- Planting Tomatoes Sideways: A Guide to Trench Planting - April 8, 2022
- How to Tell if Potting Soil is Bad - January 22, 2022
- Herbs That Don’t Grow Well Together - October 16, 2021
Alkaline soils are difficult. They have a chalky texture, which makes them hard to work, and creates an unhealthy soil structure. They make up the majority of the soils in U.S. deserts, which shows how frustrating they can be to manage.
If you have alkaline soil on your property, don’t despair. It can be managed but, where do you start?
How to correct alkaline soil? Alkaline soil can be managed with consistent applications of compost, which helps plants adapt to higher pH levels. DIY pH tests can help identify alkaline soils, and local nurseries can help you find plants that tolerate high pH levels in your climate zone.
We’ve put together an in-depth review of the science behind pH values to help you understand why high pH causes so many problems.
Don’t care about the science? That’s fine. Jump to the section you want to learn about:
- How Does pH Affect Soil?
- What Makes A Soil Alkaline?
- How to test your soil’s pH
- Solutions for alkaline soils
- Plants that tolerate high pH
What Is pH?
In short, pH is a scale that represents the acidity or basicity (alkalinity) of a substance.
The acronym pH stands for power of hydrogen, or, according to some sources, percentage of hydrogen. This percentage is represented on a scale of 0-14, with 7 as neutral.
Water is composed of hydrogen and oxygen, but certain substances can break or change the bonds between the H and O atoms, and cause hydrogen ions to form.
When a hydrogen atom breaks off and joins a water molecule, two new molecules are formed. The water molecule that lost a hydrogen atom becomes OH– (hydroxide), and the water molecule that added a hydrogen atom becomes H3O+ (hydronium). (source)
When there is an even number of hydroxide and hydronium, the power of hydrogen (pH) is neutral. When hydroxide ions (OH–) outnumber hydronium ions (H3O+), the pH is acidic. When hydronium ions (H3O+) outnumber hydroxide ions (OH–), the pH is basic, or alkaline.
Pure, distilled water has a pH of 7, which makes it perfectly neutral. One of the most extreme acids, battery acid, has a pH of 0, while an equally-destructive chemical, drain cleaner, has a pH of 14.
Acidity and alkalinity determine whether or not certain enzymes can function. Within a person’s digestive system, different enzymes digest different types of foods. Stomach acid allows certain enzymes to work, whereas the large intestine has a different pH, which allows different enzymes to work. Each enzyme has an ideal pH range to operate within, and each enzyme will denature, or fall apart, outside of that pH range.
Enzymes are responsible for performing certain chemical processes within living organisms. Therefore, the pH of what goes into an organism, and the pH of the surrounding substances, will affect the chemical processes that support that organism.
How Does pH Affect Soil?
Soil is an ecosystem. Within that ecosystem, micro and macro organisms work to break down organic material into forms that plants can absorb and use for growth.
Nitrogen is the most important nutrient for plant growth, but plants don’t actually absorb nitrogen.
Enzymes within the soil latch on to nitrogen (N2) and transform it into ammonia (NH3) through a process known as nitrogen-fixing. (source) This process changes an inert form of nitrogen into a form that plants can absorb and use for growth.
No matter what form of fertilizer you apply to your lawn or garden, you are never actually feeding plants. You are adding nitrogen to the soil, and then relying on the enzymes within the soil to transform it into nutrients that your plants will use.
There are hundreds of enzymes involved in transforming inert vitamins and minerals into forms that your plants can absorb and use. Each enzyme has an ideal pH range to perform this process, and each enzyme will denature outside of this range.
For plants, the ideal pH range is 6.0-6.5, or slightly acidic.
Too acidic, and the enzymes that transform many key nutrients begin to decrease, causing deficiencies. At the same time, enzymes that transform trace minerals are more active, causing mineral toxicities.
Too alkaline, and the enzymes that transform minerals are less active, causing iron and mineral deficiencies. Extremely alkaline soils (8.5 pH) denature the enzymes that transform key minerals, causing deficiencies in nitrogen, phosphorus, calcium, and magnesium. (source)
Soil structure is also affected by pH values.
The enzymes that break down organic material operate best within a pH range of 5-8, but severely acidic or alkaline soils break down these enzymes, which brings decomposition to a halt.
The decomposition process is what turns organic material into organic matter; a huge influencer on soil structure. When organic material is unable to decompose, the soil structure is compromised and can become compacted and form thick, impenetrable layers.
Soil aggregates, or clods, are held together by the cation-exchange capacity (CEC) of elements within the soil. Certain elements are attracted to each other, which helps soil hold its shape.
Clay soils generally have a higher CEC, which causes it to form strong aggregates. This strong bond is aided by a higher pH value, which means generally, clay soils have a higher pH, and are slightly alkaline.
But this can vary depending on region. A simple and inexpensive soil test (link to Amazon) will provide definitive answers specific to your area.
Sandy soils tend to have a lower CEC, which causes it to be grainy and prone to erosion. This lack of a bond is partly due to a low pH value, which means generally, sandy soils have a low pH, and are slightly acidic.
There are many different factors that affect pH, and only a few are within our control. Some soils would require so much intervention that it’s nearly impossible to alter the pH values. For these soils, it is more practical to find plants that tolerate pH extremes and work on slowly improving the soil quality.
What Makes A Soil Alkaline?
