates, and for good reason. Sulfur is an essential plant nutrient. Sulfate sources of major and secondary nutrients tend to have a lower salt index than chloride sources. Third, sulfate micronutrient sources tends to be more available to plants than oxides or sucrates.
Interestingly, whenever I get one of these concerns about high sulfates, I ask how the plants are doing. Invariably, the answer I get is “Oh, no, the plants are fine”. Well, yes they are. Because high sulfate levels are a perceived problem, not an actual problem to the plant. But, let’s ask a fair question: Are high sulfates ever a problem? Yes, at least in a couple of ways. One, sulfate is an anion. Negatively charged. High levels of anionic sulfates can restrict absorption of another key anion, nitrate. Second, sulfate is a soluble salt. High sulfates will generally lead to soluble salt issues before leading to any sort of toxicity. The short version is this: if sulfates are elevated in the soil test, you and your customers don’t need to worry about it as long as soluble salts are not excessive.
So, we’ve established that potassium sulfate, calcium sulfate, magnesium sulfate and K – mag are not acidic at all. Zero. Ammonium sulfate is acidic, but not because of the sulfate, because of the ammonia and the hydrogen ions it generates in soil. Interestingly, urea is 76% as acidic as ammonium sulfate. I once shocked a sales rep from a major ammonium sulfate producer when I pointed that out. So, if a customer comes to you concerned about elevated sulfate levels in their soil, I would remind them that the sulfate ion is not acidic and that sulfate nutrient levels are often more available to plants than their counterparts.