Ammonium sulfate is a good source of iron for your plants, even though it doesn’t contain any. How can that be? Read on!
Ammonium sulfate (AS) is the oldest form of manufactured nitrogen fertilizer. Chemically, ammonium sulfate [(NH4)2SO4] is a salt that contains 21% nitrogen and 24% sulfur.
What’s in the fertilizer bag?
Most people know that plants use nitrogen to grow. If you buy a 10-pound bag of 5-5-5 fertilizer, that means you are getting 5% of each of the primary nutrients - nitrogen (N), phosphorus (P), and potassium (K). This works out to 1/2 pound of each nutrient and 8-1/2 pounds of filler. If you buy a 10-pound bag of ammonium sulfate, you get 2.10 pounds of nitrogen, 2.40 pounds of sulfur, and 5.5 pounds of filler. Now, don't think that those fillers simply take up space, though sometimes that’s exactly what they do. Mostly, these fillers are sand or granulated limestone. Whether or not those are good for your soil depends on your unique situation. Personally, I prefer less filler and more substance. Also, here in the Bay Area, we generally don’t need anything besides nitrogen for plant growth. My soil had nearly 10 times the optimal amount of phosphorus, twice as much potassium and calcium, and 8 times more magnesium than my plants need, the last time I had it tested. Adding more would be a complete waste of money. My problem was iron. I had less than one-third of the optimal amount. And my plants couldn’t even get to what little there was because of sol pH.
Ammonium sulfate and soil pH
In areas with alkaline soil, sulfur acts as an extremely mild acidifier. If you want to grow acid-loving plants, such as blueberries, artichokes, or potatoes, lowering the soil pH can seriously improve your harvest and the overall health of your plants. Ammonium sulfate has a pH value of 5.5 and the sulfur it contains will provide a tiny bit of help. The real pH reduction occurs when soil microbes convert the ammonium into nitrate, in a process called nitrification. If your soil has a pH of less than 6.0, you should not use ammonium sulfate. Most Bay Area soils are closer to 7.5 (untreated, mine was 7.7). Soil that is too alkaline or too acidic make it difficult for plants to absorb nutrients and thrive.
Ammonium sulfate and food safety
Unlike many fertilizers, which can be dangerous, ammonium sulfate is a food additive. The U.S. Food and Drug Administration lists ammonium sulfate as “generally recognized as safe”. [I still wouldn't eat it out of the fertilizer bag!] It is commonly added to flours and breads to regulate acidity. It is also added to many vaccines, to improve their effectiveness. Ammonium sulfate is hygroscopic, which means it absorbs moisture from the air, so be sure to keep the bag tightly closed.
Applying ammonium sulfate
As with any soil treatment, read the label and follow the directions. Seriously. When applying ammonium sulfate to your lawn or garden, be sure to water or work it into the soil right away. If it sits on top of everything, much of the ammonia (nitrogen) will be lost to the atmosphere.
Ammonium sulfate and iron
So, how can ammonium sulfate provide your plants with iron if it doesn’t contain any? Here’s the rub: soil pH dictates the absorbability of many nutrients. Slightly acidic soil makes it easier for plants to absorb the available iron. Of course, if your soil is low on iron, all the ammonium sulfate in the world won’t help. Get your soil tested so that you KNOW what you are working with. And if you live in an area with alkaline soil, ammonium sulfate can be an excellent way to add nitrogen and reduce soil pH for healthier plants.
You can grow a surprising amount of food in your own yard. Ask me how!