How to Lower Soil pH

How to Lower Soil pH - article summary

Biology and gardening go hand-in-hand, but everybody forgets about chemistry, the group’s awkward third wheel. Not this time, though, because we’re talking about pH, acidity, buffering capacity, and all that other chemistry gibberish that you definitely paid attention to in class (or not).

To lower soil pH, first test its pH. Next, decide what type of substance (organic matter, chemical compounds, etc.) to use, based on how much and how quickly the pH needs to be lowered. Finally, mix/add in the appropriate amount of acidifying soil additive for the size of the garden being modified.

If you want your plants to be happy and healthy, the soil has to be at the right pH level. So, put your lab goggles on, and make sure you know where the fire extinguisher is, because we’re about to do some chemistry.

How to Lower Soil pH - article summary

What is soil pH?

Right, so maybe you nodded off for a minute at this part of the lecture. The term itself stands for “potential hydrogen,” and the scale, ranging from 0 to 14, measures how acidic or basic a substance is.

To get an idea of the scale, 7 is completely neutral, like water.

Substances with a pH higher than 7 are called basic, and not because they wear Ugg boots and like pumpkin spice lattes. Bleach, for example, has a pH of around 12.5.

Acidic substances have a pH lower than 7, like stomach acid coming around with a pH as low as 1.5. To summarize: don’t drink anything on either end of the spectrum.

As the name implies, the measured number comes from the presence of the hydrogen ion, H+. Those are tiny, even by small standards, so instead of having to say that something has a concentration of 10e-7 H+, which requires its own explanation, we just say it has a pH of 7.

That means, though, that the pH scale is logarithmic. A one-number change is actually multiplied by a factor of 10. Meaning, a pH of 5 is 10 times more basic than 4, and 100 times more basic than 3. Yeah, and you thought we were just going to dive into chemistry--not math too.

So, the difference between having soil with a pH level of 7 vs. 6.5 isn’t as minuscule as you might initially think. This is why it’s important to check and regulate your soil’s pH.

There are some plant types that prefer to live outside of the norm, but most plants thrive in a pH between about 6.0 and 7.2; in other words: slightly acidic conditions

When the pH rises above (or falls below) the ideal range, it affects the growth of the plants. The good, nitrogen-producing bacteria have a harder time, the texture and structure of the soil might change, and the nutrient availability goes down.

Not good, but preventable.

Causes of high pH

How soil type affect pH

There are three basic soil types:

  • sand,
  • clay, and
  • silt.

Each has a different structure, texture, and composition, which affects its ability to cope with changes in pH.

The particles in clay soil are very small but closely packed together, making it extremely dense.

This density and slow-draining quality helps clay soils to be able to resist pH changes in either direction. That’s a good thing if it’s already at the pH you want it to be, and not so good if you want to shift it up or down.

On the other hand, loose, dry, sandy soils have larger particles, and are known for being more susceptible to pH changes.

Weathering & Leaching

Through weathering and erosion of rocks and minerals, soil begins to form.

From the parent materials (the rocks), the soil inherits qualities like texture, fertility, and pH, and the way that the rocks are broken down affects the soil too.

The deterioration of parent material like limestone bedrock or caliche--which have a high concentration of calcium carbonate--will create alkaline soil. That’s because calcium carbonate is a salt that comes from calcium hydroxide, one of the common strong bases.

This is also why you might end up with high-pH soil from over-liming.

Leaching happens when water gets caught in the space between soil, and that water starts to sink downward because of gravity. It takes away soluble salts with it--specifically, it can take nitrates.

We all know and love nitrogen. It’s what fills most of the air around us, it’s in most fertilizers in the form of ammonium nitrate, and it keeps gardens going strong. When leaching occurs, those lovely nitrates are displaced, and it makes the pH go up.

Climate & Location

Organic matter is one of the ways to give soil a greater buffering capacity. It will make the pH of really acidic soils rise and make the pH of basic soils fall.

In the western United Sates, where the environment is arid and dry, the average soil is around 8.0.  This is partially because of the parent material, lots of calcium carbonate, and partially because there’s not much organic matter around.

You know, because it’s a desert. All you get is a cactus and maybe a tumbleweed.

On the other hand, in places where the rainfall is heavy, making a warm and humid environment, the pH of the soil decreases, acidifying it

The climate and location are essentially the most significant factors in what makes the soil have certain qualities, because it determines what it will be made from and the big forces that are going to change it.

Salt Accumulation

Many of the salts that end up in soil can be taken up by the plants and are used as nutrients. Still, too much of a good thing can make it not so good anymore.

By the way, in chemistry, a salt is any compound formed from an ionic bond between a metal and a nonmetal. Got your nerd hat on yet?

