If your garden is too alkaline (high pH) or you are planting acid-loving crops (like blueberries), then you may want to make your soil more acidic (that is, lower your soil pH). Luckily, there are many ways to do this.
So, how do you make your garden soil more acidic? Adding sulfur, to your soil will lower pH, but this method works slowly. Iron sulfate, aluminum sulfate, and ammonia based fertilizers also lower soil pH, but they work fast, so they can burn plants. Compost also makes soil more acidic, since organic material lowers pH as it decomposes.
Of course, the amount of acidifying material to add will depend on the current soil pH, desired soil pH, and the type of soil you have (clay, sandy, or otherwise). However, before you add anything to your garden, you should get a soil test to determine the current pH of your soil.
In this article, we’ll talk about ways to make soil more acidic to lower pH and what to know about each method. We’ll also talk about how to do a soil test to make sure you really do need to amend your soil.
Let’s get started.
How To Make Garden Soil More Acidic (4 Ways To Lower Soil pH)
There are lots of ways to make garden soil more acidic, depending on what you have available and how much you can find. Here are some methods to lower soil pH:
- Elemental Sulfur
- Sulfates (Iron Sulfate or Aluminum Sulfate)
- Ammonia-Based Fertilizers (Ammonium Nitrate or Ammonium Sulfate)
However, before you put any of these additives in your soil, it is important to do a soil test. That way, you will know for sure whether you really need them (and also if there is anything else wrong with the soil in your garden!)
Why To Test Soil pH
There are some good reasons to test your soil pH before you put any supplements in your garden.
First, soil testing may show that the pH is already within an acceptable range. In that case, you don’t need to add anything to alter the acidity.
Even if the soil pH is not where you want it to be, you still need to know the current pH. That way, you can calculate how much acidifying material you need to get the desired soil pH.
Finally, a soil test reveals nutrient deficiencies in soil. A home test kit (which you can buy online or at a garden center) allows you to test the following:
- Soil pH
How To Test Soil pH
A home test kit is one option for testing your garden soil. As mentioned earlier, it tells you about nutrient and pH levels in the soil.
You also have the option to use a digital soil tester. The advantage is that you can use it repeatedly, without waiting a long time for results.
Yet another option is to send soil for lab testing at your local agricultural extension office. For a small fee, the lab analyzes the soil and sends back a report, including recommendations on how to treat your soil.
If you tell the lab what you are trying to grow, they will give you more specific feedback about how to improve your soil. For more information, check out my article about how to do a soil test.
How to Make Your Garden Soil More Acidic (Lower Soil pH)
If you did a soil test and found the soil pH to be too high (alkaline), it is time to make it more acidic. There are a few ways to do it, so let’s review them all now, starting with the most basic one: elemental sulfur.
Elemental sulfur is recommended for lowering soil pH if you have clay soil (that is, soil that sticks together and drains poorly). Elemental sulfur has a yellowish-green color, and it normally comes in pellet form for use in gardening.
You can buy a bag of elemental sulfur pellets online or at garden centers, such as Home Depot, Lowe’s, or even WalMart for less than $1 to $2 per pound if you buy in bulk. There are 50 pound bags available, which is useful if you have a large garden.
Remember that elemental sulfur works slowly to lower soil pH. As a result, you will need to wait at least a few months for it to take full effect.
- Bacteria (they convert the sulfur to sulfuric acid to lower soil pH)
- Size of Sulfur Pieces (fine powder, coarse powder, small pellets, or large pellets – smaller will work faster)
- Soil Moisture (this affects bacterial activity)
- Soil Temperature (this affects bacterial activity)
Powdered sulfur will work a little faster than pelleted sulfur to lower pH, but it will still be a slow process, taking months to complete. Resist the urge to apply more sulfur if you don’t see results after a few weeks!
It is best to apply elemental sulfur to your soil in the fall after the last season’s harvest is over. That will give the sulfur enough time (several months) to make your soil more acidic before spring planting.
Distribute the sulfur evenly over the surface of the soil you wish to acidify. Then, mix it into the soil (it is best to do this before planting, in the fall or spring, rather than during the growing season).
