Alkaline soil refers to any soil with a pH level above 7. Making your soil more alkaline means raising your soil pH.
The pH level indicates the soil’s hydrogen ion concentration and affects how well plants can or cannot absorb necessary nutrients. The pH scale runs from 0 to 14, with seven being neutral. The lower the pH, the more acidic the soil; the higher the pH, the more alkaline the soil.
Rainfall leaches out alkaline. This means you typically find alkaline soil in dryer regions like the American West and Southwest. Conversely, areas with more significant rainfall have more acidic soil, such as the East Coast and Pacific Northwest. (The Midwest, meanwhile, tends to have neutral soil.)
Read on to learn why you might want to increase your soil’s pH and how to make soil more alkaline.
Benefits of Alkaline Soil
Alkaline soil is also known as “sweet soil” and contains higher sodium, calcium, and magnesium levels than neutral or acidic soils.
Although most plants prefer a neutral to slightly acidic pH level, some plants need alkaline soil to thrive:
- Vegetables: asparagus, okra, beets, cabbage, cucumber, celery, cauliflower
- Herbs: thyme, oregano, parsley, lavender
- Flowers: geranium, daffodils, hibiscus, Shasta daisies, echinacea, sweet pea, phlox
Here is a quick spotlight on how more alkaline soil can positively affect garden favorites lavender and hydrangeas.
Hydrangea blooms can range from pink to blue to white. The color of these generous blooms is affected by soil chemistry, particularly pH level.
In soil with a pH of 5.2-5.5, hydrangeas are likely to produce blue blossoms. If you prefer pink blossoms, you will need to raise your soil’s pH to 6.0-6.2.
Essentially, the relatively higher alkalinity blocks the plant’s ability to uptake aluminum which greatly influences bloom color. (The less aluminum present, the more likely your blooms will be pink.) You can further aid this pinking process by applying a high-phosphorus fertilizer to your soil.
Lavender is native to North Africa and southern Europe, where it grows in stony soil containing limestone. To reflect the conditions of lavender’s native growing soil, you will need to maintain a soil pH between 6.5 and 7.5 if you are planting lavender in your garden.
Lavender planted in more acidic soil probably won’t die immediately; however, it is unlikely to live long. Proper growing conditions (sunny in well-draining poor soil) will also increase the fragrance of your lavender plant.
If you’re interested in growing any of these plants but your soil is not naturally alkaline, you can follow a few simple steps to increase the soil’s pH.
4 Steps to Make Soil Alkaline
The four steps to making soil alkaline:
Step 1: Test Your Soil Quality
Before you adjust the composition of your soil, you need to determine its preexisting pH level and its texture: sandy, loamy, or clayey. (Poor sandy soil, peat, and muck soil are often acidic. Clay soil, meanwhile, will more often be alkaline.)
There are three methods for testing your soil’s pH: a lab test, an at-home test, or a DIY test. Which method you should use depends on your timeline, budget, and desired accuracy of results.
Remember that your backyard or garden may host several different soil types. Consider taking multiple soil tests if you want to plant in several locations.
Ordering a soil test from your local agricultural extension office will provide the most accurate evaluation of your soil.
In addition to analyzing the pH level of soil samples, these labs can offer recommendations on how to treat soil based on the desired pH level, soil composition, and buffering capacity (the soil’s level of resistance to changing pH). A soil test from a lab can also give you information on the soil’s Nitrogen, Phosphorous, and Potassium levels.
These tests typically cost $11 – $20, not including shipping, but as the gardening aphorism goes, “It’s better to plant a $2 tree in a $25 hole than a $25 tree in a $2 hole.”
The downside to getting lab-tested samples is that they take the most time to get results. However, this wait will yield more specific results than merely generalized information.
Using an at-home test such as a soil meter or pH test strip will give you results more quickly than sending your sample to a lab.
If you need specific information about your soil’s nutrient makeup, you will be better off using a lab test. Some soil meters claim to identify nutrients, but they are not reliable. However, an at-home soil test will do the job if you simply need a general read of your soil’s pH (i.e., acidic, neutral, or alkaline).
