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? One way to raise your soil pH is to mix limestone or dolomitic limestone into the soil. You can also add wood ashes, bone meal, clamshells, or eggshells to raise your soil pH.
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. However, before you do anything to your garden, you should get a soil test to determine the current pH of your soil.
Why and How 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 soil pH is already at an acceptable level, and you won’t need to do anything at all to change it.
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 to add to get to 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 pH, and also for levels of nitrogen, phosphorus, and potassium. You can also buy a digital soil tester to test your soil.
You can also send your soil to the lab at a local agricultural extension 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 soil testing.
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 – let’s review them now.
The most common way to make your garden soil more alkaline is to add lime and mix it in. There are a few different options for lime.
You can use “ordinary” limestone, or calcium carbonate, which supplements calcium in your soil. You can also use dolomitic limestone, which supplements both calcium and magnesium in your soil.
Dolomitic limestone is useful if your soil test indicates a magnesium deficiency in your soil, although you can also use Epsom salt to supplement magnesium in your soil.
You also have the choice of powdered or pelletized lime. Powdered lime is finer, and so it will work more quickly to raise the 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.
So, how much lime should you add to your soil? This depends on three things:
- Current pH (determined from a soil test)
- Desired pH (the pH that your plants will grow best in)
- Type of soil (sandy, loamy, or clay)
Let’s go through an example to get a sense of how to determine the amount of lime you will need. For instance let’s say you do a soil test, and your current pH is 5.5, which is a little too acidic for some plants.
Let’s also say that you want to raise the soil pH to 6.5. Finally, let’s say that the soil in your garden is loamy.
Based on this lime calculation table from Iowa State University, if our current pH is 5.5 (2nd row of table) and the soil is loamy (3rd column of table), then 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 about $5 online, so you could treat your entire garden (80 pounds worth) for $10 or so.
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 if 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.
Adding wood ash (remnants of wood after a fire) to your soil will also raise pH. According to Michigan State University, wood ash will contain, on average, 0% nitrogen, 1% phosphorus, and 3% potassium by weight.
Wood ash also has high calcium content. 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.
Eggshells, clam shells, and oyster shells all contain plenty of calcium. If you grind them up and add them to soil, they will raise the pH, and also supplement calcium.
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 a bit into a fine powder.
Bone meal is another high-calcium garden supplement which can raise the pH of soil. Bone meal also supplies phosphorus, which is a necessary nutrient for plants and promotes flowering in particular.
What to Avoid to Keep Your Garden Soil Alkaline
Like most gardeners, you probably to maintain the pH of your soil at a steady level without needing to use lime or other amendments constantly. 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.
As an alternative to fertilizers, you can add compost or soil to your garden to supplement nutrients.
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.
Compost and mulch also supplement nutrients, including nitrogen and other elements 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.
You can put grass clippings, leaves, and fruit or vegetable scraps in your compost pile. You can also add 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.
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 yellow or light green leaves and poor growth.
If you see these signs or suspect your pH is too low, get a soil test (details provided above). 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 (which like more acidic soil, with a pH of 5.0 to 6.0) and asparagus (which prefer more alkaline soil, with a pH above 7.0).
When soil pH is outside this 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.
Hopefully, this article gave you some idea of what you can do to raise and maintain your soil pH. Always remember to test your soil pH before adding anything to your soil!
I hope this article was helpful. If you have any questions or advice of your own, please leave a comment below.