How Much Sulfur To Add To Soil?
So, how much sulfur should you add to your soil? This depends on three things:
- Current pH (determined from a soil test)
- Desired pH (the pH that your plants will grow best in – this varies by species)
- Type of soil (sandy, loamy, or clay)
Let’s go through an example to get a sense of how to determine the amount of sulfur you will need.
Example: Adding Sulfur To Make Soil More Acidic
Let’s say you do a soil test, and your current pH is 7.5 (slightly alkaline). This is a little too alkaline for some of your plants, so you decide to add some sulfur to lower the pH.
Let’s say that you decide to lower the soil pH to 6.5. Also, let’s say that the soil in your garden is loamy (you can find out what type of soil you have with this article from Gardener’s Supply Company).
We will use this sulfur calculation table from Oregon State University (on the 4th page of the PDF) to help with our calculations. The current soil pH is 7.5 (2nd to last row of the table) and the soil is clay (far right column of the table), and this cell has a value of 25.
This means we would need to add 25 pounds of elemental sulfur to 1000 square feet of soil (an approximately 32 foot by 32 foot square area) to decrease the pH to 6.5.
If your garden is 500 square feet, then you would need to add 0.5*25 = 12.5 pounds of sulfur, spread evenly over the entire area. (Note that you will want to work the sulfur into the top 6 inches of soil).
When I checked online, you can get a 50 pound bag of lime for $50 or less. So, you could treat your entire garden (12.5 pounds worth) for around $12.50 or less.
If you don’t feel comfortable doing the calculations or working with the table mentioned above, you can use this sulfur calculator at garden.org.
One final note on elemental sulfur: it can make soil pH quite low if you use too much. A low soil pH can prevent plants from absorbing nutrients from the soil.
Be careful that you don’t add too much sulfur all at once.
Also, be careful not to inhale sulfur dust, or any other soil amendments you may use in your garden. Wear gloves and a mask when you handle it and apply it.
Sulfates are another option if you want to make your soil more alkaline in a shorter time frame. According to the Iowa State University Extension, sulfates will lower soil pH faster than elemental sulfur.
The are two types of sulfates you can use to lower soil pH: iron sulfate and aluminum sulfate.
Iron sulfate has the appearance of light greenish-blue powder or crystals. It sells for $1 to $2 per pound.
Aluminum sulfate has the appearance of white crystals or powder. It sells for $1 to $3 per pound.
No matter which sulfate you choose, you start by working the powder or crystals into your soil, spreading evenly over the area you want to acidify.
Iron sulfate and aluminum sulfate both work much faster than elemental sulfur to lower soil pH. However, one drawback is that you will need a much greater amount to get the same change in pH.
Another drawback of using this method: you may end up with too much heavy metal (iron or aluminum) in your soil. This can lead to iron toxicity in your plants, especially when soil pH is low.
Another way to lower your soil pH is to use ammonia based fertilizers. Two options are ammonium nitrate and ammonium sulfate.
Both of these fertilizers go for about $1 to $2 per pound, and they have the appearance of white pellets. You can purchase these ammonia based fertilizers online or at garden centers.
There are a couple of cautions to keep in mind when using ammonia-based fertilizers to lower your soil pH.
First, adding too much ammonium too fast can burn your plants, due to a rapid change in pH. For more information, check out my article on over fertilizing your plants.
Also, excessive ammonia-based fertilizers can lead to excessive nitrogen in your soil. Some nitrogen is necessary for plants to grow green and healthy.
However, too much nitrogen can actually prevent plants from flowering. No flowers means no fruit or vegetables!
If you need to encourage flowering or fruiting without adding more nitrogen to your soil, check out my article on low-nitrogen fertilizers.
Add ammonia fertilizers gradually to avoid quick changes in soil pH, especially if you have already planted your crops. Buy a soil test kit or digital tester to track changes over time until you get to the desired pH.
Otherwise, you will make the soil too acidic. Then, you will have to add lime or other amendments that raise your pH – what a waste!
