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 elemental sulfur to your soil will lower pH, but this method works slowly and can take months. Ammonia based fertilizers, such as ammonium nitrate or ammonium sulfate, will also lower soil pH, but they work quickly, and so they can burn your plants. You can also mix sphagnum moss into your soil to lower pH.
The method you use will depend on your situation, including soil composition, current soil pH, desired soil pH, and how soon you need to lower the soil pH. The first thing you should do is to get a soil test.
Why and How to Test Soil pH
It is important to start by testing your soil to find your “baseline” or starting soil pH. The higher the soil pH, the more sulfur, fertilizer, or sphagnum moss you will need to mix into your soil.
There are two methods to test your soil pH. One option is to buy a soil test kit, either online or at a garden center.
The other option is to take a soil sample and send it away for testing at your local agricultural extension. There is normally a fee for this testing, but you will get more detail than you would from a store-bought soil test. This includes information about nutrient levels, in addition to soil pH.
Even better, the soil testing lab will send you detailed results, along with recommendations for treatments to improve your soil. Make sure to include information about what you are trying to grow, so that they can give you specific recommendations.
For more information, check out my article on soil testing.
How to Make Your Garden Soil More Acidic (Lower Soil pH)
There are a variety of methods you can use to make your soil more acidic. Let’s look at them more closely.
Elemental sulfur is recommended for lowering soil pH if you have clay soil (poorly draining, clumpy soil). Elemental sulfur has a yellowish-green color, and 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 around $1 per pound.
Remember that elemental sulfur works slowly. As a result, you will need to wait at least a few months for it to take full effect. 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 to make your soil more acidic before spring planting. Distribute it evenly over the soil you wish to acidify, and mix it into the soil.
It is hard to pin down exactly how much sulfur to add to your soil, but it will depend on the starting soil pH and the desired soil pH. For example, let’s say you have a soil pH of 7.0 (neutral), and you want a soil pH of 5.0 (somewhat acidic, perhaps for blueberries).
You would need to add 0.4 pounds (6.4 ounces) of elemental sulfur per 10 square feet (4 pounds per 100 square feet) of soil to lower the pH from 7.0 to 5.0. If your garden is 30 feet by 20 feet (20*30 = 600 square feet), then you would need 6*4 = 24 pounds of elemental sulfur for your entire garden.
For more information, check out this article from the Iowa State University Extension, regarding how to change your soil’s pH. (They even have a table that tells you how much sulfur to add, depending on your starting soil pH and desired soil pH!)
Ammonia Based Fertilizers
Another way to lower your soil pH is to use ammonia based fertilizers, such as ammonium nitrate or ammonium sulfate. Both of these fertilizers go for about $1 to $2 per pound, and have the appearance of white pellets. You can purchase these ammonia based fertilizers online or at garden centers.
There are a couple of cautions when using ammonia based fertilizers to lower your soil pH. First of all, adding too much too quickly 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. Nitrogen is necessary for plants to grow green and healthy, but too much nitrogen can actually prevent plants from flowering. No flowers means no fruit or vegetables!
For more information, 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 additives that raise your pH – what a waste!
Sphagnum moss is a more “natural” way of lowering your 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 $3 to $10 per cubic foot, 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.
Iron or Aluminum Sulfate
Iron sulfate has the appearance of light greenish-blue powder or crystals. It sells for $1 to $2 per pound. You can also use aluminum sulfate as an alternative. You work the powder or crystals into your soil, spreading evenly over the area you want to acidify.
Iron sulfate or aluminum sulfate both work much more quickly 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.
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!
However, coffee grounds can go into your compost pile, although 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 household cleaning, and 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, or 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: 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
We already mentioned that acid-loving plants prefer a lower soil pH, but there is another possible reason to make your soil more acidic.
If your soil pH is too high, it can disrupt a plant’s ability to absorb nutrients. This can lead to nutrient deficiencies in a plant – even when there is plenty of the nutrient in the soil! For more information, check out my article on identifying nutrient deficiencies in your plants.
Hopefully, this article gave you a good idea of the methods you can use to make your soil more acidic, along with some details on each one.
I hope this article was helpful. If you have any questions or advice of your own, please leave a comment below.