How To Make Garden Soil More Alkaline (6 Ways To Raise pH)


If your garden is too acidic (low pH) or you are planting alkaline-loving crops (like asparagus), then you may want to make your soil more alkaline (that is, raise your soil pH).  Fortunately, there are a few ways to do just that.

So, how do you make your garden soil more alkaline?  To make garden soil more alkaline, add lime or dolomite lime to raise pH.  Wood ash, bone meal, and ground eggshells or clamshells also work, since they contain calcium carbonate to make soil more alkaline by raising pH. Hydrated lime is another option that works fast, but it can burn plant roots.

Of course, the amount of limestone or other materials 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 alkaline to raise 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 Alkaline (6 Ways To Raise Soil pH)

There are lots of ways to make garden soil more alkaline, depending on what you have available and how much you can find. Here are some methods to raise soil pH:

  • Lime (calcium carbonate)
  • Dolomite Lime (calcium carbonate + magnesium carbonate)
  • Wood Ash
  • Bone Meal
  • Crushed Shells
  • Hydrated Lime (calcium hydroxide)
calcium carbonate
Lime (calcium carbonate) is one of the most common ways to make garden soil more alkaline.

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 a few good reasons to test your soil pH before you add anything to your garden.

First of all, a soil test may reveal that the pH is already at an acceptable level. In that case, you won’t need to do anything at all to change it.

soil test kit
A soil test kit will tell you if the pH in your garden is off.

Also, even if the soil pH is not at an acceptable level, you still need to know the current pH in order to calculate how much lime you must add to arrive at your desired soil pH.

Finally, a soil test will reveal nutrient deficiencies in your soil.  A home test kit (which you can buy online or at a garden center) allows you to test the following:

  • Soil pH
  • Nitrogen
  • Phosphorus
  • Potassium

How To Test Soil pH

A home test kit is the first option for testing the soil in your garden. As mentioned above, it can tell you all about nutrient and pH levels in the soil.

You can also buy a digital soil tester, which you can use repeatedly without waiting a long time for results.

soil
Testing your soil helps you to figure out what to add (if anything) for optimal results.

Another option is to send your soil to the lab at a local agricultural extension office for testing.  For a small fee, the lab will analyze your soil and send back results, along with recommendations about how to treat your soil.

If you tell the lab what you are trying to grow, they will give you specific feedback about what steps to take to improve your soil.  For more information, check out my article about how to do a soil test.

How To Make Garden Soil More Alkaline

If you did a soil test and found the soil pH to be too low (acidic), it is time to make it more alkaline.  There are a few ways to do it, so let’s review them all now, starting with the most common one: lime.

Lime (Calcium Carbonate) Or Dolomite Lime (Calcium Carbonate + Magnesium Carbonate)

The most common way to make your garden soil more alkaline is to add powdered lime and mix it in.  There are a few different options for lime.

limestone
A limestone pillar in England. Crushed limestone can raise soil pH and supplement calcium when added to your garden.

You can use “ordinary” limestone (calcium carbonate), which supplements calcium in your soil.  You can also use dolomitic limestone (calcium carbonate + magnesium carbonate), which supplements both calcium and magnesium in your soil.

Dolomitic (dolomite) limestone is useful if your soil test indicates a magnesium deficiency in your soil. You can learn more about dolomite lime in my article here.

dolomitic lime
Dolomite lime contains both calcium carbonate and magnesium carbonate.

However, you can also just use ordinary lime along with Epsom salt to supplement both calcium and magnesium in your soil while raising pH.

You also have the choice of using either powdered or pelletized lime.  Powdered lime is finer, and so it will work more quickly to raise soil pH.

However, keep in mind that whether you use powder or pellets, lime can take 2 to 3 months to have full effect.  Thus, it is a good idea to mix lime into your soil in the fall of the previous growing season.

This will give the lime enough time to work before you do your planting in the spring.  It also gives you time to add more lime if the pH is not quite high enough.

How Much Lime To Add To Soil?

So, how much lime should you add to your soil?  This depends on three things:

clay soil
Clay soil holds more water and requires more lime than sandy or loamy soil to raise the pH.

Let’s go through an example to get a sense of how to determine the amount of lime you will need.

Example: Adding Lime To Make Soil More Alkaline

Let’s say you do a soil test, and your current pH is 5.5. This is a little too acidic for some of your plants, so you decide to add some lime to raise the pH.

Let’s say that you decide to raise 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 lime calculation table from Iowa State University to help with our calculations. The current soil pH is 5.5 (2nd row of table close to bottom of page) and the soil is loamy (3rd column of table), and this cell has a value of 8.

This means we would need to add 8 pounds of limestone to 100 square feet of soil (a 10 foot by 10 foot square area) to increase the pH to 6.5.

If your garden is 500 square feet, then you would need to add 5*8 = 40 pounds of limestone, spread evenly over the entire area.  (Note that you will want to work the limestone into the top 6 inches of soil).

When I checked online, you can get a 40 pound bag of lime for under $10. So, you could treat your entire garden (40 pounds worth) for $10 or less.

If you don’t feel comfortable doing the calculations or working with the table mentioned above, you can use this lime calculator at garden.org.

