As a gardener, there are a number of factors you have to keep in mind to grow healthy plants, such as if your plants are getting enough water if you’re adding enough or too much fertilizer and how many hours of sunlight your garden is receiving.
Another equally important factor that is often overlooked, however, is the pH of your garden soil.
If you’ve ever tried growing acid-loving plants such as azaleas or blueberry bushes only to watch them wither away, then you’ve probably been ignoring the pH requirements of your plants too.
If you need to lower the pH of your soil in order to grow acid-loving plants, you have quite a few options available to do so.
You’ll have to weigh the needs of your plants and your garden against the benefits and detriments of each one in order to choose the best additive for your garden and your family.
What Is Soil pH And Why Is It Important?
Garden soil’s measure of acidity or alkalinity is known as pH and is measured on a scale from 0 to 14. A pH of 7.0 is considered neutral, and like water is neither acidic nor alkaline.
A measure below 7.0 is acidic while anything above 7.0 is alkaline.
Most garden plants do well in soils between 6.0 (slightly acidic) and 7.5 (slightly alkaline), but there are a handful of plants that need more specific pH ranges in order to grow well.
You may even find that your garden soil is too alkaline to tolerate plants that just need a neutral soil to grow.
Soils that are formed from limestone are often alkaline and may have a pH as high as 9.5.
If the topsoil in your area was removed and replaced with the more alkaline subsoil during construction, or if alkaline building materials such as limestone, gravel or concrete were used in your area you may find high pH levels as well.
Before you begin trying to lower your pH, however, it’s important to conduct a soil test to find out what your current pH is.
You can easily purchase an inexpensive soil pH testing kit that will give you a pretty accurate reading.
If you’re interested in getting a more exact measurement, you can contact your local county Extension office to find out how to get a professional soil test done, often for free.
Once you know where your soil stands, you can begin to make the changes necessary to lower your pH, which will increase your acidity.
What Can You Add To Soil To Make It More Acidic?
There are more than a handful of substances that will make your soil more acidic.
Some are more natural, more fast-acting or more cost-effective than others, so you’ll have to decide which one is best for the size of your garden or how quickly you want your soil acidified.
Here are some pH-lowering additives you can use in your garden:
- Sphagnum peat
- Aluminum sulfate / Ferric sulfate
How To Acidify Soil
How to go about acidifying your soil will depend on the additive you choose to use. Each one has its own benefits and drawbacks, so make sure to consider all the needs of your garden before settling on one or another.
Keep in mind that soil with a pH higher than 7.5 may contain very high amounts of free calcium carbonate, a compound that is very resistant to changes in pH.
Attempting to change the pH in this kind of soil may be very difficult, expensive, or even impossible.
If your soil tests high for calcium carbonate, you may be better off working with your soil instead of against it and growing alkaline tolerant plants.
Here’s how you can acidify your soil:
1. Compost and Other Organic Matter
Adding organic matter, such as compost, composted manure, and certain mulches, can lower the pH over time, but it’s not a quick fix.
As organic matter further breaks down in your soil, the decomposing bacteria will produce acidic by-products, which will lower the pH of your soil over time.
While organic matter will add other benefits to your soil right away, its pH-lowering effect will take a few seasons to a few years to become apparent.
2. Sphagnum Peat
If you’re only working with a small area of your garden, adding sphagnum peat is a great all-natural way to lower its pH.
Not only is it a source of organic matter, sphagnum peat actually has a low pH itself, from 3.0 to 4.5, and will lower the pH of the surrounding soil within a season. Its effects can even last for a couple of years.
To use sphagnum peat, add 1 to 2 cubic feet per plant for each full point of pH reduction you want to achieve (to go down from 7.0 to 6.0, for example). The sphagnum peat should be worked at least 6 inches into the soil for it to be effective.
Because of the price of sphagnum peat, it isn’t cost-effective for larger areas and is often used only for small garden beds or on a per plant basis instead.
Sublimed sulfur is probably one of the least expensive as well as the safest ways to reduce the pH of your soil. It is relatively slow-acting, though, as the sulfur has to be metabolized into sulfuric acid by soil bacteria before it can begin to lower the pH.
Sulfur can take up to a few months to bring significantly acidify your soil, but you’ll only need a very small amount of it overall. Generally, 0.2 pounds of sulfur is enough to bring down the pH of a 10 square foot area by one full pH point.
For the safety of your plants, it’s recommended to not go over 2 pounds of sulfur per 100 square feet.
4. Aluminum Sulfate & Ferric Sulfate
If you’re looking to quickly reduce the pH of your soil, aluminum or iron sulfate can get the job done.
They will turn your soil acidic as soon as they dissolve, but unlike pure sulfur, you’ll need a whole lot more for it too work which may be detrimental to plants.
Be sure to not apply more than 5 pounds of either sulfate per 100 square feet at one time.
Here’s a reference chart to help you decide how much aluminum sulfate you’ll need:
|Present pH||Desired pH|
Some fertilizers which contain additives such as diammonium phosphate, ammonium nitrate, urea and others will acidify your soil and are considered safe to use.
Read the fertilizer label to find out if it’s acidifying and for application instructions.
Frequently Asked Questions
Do Coffee Grounds Acidify The Soil?
It’s a common myth that coffee grinds can be used to acidify garden soil.
The truth is that while fresh (unused) coffee grounds are acidic and will have a temporary acidifying effect, used coffee grinds are practically neutral (6.5) and won’t significantly change your soil’s acidity.
Can You Use Vinegar To Acidify Soil?
Vinegar is a natural acid with a pH of around 2.4 and can be used to naturally reduce the pH of your soil as well. To do so, combine a cup of vinegar with a gallon of water and poured over the soil.
Keep in mind that the effect won’t be permanent, and as the vinegar solution is washed out of the soil, the pH will bounce back.
When using this method, recheck your soil every week or so and add more vinegar as needed.
Does Epsom Salt Make Soil More Acidic?
Epsom salts (magnesium sulfate) generally have a neutral pH and won’t affect the acidity of your soil. However, they are rich in magnesium and can be beneficial to your plants if your soil has a magnesium deficiency.