You can have the best soil in the world and give your plants the right amount of sunlight and water, but if they don’t have the pH they need, they won’t be happy.
While most plants aren’t too picky about pH, certain plants need acidic soil to thrive. But what if you don’t have acidic soil or you only want to increase the acidity in certain parts of your garden?
There are several methods you can use that are simple and perfect for the home gardener. You can use them on targeted areas or amend your garden as a whole.
Here’s everything you need to know about how to make soil acidic using natural methods.
What Is Soil pH?
Before we get into how to acidify your soil, let’s take a look at what soil pH is.
pH is a measurement of how alkaline or how acidic your soil is. The scale ranges from 0-14 with 7 being neutral. The closer the number gets to 14, the more alkaline your soil is. The closer it gets to 0, the more acidic it is.
Most plants, including common garden vegetables, grow well in a pH range of 6-7.5, which would be slightly acidic to slightly alkaline. However, certain plants need a pH of 4.5-5.5 in order to be healthy.
Most ornamental and edible plants will happily grow in a pH close to neutral, but certain plants (like blueberry bushes) thrive on acidity.
If you have an alkaline soil of 7+, you’ll need to amend it to make it acidic. Even a slightly acidic soil in the range of 5.5-7 will need some adjusting to get it below 5.5. Keep in mind that the more alkaline you soil is, the more work it will be to amend.
Which Plants Need Acidic Soil?
If you’ve ever tried to grow certain plants only to see them turn yellow or fail to bloom with no apparent cause, it could be due to pH.
Here’s a list of common plants that love acidic soil:
- Azaleas and rhododendrons
- Hydrangeas (if you want them to be blue)
You may be surprised to see some common vegetables on that list. While they do prefer more acidic soil, most vegetables will be fairly happy with a pH of up to 7, so you don’t need to amend unless you notice a problem.
Azaleas and rhododendrons (two closely related plants) are another good example of acid-loving shrubs that won’t do well in alkaline soil.
The fruiting bushes and ornamental shrubs on this list absolutely do need acidic soil to thrive. Also, please note that hydrangeas only need acidic soil if you want the blooms to be blue. For pink blooms, your pH should be closer to neutral.
How to Test Your Soil
Before you can start adjusting the pH of your soil, you need to know what your starting point is. Here are a few ways to find out.
One of the easiest ways to test your pH is to contact your local County Extension Office. They can send you a testing kit and instructions for how to use it. You then collect your samples and mail them back to be tested in a lab.
This is the most accurate testing you can do, and you’ll also likely get an analysis of the fertility of your soil, which will tell you what kind of fertilizer you need (if any). The kits usually cost between $9-$20.
Home Testing Kit
If you aren’t as worried about precise measurements, you can order your own home testing kit and follow the instructions to test your soil yourself.
Most often, you’ll need to take a sample and mix it with water and a chemical to find out the pH. Many kits will also test the amounts of nitrogen, phosphorus, and potassium present in the sample.
These kits vary widely in price, and you’ll likely have to pay more to get one with good accuracy. Though less accurate than a lab test, you can typically use a good home testing kit 40 times or more.
For most soil tests, you’ll need to collect a sample and either send it off to a lab or test it yourself at home. Digital meters are easier to use because all you need to do is stick them 4-6” into the dirt where you want to measure pH.
A third solution is to buy a digital soil meter. These are by far the quickest testing method to use.
All you need to do is insert the probes of the meter 4-6” into the soil you want to test. The pH reading will show up automatically. This can be a good method for testing the soil in container gardens and established plantings where you don’t want to dig up any dirt for samples.
Like other home testing kits, the accuracy and price of digital meters varies widely. Unlike home testing kits that use samples, meters can typically only test for pH, moisture, and light- not soil fertility.
There are other ways to discover if you have acidic or alkaline soil, like using pH test strips or a baking soda/vinegar test, but none of these will give you an exact measurement and aren’t very helpful for telling you how much amending your soil needs.
How to Make Soil Acidic
Once you’ve completed your pH test, it’s time to choose your method for making your soil more acidic.