You can purchase a soil meter for about $13 from your local garden center. Note: test kits at this price will not analyze your soil texture.
Unless you are a pro-gardener, a DIY test will not give you a specific pH level but rather a general assessment of whether your soil is on the acidic or alkaline end of the pH scale.
Place a soil sample in a clean tub to determine if your soil is already alkaline. Then, pour vinegar on top of your sample. If your sample begins to fizz, the soil is alkaline. Conversely, check if your soil is acidic by placing the sample in baking soda. If the soil fizzes, it is acidic.
The more severe the fizz, the farther your soil is on either end of the pH scale.
If you’re plant-savvy, another method of determining your soil’s pH is to examine the weeds and plants already growing in the soil.
Are there weeds present that grow exclusively in alkaline soil? Then you can deduce that your soil is likely alkaline.
If you’re not sure if your soil is more sandy, loamy, or clayey, consider doing a percolation test (essentially measuring how quickly water is absorbed).
Step 2: Choose Your Additive
There are several ways to make soil alkaline naturally and organically. Lime is the go-to additive for most farmers and gardeners. However, if you want to make your soil more alkaline without using lime, there are other options.
In addition to applying an alkaline additive, it’s always a good idea to add compost. Compost has a neutral pH and acts as a buffer, allowing you to alter change-resistant soil more easily.
Garden lime (also known as agricultural lime) is the most common additive to raise soil pH (PDF). Its basic chemical makeup is calcium carbonate.
You can buy this lime in various textures: pellets, powdered, or hydrated.
The finer the lime, the more quickly it works. Powdered lime will work faster than pelletized lime, but some people prefer working with pellets as they’re less messy and easier to spread.
Hydrated lime works the quickest of the lime options but should be used with caution. It can burn the roots of plants and is dangerous to breathe. Wear a mask and gloves if handling it.
Dolomite lime is a naturally occurring mix of calcium carbonate + magnesium carbonate. If you don’t have dolomite lime to hand, you can use ordinary lime mixed with Epsom salt to increase your soil’s magnesium levels. Be careful not to use dolomite lime too frequently, as doing so can cause a nutrient imbalance.
Regardless of which type you use, lime can take 2-3 months to achieve full effect. If you’re able to prepare your garden ahead of time, try mixing lime into your soil during the autumn of the previous growing season.
Wood ash contains calcium, potassium, magnesium, and phosphorus and can increase pH up to 59% when applied to the soil. Using wood ash to raise your soil’s pH is a low-cost option that recycles a material that would otherwise go to waste.
Wood ash raises soil pH quickly; however, it doesn’t have long-lasting effects, so you will likely need to reapply ash throughout the growing season.
For more lasting results, spread a ¼ inch layer of ash on the soil surface during the winter before your growing season. Then, incorporate the ash into the soil using a till or rake at the beginning of spring.
A few things to remember when using wood ash:
Make sure wood ash is dry before applying it to the soil.
Don’t use ash from chemically treated wood, or these chemicals will seep into your soil.
Coal ash is not an acceptable substitute for wood ash. Applying coal ash could acidify your soil.
Moderation is vital when using wood ash. Applying wood ash in excess can raise your soil’s pH to a very high level. It can also damage germinating seeds or plant roots when it comes into contact, so spread carefully.
Ground Shells & Bone Meal
Eggshells, clamshells, oyster shells, and bone meal can make the soil more alkaline while supplying phosphorus and calcium. However, if these materials are not finely ground, they will likely be too coarse to be effective.
Also known as sodium bicarbonate, baking soda has a pH of 8.2 and can produce results in a few days when applied to the soil.
The effects of baking soda will not last as long as those of lime, and you will need to reapply the baking soda every few months.
Baking soda is a cost-effective option when bought in bulk.
Plus, it’s gentle on soil, plants, and gardeners. However, applying too much baking soda could lead to the build-up of bicarbonate in your soil, which will slow plant growth. The sodium in baking soda can also burn plant roots and leaves if overapplied.