Sodium (a metal) and chlorine (a non-metal) make up sodium chloride--common table salt--and it makes all your food yummy too.

Back to soil: a build-up of salt can come in a few different ways. We’ve already mentioned one: little rainfall. It can also happen from the water that irrigates the area, which may itself be high in salt concentration.

Strangely enough, this problem is unique to the arid and dry regions, as well as coastal areas, where the salt from the nearby water lingers in the soil

Try to limit your garden’s salt intake like you would your own--too much salt isn't healthy!

bacteria and microbes in soil

Presence/absence of bacteria & microbes

Just as you need the beneficial bugs in your garden, you need the beneficial micro-bugs too. We’ll call all those micro-critters microbes for simplicity sake--even though there's all kinds of microbial life down there: bacteria, protozoa, protists, fungi, viruses.

Microbes uses organic matter, or even the nitrogen straight from the atmosphere and produces nitrogen that the plants can consume.

Microbes & soil bacteria function best within a pH range of 5.5 to 7.0. That’s just slightly under the range that’s best for the plants, with a lot of overlap between the two.

Having a sufficient amount of bacteria in your garden is more of an after-effect of making it look homey to those microbes than it is something that you can control. You could always try playing some nice music, though. And put out some organic matter.

Testing soil pH

Before making any changes to your garden, you should run a pH test. There are a few ways you can do that.

One option is to buy a test kit online or at a local store--big-box hardware stores typically have them for about $14. With that, you’ll probably get either a probe, pH strips, or maybe even a liquid solution that will change color after you mix it in with your soil in a test tube.

In any case, you don’t want to take only a single section of soil out for the test. It might give you results that aren’t really indicative of the whole garden.

This is starting to sound like social science, let’s get back to chemistry.

Taking a soil sample

Dig a few small holes all around the area, making sure to dig at least half a foot for the sample. Mix the samples together to get one all-inclusive pot of dirt.

Here’s where the other options come in. So, whether you have a test kit or not, you can use a pH probe to get an immediate measurement.

Add about the same amount of water as there is soil and mix it together to make a fun mush. It’s probably best to way 5-10 minutes after that, then insert the probe and wait for it to display the pH.

IMPORTANT: Use distilled water. Bottled water and tap water have extra minerals in them, making them slightly alkaline. Get distilled/deionized water, or else it will interfere with the results.

Using pH test strips or a probe

Another method is to use pH strips. These aren’t expensive, and they can be useful pretty often, so don’t be afraid to grab ‘em next time you’re at the store.

Litmus paper, or just pH strips, will change color when it comes in contact with a solution.

Sometimes they’ll change one color to indicate a basic solution and another color to show that something is acidic. Other times, they have a spectrum of colors to reveal a specific pH range that the solution falls into.

To know what colors means what, the package will likely include a chart that the color can be compared to.

The method here is pretty much the same as with a probe. Add in water – DISTILLED water – let it sit for a little while, then put the litmus paper into the mushy mud. Compare it to the chart to see where the pH resides.

The problem with this is that it won’t give you an exact result, just a range. If the pH is drastically far from where it should be, then it will be helpful. But remember, the pH scale is logarithmic. Even a half of a unit off can be quite significant.

Either get pH strips that have a lot of colors to show the ranges, or go with a pH probe.

Measuring soil pH by using vinegar & baking soda

A final option is to use vinegar and baking soda. This will only tell you if the soil is basic (pH above 7) or acidic (pH lower than 7). It’s even less definitive than the litmus paper.

But, since you probably already have both of these at home, it can be a quick way to test if your soil is really alkaline, and if it is, you can get a probe or the litmus paper to figure out where exactly it’s at and what to do from there.

Combine the sample to make your mishmash sample, then scoop out a little bit into two separate containers.

This is where it gets fun. Add ½ a cup of vinegar to one, and ½ a cup of baking soda to the other. Whichever one reacts like a 1st-grader’s papier-mâché volcano will tell you if it’s basic or acidic.

That’s because baking soda is a base and vinegar is an acid… kind of. Baking soda is a weak base, and a main part of vinegar is acetic acid, which is also weak. When they combine, they react violently and release carbon dioxide bubbles, making it all fizzy.

So, if the vinegar container starts to get fizzy, that means it reacted with a base – your soil is alkaline. If the baking soda container is the one to go off, the soil is acidic.

In the really boring case, the soil is at, or very close to, neutral. That’s not such a bad thing, but it means you won’t get a reaction, which is super lame.

Whatever you decide to do, you should now have a good idea of what level the pH of your soil is at. If it does need to modified, you’ve got a lot of options to choose from.