One final note on lime: it adds lots of calcium to your garden, which is a necessary nutrient for plant growth.  However, too much calcium can prevent a plant from absorbing magnesium (even when there is plenty of magnesium in the soil!)

Be careful that you don’t add too much lime. If you are worried about this, use dolomitic limestone to add magnesium along with calcium.

Also, be careful not to inhale lime dust, or any other soil amendments you may use in your garden. Lime can cause cracked skin as well, so wear gloves and a mask when you handle it and apply it.

Wood Ash

Adding wood ash (remnants of wood after a fire) to your soil will also raise pH. The reason is that wood ash contains a large amount of calcium carbonate, which is the same thing as lime.

According to the Purdue University Extension, wood ash contains about 25% calcium carbonate by weight. Wood ash will also contain, on average, 0% nitrogen, 1% phosphorus, and 3% potassium by weight.

wood ash
Wood ash can provide nutrients to soil, and can be used to raise pH.

For more information, check out this article about wood ash from Michigan State University.

It is difficult to pin down how much wood ash to use to increase your soil pH by a certain amount.  For one thing, different trees have different chemical compositions, and so the resulting wood ash will have different contents.

Wood ash is somewhere between 25 and 59 percent as effective as lime when you use it to raise the pH of soil.  This suggests that you might need 1.7 to 4 times as much wood ash as you would need lime to raise soil pH by the same amount.

For more information, check out this article on wood ash and soil pH from the University of Maine Extension.

You can also check out my article on putting wood ash in your garden.

Bone Meal

Bone meal is another high-calcium garden supplement containing calcium carbonate, which can raise the pH of soil.  Bone meal also supplies phosphorus, which is a necessary nutrient for plants and promotes flowering in particular.

bone meal
Bone meal is a good source of both calcium and phosphorus for your garden.
Image from: https://commons.wikimedia.org/wiki/File:%D0%93%D0%BE%D1%80%D1%81%D1%82%D1%8C_%D0%BA%D0%BE%D1%81%D1%82%D0%BD%D0%BE%D0%B9_%D0%BC%D1%83%D0%BA%D0%B8_2014-05-07_13-57.jpg

You can learn more about high-phosphorus fertilizers in my article here.

Shells

Eggshells, clam shells, and oyster shells all contain plenty of calcium carbonate.  If you grind them up and add them to soil, they will raise the pH and also supplement calcium.

clam shell
A giant clam shell. Break clam shells into pieces or grind them into powder to make them act faster to raise soil pH.

If you are worried about too much calcium in your soil blocking uptake of magnesium, be sure to add some Epsom salt to counteract the effects of shells. If you want them to take effect faster, then grind them up into a fine powder (the finer the powder, the faster the effect).

Hydrated Lime

Hydrated lime (calcium hydroxide) is another option for making garden soil more alkaline. However, it has its downsides as well.

Hydrated lime works faster than ordinary lime to raise soil pH. However, it is dangerous to breathe or handle, so you should use a mask and gloves when working with it.

Hydrated lime can also burn the roots of plants, so be careful when using it right before planting or during the growing season.

What To Avoid To Keep Your Garden Soil Alkaline

Like most gardeners, you probably want to maintain the pH of your soil at a steady level without needing to use lime or other amendments too often.  The best way to do this is to avoid adding high-acid compounds to your soil.

In particular, avoid adding ammonia based fertilizers to your garden if you want to keep the pH up.  Ammonium nitrate and ammonium sulfate are both commonly used fertilizers that will lower the pH of your soil.

For more information, check out my article on low-nitrogen fertilizers.

ammonium nitrate
Ammonium nitrate will fertilize your garden, but it will also lower the pH, making your soil acidic.

As an alternative to fertilizers, you can add compost or soil to your garden to supplement nutrients. These amendments will also act as a buffer to maintain soil pH.

What To Do To Maintain Soil pH

There are several benefits to adding compost and mulch to your garden.  First of all, they help to maintain the soil pH around neutral (7.0).

Compost and mulch also supplement nutrients, including nitrogen, phosphorus, potassium, and other elements that are necessary for plant growth.  Finally, they add organic material to soil, helping the soil to drain in wet weather and to retain moisture in dry weather.

Compost
Compost is a great way to supplement nutrients and organic material in your garden, without causing huge changes in soil pH.

You can put grass clippings, leaves, and fruit or vegetable scraps in your compost pile.  You can also add aged manure from cows, pigs, chickens, or other livestock.

Just make sure to let everything break down before you add the mixture to your garden. For more information, check out my article on how to make your own compost and my article on where to find manure.

One Sign That You May Need To Raise Your Soil pH

If any of your plants are showing signs of nutrient deficiency, then it may be due to acidic soil.  Some of the most common signs of nutrient deficiencies in plants are poor growth along with leaves that are yellow or light green.

chlorosis
Plants with yellow leaves (chlorosis) could be suffering from a nutrient deficiency caused by acidic soil (low pH).

If you see these signs or suspect your pH is too low, get a soil test.  If the pH is too low, then use lime or one of the other methods mentioned earlier to raise 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!

For more information, check out this article from Research Gate showing the relationship between soil pH and nutrient availability.

Conclusion

Now you know what you can do to make your soil more alkaline and maintain your soil pH.  Always remember to test your soil pH before adding anything to your soil!

I hope that you found the article helpful – if so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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