Step 3: Measure & Mix
The amount of additive you’ll need to apply depends on a few factors: the type of additive you’re using, the soil’s current pH, the desired pH, and the texture of the soil (sandy, loamy, clayey).
Follow your additive’s packaging guidance to determine how much you’ll need to apply.
Use a rake, shovel, or till to mix the additive through the top 6 inches of the soil. You can top-dress additives such as lime on the soil surface.
However, this low-effort method will only increase pH levels an inch or two from the top of the soil surface.
Water well, making sure soil is saturated to at least 6 inches deep. After the initial application of your alkaline additives, do not overwater as this will contribute to soil leaching.
Keep in mind that sandy soil drains quickly and requires more frequent re-application.
Again, beginning this process in late autumn of the previous growing season will produce better results.
Step 4: Monitor & Maintain
Soil acidification occurs naturally over time, accelerated by crop production practices and rainfall.
Retest the soil every few months or after any significant changes (abnormally heavy rainfall, fertilizer application).
If you’re using a fast-acting but not long-lasting additive, you’ll need to test and reapply more frequently.
What Happens If Soil Is Too Alkaline
The majority of common garden plants prefer slightly acidic soil (pH 5.5-7), and some plants (blueberries and azaleas, to name a few!) prefer acidic soil.
Plus, alkaline soil is less soluble than acidic or neutral soil. This means that available nutrients are limited in comparison and can exacerbate zinc and iron deficiencies in the soil.
If alkaline contributors such as limestone are naturally present beneath the soil, it will be challenging to decrease soil alkalinity.
Concrete foundations or walkways will similarly leach alkaline into the adjacent ground. Runoff from farms and mountains will also contribute to higher alkaline levels.
Making soil less alkaline is more challenging than making the soil less acidic, but it can be accomplished.
How to Make Soil Less Alkaline
Elemental sulfur (typically listed as soil sulfur at garden centers) naturally lowers soil’s alkalinity. Like using lime to increase soil’s alkalinity, adding sulfur will not immediately increase the soil’s acidity. Soil bacteria will take several months before successfully converting the sulfur to sulfuric acid.
For gardeners growing on a smaller scale, you can use vinegar to make the soil more acidic (use the guiding ratio of 1 gallon of water to 1 cup of vinegar, adjusting depending on how alkaline your soil is). This method will require much more frequent reapplication.
Other methods of lowering soil pH include mulching your soil using composted wood chips, sawdust, pine needles, or fresh (unused) coffee grounds. Keep in mind that these methods might provide a short-term fix but won’t change the soil’s composition in the long term.
Some gardeners recommend applying peat to lower the soil’s pH. However, using peat is environmentally destructive. (It is impossible to extract peat from peat bogs, which are vital carbon sinks, without damaging them.)
Also, consider how the water you’re using to irrigate your plants might be contributing to alkalinity. Hard water is more alkaline. As such, sprinkling hard water on acid-loving soil plants can injure them.
Changing the pH level of your soil takes time and maintenance. If you’re not interested in maintaining alkaline levels for an entire growing season, consider selecting alkaline plants for your garden based on your soil’s naturally occurring pH levels. Or, consider planting in containers and raised beds, as doing so will allow you greater control over soil composition.
Frequently Asked Questions
Do eggshells make soil acidic or alkaline?
As stated above, eggshells if finely ground will make the soil more alkaline.
What plants grow in alkaline soil?
Now that you’ve followed our guide on how to make garden soil more alkaline, check out the article on alkaline plants.
Changing pH in soil: vric.ucdavis.edu/pdf/soil/ChangingpHinSoil.pdf
How To Change Your Soil’s pH – Iowa State (PDF): hortnews.extension.iastate.edu/1994/4-6-1994/ph.html
Davin is a jack-of-all-trades but has professional training and experience in various home and garden subjects. He leans on other experts when needed and edits and fact-checks all articles.