The best method for your garden depends on several factors: how large of an area you want to amend, your soil type, how quickly you want results, and the pH you’re starting with.
Heavy clay soils and ones with a starting pH of over 7.5 are the most difficult to acidify. It can still be done, but you should plan to make consistent amendments over a long period of time to get results.
The method that will work best for you depends on several factors like the area size you’re working with and your starting pH. You may need to make several amendments to get your soil as acidic as you want it to be.
#1- Sphagnum Peat Moss
Sphagnum peat moss is one of the most popular choices for home gardeners who want more acidic soil.
The pH of peat moss is usually from 3 to 4.5, which makes it very effective at lowering pH. It also adds organic matter to your garden and works fairly quickly.
There are two ways to amend your soil with peat moss. You can add it in before planting acid-loving plants like blueberry bushes, or you can add it around existing plants that need a lower pH.
To add it before planting, spread out a 2-3” deep layer of peat moss over your planting area. Then, work it into the soil to a depth of about 8-12”. You want to make sure the peat moss will be as deep as the roots of the plants you’ll be planting.
This method is the most effective because it almost instantly acidifies your soil and establishes a low pH around the roots of your plants, which is exactly where it needs to be.
To add peat moss to existing beds, mulch around plants with a 1-2” layer of peat moss. Work the peat moss into the top few inches of dirt if you can do so without disturbing the roots of your plants.
This method is also effective, but it will take longer for the acidity from the peat moss to work its way down to the roots of your plants.
If you can do so without disturbing any roots, use a rake or a small hand trowel to work peat moss into the top 1-2” of soil surrounding existing plants.
Whichever method you use, be sure to water the peat moss thoroughly before and after adding it to your soil. Dry peat moss can soak up a lot of water, taking it away from your plants.
Advantages of peat moss: Sphagnum peat moss is widely available for sale at nurseries and garden centers. It will only take you an afternoon to add it to a normal garden area, and it can acidify your soil almost instantly.
Peat moss can work well for all different types of soil, although it will be more difficult to work it into heavy clay. It’s a long-term solution that will keep soil acidic for years.
Disadvantages of peat moss: Peat moss works well for raised beds and small to medium garden areas but is not very cost effective for large garden areas. It can also be labor intensive depending on the size of the area you’re working with.
There’s also concern about the sustainability of peat moss. It grows very slowly in peat bogs, and overharvesting has become a problem. The best solution is to use peat moss for small areas that need acidified and use an alternative like coconut coir for other uses.
#2- Vinegar Solution
Vinegar is a very acidic natural substance that can be used for a quick pH adjustment.
That being said, never put undiluted vinegar on or around your plants. It needs to be mixed with water first so that it won’t harm either your plant roots or beneficial soil bacteria.
To use this method, add two tablespoons of vinegar per gallon of water. Water existing acid-loving plants with this mixture a few times until the pH is where you want it.
Using a diluted vinegar solution can be a very effective method for quickly lowering pH in containers. It won’t be effective for large areas of your garden and is a short-term fix.
Vinegar solutions work best for container gardens where other methods are more difficult to use. The vinegar will quickly make the soil more acidic, but it won’t last long-term. You’ll likely need to keep an eye on the pH and use the vinegar solution again later in the season.
Advantages of a vinegar solution: The biggest advantage of this method is that it works quickly and uses ingredients you likely already have on hand. It can almost instantly adjust pH in containers, and all you have to do is water your plants.
Disadvantages of a vinegar solution: While this can be a good solution for plants in pots, you probably don’t want to use this method out in your garden. It’s a short-term solution that will disappear when all the vinegar is used up and is difficult to use on a large garden area.
Lowering the pH too quickly can harm both your plants and good bacteria that live in the dirt, so be sure you properly dilute the vinegar with water before using.
#3- Elemental Sulfur
Sulfur is another natural substance that can very effectively acidify your soil.
Elemental sulfur is safe and easy to apply to a large garden area. In fact, it’s the favored method of large scale growers, commercial gardens, and farmers for making soil more acidic, but it can be used by home gardeners as well.
