When you use acidic fertilizers your garden, you want to make sure that the soil pH stays in the proper range. Otherwise, some of your plants may suffer nutrient deficiencies.
So, which fertilizers are acidic? Ammonium-based fertilizers (such as ammonium nitrate, ammonium sulfate, and urea) are acidic. Sulfur-based fertilizers (such as elemental sulfur, iron sulfate, and aluminum sulfate) are also acidic. Natural acidic fertilizers (such as peat moss, compost, and manure) acidify soil as they decompose.
Of course, the fertilizer you use depends on the nutrients you need to add to the soil and how much you need to change the pH.
In this article, we’ll talk about some acidic fertilizers and the effects they have on soil. We’ll also answer some common questions about which materials make soil acidic and which plants need acidic soil.
Let’s get started.
Which Fertilizers Are Acidic?
There are various acidic fertilizers, including:
- Ammonium-based fertilizers: these fertilizers are primarily used to deliver nitrogen to plants. They have a slight to strong acidifying effect on soil, depending on which one you use.
- Sulfur-based fertilizers: these fertilizers are primarily used to acidify soil (lower pH), often for acid-loving plants like blueberries. They have a moderate to high acidifying effect on soil.
- Natural acidic fertilizers: these fertilizers are primarily used to improve drainage and add organic material to soil. They have a slight acidifying effect on soil over time, due to decomposition by soil microbes.
The following table gives a summary of various acidic fertilizers, how strong they are, and how fast they work.
how strong they are, and how fast they work.
Ammonium-based fertilizers contain NH4+, which is a form of nitrogen that plants can use for growth.
- Urea is an ammonium-based fertilizer that is 46% nitrogen by weight. It is found naturally in animal urine, but it can be created from ammonium carbamate. It has a slight acidifying effect in soil.
- Ammonium nitrate is an ammonium-based fertilizer that is 34% nitrogen by weight. It is highly water-soluble, and it is used in fertilizer and industrial explosives. It has a slight acidifying effect in soil.
- Diammonium phosphate (DAP) is an ammonium-based fertilizer that is 18% nitrogen by weight. It is a water-soluble salt that is used as a fertilizer, fire retardant, or yeast nutrient in wine-making. It has a moderate acidifying effect on soil.
- Ammonium sulfate is an ammonium-based fertilizer that is 21% nitrogen by weight. It has a severely acidifying effect on soil.
- Monoammonium phosphate (MAP) is an ammonium-based fertilizer that is 11.3% nitrogen by weight. It is a water-soluble salt that is used as a fertilizer and also in fire extinguishers, optics, and electronics. It has a severely acidifying effect on soil.
Sulfur-based fertilizers contain sulfur, which helps to acidify soil.
- Elemental sulfur is “pure” sulfur that is not bonded to anything else. It is bright yellow, and it is an abundant element. Elemental sulfur is not water soluble, and plants cannot use it in its pure form. However, bacteria in the soil can convert sulfur to forms that plants can use. It is more acidifying than sulfates, but works more slowly. You can find out how much sulfur to use with this table from Clemson University.
- Aluminum sulfate is a water soluble salt used to acidify soil and also to purify drinking water, make paper, and in concrete and firefighting foam. It is not as acidifying as sulfur, but it works faster.
- Iron sulfate (Ferric sulfate) is used to acidify soil and also in dyeing. It is not as acidifying as sulfur, but it works faster.
Natural Acidic Fertilizers
Natural acidic fertilizers make the soil slightly acidic as they decompose, thanks to the action of soil microbes.
- Peat moss (sphagnum) can store water (16 to 26 times their dry weight), and it adds organic material to soil. It decays very slowly, forming organic acids as soil microbes work to decompose it. It has a slight acidifying effect on soil.
- Compost is made from grass clippings, leaves, plant matter, fruit or vegetable scraps, and other organic matter. It decays and forms organic acids slowly, depending on the activity of bacteria in the soil. It has a slight acidifying effect on soil.
- Manure is the waste of animals. It can burn plants if applied fresh, so it is aged before use in gardening or farming. It decays and forms organic acids slowly, depending on the activity of bacteria in the soil. It has a slight acidifying effect on soil.
What Fertilizer Is Most Acidic?