Options to lower pH

Choosing what substance to add in order to lower the pH of your soil should be based on three things:

  • We’ll call the first one “the power.” The intensity of the pH change varies from one substance to another. If you only want to lower your garden’s pH slightly, you don’t want to use something that’s really strong.
  • The second aspect to consider is time. Are you checking the pH months ahead of the growing season, or just a short while before? If you’ve got time, you also have the luxury of using more moderate, slow-acting techniques.
  • The last area is kind of a jumble of things. Check how much of the substance you’ll have to add based on how much the pH needs to be lowered, how big your garden is and what type of soil you have, and your budget. This is also when you should look at any possible side effects that it might cause.

Method to lower soil ph

Pros

Cons

Elemental sulfur

  • Inexpensive
  • Strong-acting
  • Strong-acting
  • Takes 1-3 months

Aluminum Sulfate

Immediate effect

  • Medium strength
  • Potential for aluminum toxicity

Ferric sulfate, phosphates

Strong-acting

Potentially dangerous to handle

Ammonium fertilizers

Inexpensive

  • Weak effect on pH
  • Potential for ammonium toxicity


Organic matter

  • Inexpensive
  • Increases soil tillage
  • Increases soil water retention
  • Longest-lasting effect on pH

Slow-acting

Lowering the pH with sulfur

The first option we’ll discuss is sulfur. When you add elemental sulfur to a garden, the bacteria can convert it to sulfuric acid, which makes the whole thing a little more acidic and lowers the pH.

On the power scale, sulfur is up pretty high. You don’t need to add very much of it, and it’s relatively inexpensive compared to some other methods.

To get specific, you’d need 0.3 pounds of sulfur to take a 10-square foot garden with a pH of 8.0 all the way down to 6.5. That’s some serious stuff.

Despite its strength, this one does require some time to see the full effects. The bacteria is doing the work here, so it’s not like the quick vinegar and baking soda volcano from earlier.

It can take a few months before the pH lowers all the way to the desired level. Get started with this one earlier rather than later.

There aren’t really side effects that come with adding sulfur--if you add the right amount.

Introducing too much will make your soil too acidic, and that will cause its own problems. The plants will have a hard time getting nutrients, the bacteria won’t do as well, and you’ll be stuck having to look up the article “How to Raise Soil pH.”

Because the sulfur is strong, don’t get impatient with this method by adding more before it’s fully done its job. If you add the sulfur ahead of time, as you should, it would be a good idea to regularly test the pH every now and then to make sure that it’s going down, slowly but surely.

Lowering soil pH with aluminum sulfate

Let’s say that you’ve been living in blissful ignorance of soil pH before seeing this article headline. Or, at least, you’ve got other things on your mind.

You dutifully check your soil pH only to find that it’s way. too. high. But you were going to start planting so soon! What do you do?

That’s right, you panic.

Wait, no. You just go to the store.

And you buy a bag of aluminum sulfate.

Aluminum sulfate is highly soluble, it will just soak right into the soil. And even though you won’t see all the fizz and explosions or pomp and circumstance characteristic of chemical reactions, it basically works immediately.

It isn’t quite as powerful as sulfur – meaning, you’ll have to add a little bit more – and it’s just slightly more expensive too.

But aluminum sulfate will still do the trick without needing pounds and pounds even though it’s not quite as intense as pure, elemental sulfur.

For a 10-square foot garden, add 1.8 pounds of aluminum sulfate to take the pH from 8.0 to 6.5. Not bad.

Now, this is where the side effects come in. Adding too much aluminum sulfate will give you aluminum toxicity, which is just as bad as it sounds.

Aluminum isn’t one of the elements that's needed for plant growth, and having a lot of it present will only bring bad news for your plants.

Excessive aluminum concentration in the soil will make it harder for plants to absorb nutrients as well as water, and the roots of the plants plagued by aluminum toxicity tend to shrivel to about half of what they should be.

Yikes!

That’s the drawback that comes with having such a fast-acting, pH-lowering substance. If it were all rainbows and sunshine, it’d be more widely used and you probably wouldn’t need to look for anything else to drop the pH.

Anyway, just be mindful when you’re adding it. Double or triple check your math.

lowering soil pH and other chemical compounds

Lowering soil pH with other chemical compounds

Elemental sulfur and aluminum sulfate are commonly used to lower pH in soil, but they definitely aren’t the only options.

A sulfate is simply a chemical compound that includes the anion SO42- in it. Sulfuric acid, which is what the bacteria make out of elemental sulfur, is H2SO4.

You could speed up this process and save time by adding the sulfate part yourself, right from the beginning – no more waiting on the bacteria. This is also why aluminum sulfate works so fast.

Quick note: though you could potentially add sulfuric acid itself to your garden or to the water you use for it, it’s not a great option. Sulfuric acid is one of the seven strong acids. In chemistry, it’s a wear-gloves-when-you’re-pouring-it kind of liquid. And not just dishwashing gloves--you'll need the serious nerd chemist kind that poison won't eat through gloves.