Using sulfur is a long-term solution that will keep the pH down for years. It’s easy to apply around existing plants but does take a longer time to work, usually several months.
If you can, apply elemental sulfur in late fall so that it can have a long period of time to start working before the next growing season. Cold weather will pause the sulfur reaction, but it will start back up as the temperature heats up.
How much sulfur you need to use will depend on the size of the area you’re amending, your starting pH, and whether you have sandy or clay soil.
For this method, you do need to have fairly accurate pH measurements, so make sure you’ve done a soil test first. Then, read the instructions carefully to determine how much sulfur to use. Once you apply it, all you have to do is let it work.
Advantages of elemental sulfur: Sulfur is a completely natural substance that will increase acidity long-term. It’s one of the best options for larger garden spaces and for using around established plants.
You can use sulfur on any soil type, and its slow-acting nature means that plants will have time to gradually adjust to a lower pH. Sulfur is also very cost effective.
Disadvantages of elemental sulfur: The main downside to sulfur is that it takes several months for it to make soil more acidic. You also need to do a little more work at calculating the pH of your soil and then determining how much sulfur to add to get to the desired pH.
#4- Acidifying Fertilizers
Acidifying fertilizers are another way to acidify your soil quickly and are easy to use.
Most acidifying fertilizers are ammonia based and many also contain some form of sulfur. The most common contain ammonium sulfate, ammonium nitrate, or sulfur-coated urea.
Acidifying fertilizers can be effective for both areas in your garden and containers. Be sure to read the instructions carefully so that you don’t apply too much fertilizer and burn plant roots.
These fertilizers can be very effective and also provide your plants with some extra fertility. They can be used on large garden areas, raised beds, around trees and shrubs, and around other existing plants.
When choosing a fertilizer, make sure that it says ‘acidifying fertilizer’ or something similar, since some ammonia-based fertilizers will actually have an alkalizing effect.
Also, be sure to read and follow the instructions carefully. Overusing ammonia fertilizers (especially ammonium sulfate) can burn plant roots, harming your plants.
Advantages of acidifying fertilizers: Acidifying fertilizers are effective, easy to use, and work quickly to lower pH. They can be used directly around the roots of plants, making them a good choice if you have acid-loving plants mixed in with other plants.
Disadvantages of acidifying fertilizers: One downside is that fertilizer becomes less cost effective if you need to use it on a large garden area. Certain acidifying fertilizers can be very strong, so you need to use them carefully.
Fertilizers are also not a long-term solution. Once the fertilizer is used up, the pH will start to creep back up, so you’ll have to keep applying it to keep your soil acidic.
#5- Iron Sulfate
Iron sulfate is an intermediate solution when it comes to how long it takes to work. It’s applied in a similar way as elemental sulfur but works more quickly: 3-4 weeks.
Iron sulfate causes a chemical reaction in the soil that makes it more acidic. It’s also an effective method for heavy clay soils that are difficult to work with.
Iron sulfate can be put down around acid-loving plants (like magnolias) or spread out over large garden areas. It typically works in 3-4 weeks.
Like elemental sulfur and fertilizers, you can apply iron sulfate around existing plants or over larger garden areas. However, you do need to apply with caution and infrequently, since the iron can cause harmful heavy metals to build up in your soil.
Advantages of iron sulfate: The main advantage of iron sulfate is that it works much more quickly than elemental sulfur while still being effective. You can use it around specific plants, and it’s a good option for heavy clay soils.
Disadvantages of iron sulfate: The biggest drawback is that iron sulfate can be harmful to plants if you use too much of it. Overuse causes heavy metals to accumulate, which can injure your plants.
Maintaining Soil Acidity
Once you’ve done the hard work of making your soil acidic, a little maintenance can keep it at the right pH level. Even with long-term solutions like elemental sulfur and peat moss, pH can naturally creep back up with time.
All you need to do is test the pH of your garden or containers about once a year to check on the pH level. If it starts going up, just add amendments as needed to bring it back down.