Sulfur and sulfates are the most acidic fertilizers. However, monoammonium phosphate also has a severely acidifying effect on soil.
This table from Purdue University shows how much of each fertilizer you would need to get the same acidifying effect as 1 pound of sulfur.
You can calculate the amount of sulfur to use in your soil and then convert to other fertilizers by using the table above.
To find out how much sulfur you would need, use this table from the Iowa State University Extension. Note that sandy soil needs less sulfur (1/3 less), and clay soil needs more sulfur (50% more).
Let’s look at some examples.
Example 1: Using Sulfur To Acidify Clay Soil
Let’s say that you get a soil test and your current soil pH is 6.5 (slightly acidic).
You want to change the soil pH to 5.5 (somewhat acidic) to grow potatoes.
The table suggests that we need 0.2 pounds of sulfur per 10 square feet.
If our garden is 25 feet by 40 feet, then the area is 25*40 = 1000 square feet.
Since 1000/10 = 100, then we need 100*0.2 = 20 pounds of sulfur.
If the soil is clay, then we need 50% more sulfur. Since 50% of 20 is 10, then we need 20 + 10 = 30 pounds of sulfur, spread evenly over the entire 1000 square foot garden.
Example 2: Using Sulfur To Acidify Sandy Soil
Let’s say that you get a soil test and your current soil pH is 7.0 (neutral).
You want to change the soil pH to 4.5 (strongly acidic) to grow blueberries.
The table suggests that we need 0.5 pounds of sulfur per 10 square feet.
If our garden is 20 feet by 30 feet, then the area is 20*30 = 600 square feet.
Since 600/10 = 60, then we need 60*0.5 = 30 pounds of sulfur.
If the soil is sandy, then we need 1/3 less sulfur. Since 1/3 of 30 is 10, then we need 30 – 10 = 20 pounds of sulfur, spread evenly over the entire 600 square foot garden.
What Is A Natural Acidic Fertilizer?
Peat moss (sphagnum), compost, and manure are natural acidic fertilizers. They do not acidify soil as fast as ammonium-based or sulfur-based fertilizers.
However, you can find peat moss at most garden centers, and you can find manure locally wherever people keep horses, cows, or chickens. You can also make your own compost from kitchen scraps and yard waste (you can learn more here).
Does Epsom Salt Make Soil Acidic?
Epsom salt will not do much to affect the pH of soil. Its pH is between 5.5 and 6.5, so you would need quite a bit to make neutral soil slightly more acidic.
Although Epsom salt (magnesium sulfate) contains sulfur (which acidifies soil), it also contains magnesium (which has the opposite effect).
You should not use Epsom salt in the garden unless you have a magnesium deficiency. Even then, you could use dolomite lime or another source of magnesium.
Do Coffee Grounds Make Soil Acidic?
Used coffee grounds do not make soil acidic. According to the Oregon State University Extension, rinsed coffee grounds have a pH close to neutral (6.5 to 6.8).
Of course, feel free to add coffee grounds to your compost pile. Eventually, they will help to fertilize and add organic material to your soil.
Do Pine Needles Make Soil Acidic?
Pine Needles do not make soil acidic, even though the needles themselves may have an acidic pH. Instead, pine trees tend to grow where the soil is already acidic.https://hort.extension.wisc.edu/articles/reducing-soil-ph/
What Plants Like Acidic Fertilizer?
There are many plants that like acidic fertilizer and prefer acidic soil, including azalea, blueberries, rhododendron, and white pine.
The following table gives some common plants that like acidic fertilizer, along with the ideal soil pH range.
and their ideal soil pH range.
Remember that it is best to lower your soil pH before planting. It is difficult to mix in soil additives around plants and their roots after they have already been planted.
Also, remember to delay planting by a month or so after acidifying soil. This will help to avoid burning the plant’s roots.
Finally, remember to get a soil test before adding anything to your soil. Don’t try to solve a problem that you don’t have!
Now you know which fertilizers are acidic and which ones have the strongest effect. You also know about some plants that prefer strongly acidic soil and how to provide it for them.
You can find some vegetables that like acidic soil in my article here.
You can learn about shrubs, ornamentals, and trees that like acidic soil here.
You can learn about trees that like acidic soil here.
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