Even in a diluted form, it can still be dangerous, and there’s really no need for it when you have a bunch of other choices.

So, using sulfates will work. Besides aluminum sulfate, ferric sulfate is not uncommon to use.

And we don’t have to limit ourselves to sulfur and sulfate. Phosphates have a similar effect. A normal form that they can be found in is diammonium phosphate. That way, you also get the nitrogen.

Phosphorous, which makes up the anion that phosphates are based around (PO43-), is necessary for healthy plant growth, and is often found in fertilizers.

Lowering soil pH with ammonium fertilizers

That brings us to our last miscellaneous option: ammonium fertilizers. One kind is ammonium sulfate – which sounds like a double win, because you get both nitrogen and the sulfate.

However, there are other ammonium fertilizers, and any kind that you decide to go with should work just fine. Because all of these other chemical compounds aren’t used quite as frequently as the first two mentioned, you’ll need to take extra care in finding how much to use and how long it will take to see the effects.

Lowering the pH with organic matter

The final choice that we’ll talk about here is the most basic one: add organic matter.

This is going to be, by far, the slowest technique of all the ones mentioned, but it’s also a very natural and long-term solution, making it fully worth including.

Like the elemental sulfur, the change that comes from organic matter is dependent on the bacteria. The pH level in the soil falls because the bacteria create acidic byproducts as they break it down.

Organic matter is typically added to gardens at least once annually anyway. Keeping up this practice is a good way to avoid having to deal with all the other compounds in an effort to lower the pH in the first place.

As for deciding what exactly to add, you can use composted manure, peat moss, acidic mulches--a whole lot of things.

If you’re patient enough for it, you could take a long-term approach to this already long-term technique: you can grow alkaline-loving plants instead. They’ll grow, and eventually lower the pH level by becoming the same organic matter for the bacteria to consume.

Even if you don’t take that approach, it’ll take months and months for the pH to drop. The plus side is that, afterwards, it will have a steady pH range because organic matter offers a strong buffering capacity. The soil won’t be as susceptible to changes.

And, when it comes to the possible side effects, you’ll find that there’s only benefits to be seen. Adding organic matter to your soil improves its structure, increases nutrient supply to the plants, and holds onto water for the plants to later soak up. Sounds like a win all around.

This method is definitely more of a “slow and steady wins the race” kind of take on the issue, but that’s okay!

Not everything has to be the baking soda volcano.

Ah, but if only it all could be.

Related Questions/Searches

Can you use vinegar to lower pH in soil?

Yes, vinegar can be used to lower soil pH, but like other substances that quickly lower soil pH, you need to be cautious about how much to apply. Do NOT pour vinegar straight onto plants or into the soil--you'll damage or burn the plants. Instead, dilute 1/8 - 1/4 cup vinegar per gallon of water, and pour over 10 - 20 square feet of your garden. In a week, retest the soil pH, and if the pH is still alkaline, repeat with another gallon of solution.

How can I make my soil more acidic naturally?

The most reliable--but time-consuming--way to acidify soil naturally is to add organic matter. You can do this any time of year, and use things like compost, coffee grounds, leaves, straw, etc. By itself, organic matter isn't very acidic, but the microbes in the soil will gradually acidify the soil by breaking down the organic matter. Also, as the organic matter holds more water in the soil, this will also acidify the soil.

Does gypsum lower soil pH?

Gypsum does NOT lower soil pH; gypsum actually has no effect on pH. Instead, gypsum can be used as a source of sulfur and calcium. Gypsum can also improve water retention in soil, as well as tillage.

How to lower pH of soil with home remedies

Adding anything acidic to your garden will work to lower the pH. When it comes to using what you likely already have in your house, vinegar is a good option. With a main constituent of vinegar being diluted acetic acid, it can bring the pH down.

How to lower soil pH after planting

The problem with waiting until after planting to make amendments to the soil is that, firstly, it might take too long. Your plants might suffer during the wait for the change. Secondly, adding something really harsh and strong could hurt the plants, too. It’s an awkward tightrope.

The best approach would be to add a generous amount of organic matter, like peat moss, to the garden, and continue to do so regularly.

How to raise soil pH

When it comes to raising the pH of soil, lime and wood ashes will do the job. Add them to your garden just as you would any of the other amendments listed here. Make sure to check the amount of the substance needed beforehand to make sure that you don’t include too much.

About the author

Greg Volente

Greg Volente holds a Naturalist Certificate from the Morton Arboretum, and worked for The Nature Conservancy leading environmental education programs and doing natural areas restoration. Besides gardening, he's an avid wildflower enthusiast, and loves botanizing, hiking, and backpacking.