Compost is a “natural” way to make soil more acidic. If you add compost to your garden every year, the pH may slowly decrease over time.
This is due to the decomposition of organic material in the compost. As an added benefit, compost adds important nutrients to the soil.
The best part about compost is that you can make your own with yard waste and kitchen scraps. You can learn more about how to make your own compost in my article here.
One notable material that you can add to your garden to acidify soil and provide organic matter is sphagnum moss.
Sphagnum moss is another “natural” way to lower soil pH. It is recommended for loose, well-draining soil (as opposed to clay soils, mentioned earlier). It has a pH between 3.0 and 4.5, meaning it is fairly acidic.
Sphagnum moss is normally sold by volume (it is very light when dry), and it holds a lot of water. When I checked, it cost $15 or more per cubic pound, so check your prices carefully!
To apply sphagnum moss, spread 1 to 2 inches over the top of the soil you wish to acidify, and work it into the soil.
You can also use coconut coir (coconut fiber) as an alternative.
One caution: sphagnum moss holds lots of water, so if you apply too much to the top of your soil without blending it in, the roots of your plants might not get enough water.
Myths About Making Soil Acidic
There are a few myths surrounding ways that you can make your soil more acidic. Let’s take a brief look at a few of them.
Coffee grounds have a pH close to 7.0 (neutral), so they will not have much effect on soil pH. For example, if your soil pH is 6.5 and you want to lower it to 5.0, do not add coffee grounds! They will likely increase your pH slightly from this point, moving you in the wrong direction!
You can still add coffee grounds to your compost pile. However, you might not drink enough coffee to make much compost this way.
You can always go to a diner or coffee shop and ask them if you can take a few buckets of used coffee grounds for your garden. They might even pay you to haul it away in large amounts!
Vinegar mixed with water will certainly lower the pH of your soil. However, it will do so quite rapidly, potentially harming or killing any plants that are already in your garden.
Even worse, vinegar will kill off beneficial bacteria in the soil. Vinegar is a good antimicrobial agent, which is why it is often used for cleaning.
Leave the vinegar for cooking or cleaning. Try one of the soil additives mentioned earlier if you need to make your soil more acidic.
Like most myths, there is a grain of truth that started this pearl of a myth. Pine needles do have a low pH (3.2 to 3.8), but you would need a huge quantity to have any measurable effect on soil pH.
However, pine trees do prefer acidic soil (much like blueberries, azaleas, and other acid-loving plants). If you are trying to decide where to plant acid-loving plants, put them near pine trees!
Thus, the nature of the myth: correlation does not imply causation. Pine needles do not make the soil acidic; rather, pine trees tend to grow in soil that is already acidic to begin with.
If the soil is already acidic enough for a pine tree to grow and thrive, then it is probably acidic enough for your acid-loving plants. This also saves you the trouble of trying to acidify your soil.
For more information on this and other myths, check out this article from the Oregon State University Extension. The take home message is this: adding pine needles to your soil will not change the pH much, but it won’t really hurt anything either.
One Sign That You May Need to Lower Your Soil pH
If any of your plants are showing signs of nutrient deficiency, then it may be due to alkaline soil. Some of the most common signs of nutrient deficiencies in plants are poor growth along with leaves that are yellow or light green.
If you see these signs or suspect your pH is too high , get a soil test. If the pH is too high , then use elemental sulfur or one of the other methods mentioned earlier to lower the pH to an appropriate level.
What is an appropriate pH level for your garden soil? Most plants prefer a pH between 6.0 and 7.0 (slightly acidic to neutral). Exceptions include blueberries and rhododendrons (which prefer more acidic soil, with a pH of 4.5 to 5.5) and asparagus (which prefers more alkaline soil, with a pH above 7.0).
When soil pH is outside of the optimal range, there is a higher chance of nutrient deficiencies in your plants. This is true even if there is plenty of a given nutrient in your soil!
Now you know what you can do to make your soil more acidic and maintain your soil pH. Always remember to test your soil pH before adding anything to your soil!
I hope this article was helpful – if so, please share it with someone who can use the information.