Soil may be alkaline for a few different reasons.
Pure water has a pH of 7, but most water sources are a product of the surrounding environment. Therefore, the pH of water will vary by region.
When water picks up minerals, it becomes more alkaline. Common sources of alkaline water are natural springs, mountain runoff, or water that passes through limestone or over and through other rock formations.
Using alkaline water to irrigate your lawn or garden can influence the pH of the soil, but not drastically.
Water is generally alkaline because the surrounding topography is alkaline. So, alkaline water is the result of alkaline soil. Chances are, it’s your soil that’s causing a rise in the pH of the water, and not the other way around.
This is the exception to water’s influence on pH values. If you live in a rural area near farms, the water may be alkaline due to modern farming practices.
In natural settings, plant material dies and is returned to the soil as organic matter, which helps maintain a slightly acidic pH value. In large-scale farming operations, crops take in the soil’s resources, but then the plant material is removed, which leaves the soil deficient in many nutrients and minerals. (source)
This causes a chain reaction that results in low pH, which farmers then try to remedy by adding lime.
Lime helps to raise pH, for a time, which allows farmers to grow another crop without the negative effects of acidic soil.
As farmers irrigate, this lime is leached down into the groundwater, which is then passed on to the water supply of rural areas. When you use this water on your lawn or veggie garden, your slightly-acidic soil gets a small dose of lime. Over time, lime buildup can cause alkaline soil, even though your soil is naturally acidic.
The foundation of any soil structure is the parent material. All soils have particles of clay, silt, and sand that were deposited by wind, water, or natural weathering of the surrounding environment.
If the parent material of a soil is calcic, the resulting soil will have large amounts of calcium carbonate. This is the main ingredient in chalk, and also the main component of lime.
Soils with a calcic parent material have chalky, clay textures, and can form thick layers of concrete within the soil. This type of soil is most common in dry, arid regions of the Western United States.
How Do I Test My Soil’s pH?
Homeowners have a variety of options for pH tests. Tests range from a ballpark guesstimate to an incredibly accurate analysis. The test you choose will be determined by what you are growing, and how much work you want to do to improve your soil.
There are few layman’s tests to determine soil pH, and these are best for the casual gardener or as a diagnostic tool for soil issues.
If you’ve noticed yellow leaves, curling leaves, purple stems, or other plant deformities, you may have a pH imbalance. This can be confirmed by a DIY soil test that can help you determine a plan of action.
Baking Soda & Vinegar
Baking soda is alkaline, and vinegar is acidic. Acids and bases react with solutions with an opposite pH value. This is why vinegar and baking soda foam when they are mixed together.
Take two small cups and a bottle of distilled (not tap) water out to the area of soil you want to test. Dig a hole about 4” deep, and remove organic debris and rocks. Fill the cup about halfway with soil, and add enough distilled water to make a muddy solution.
Next, add a few ounces of vinegar. If the soil foams or fizzes, your soil is alkaline.
If the mixture doesn’t fizz, repeat the process, but add baking soda instead of vinegar. If the mixture fizzes, your soil is acidic.
The more pronounced the reaction, the more extreme the pH value. Most plants want a slightly acidic soil, so a light amount of fizzing after the baking soda indicates your pH is right where it needs to be.
On the off-chance you have a red cabbage in your refrigerator, you can make an easy pH test with a few pots and some distilled water.
Bring distilled (not tap) water to a boil. Chop up the cabbage, and boil until the water turns a deep purple color. Discard the cabbage, and pour a small amount into two containers.
Add a splash of vinegar to one container, and a dash of baking soda to the other. One should turn pink, and the other should turn bluish-green.
If the two solutions change color, this indicates you have successfully created a pH indicator from a cabbage. You’re awesome.
Next, collect a sample of soil from the area you wish to test. Put a spoonful of soil in a clean container, and add the cabbage solution while stirring. Wait for the solution to change colors.
If the liquid turns pink, it means your soil is acidic. Bluish green liquid means your soil is alkaline. The more drastic the color change, the more extreme the pH value. A slightly pink liquid means your soil has a healthy pH range.
More Accurate pH Tests
Soil pH strips are simple to use, cheap, and deliver a much more useful result. While vinegar and baking soda can tell you whether soil is acidic or alkaline, pH strips can give a more accurate representation of the pH of your soil. This is helpful if you garden consistently, or have recurring problems with your lawn and landscape.
Tests are readily available in most hardware stores, local greenhouses, lawn & garden stores, and online.
While instructions will vary, most kits will have you mix enough water and soil together to make the texture of a milkshake. Then, dip a test strip into the mixture for about 30 seconds. Dip the strip in distilled water to clean off the mud, and look at the color to determine the pH value.
Pinks and yellows indicate acidic soils, while greenish browns indicate alkaline soils. The colors will depend on the test strips, but you should be able to determine a numerical value for the pH range.
Incredibly Accurate pH Tests
Meters are more common in commercial production when large amounts of tests are performed on a regular basis. However, if you have persistent pH issues, or you’re trying to improve a difficult soil, this may be worth the investment.
Meters range from $10-$1,000, but most homeowners only need a simple, inexpensive instrument.
Meters will need to be calibrated with a buffing solution before use, and instructions vary widely depending on which meter you choose. Results will be very accurate and can help you track progress if you are working on a long-term soil improvement plan.
Local Extension Office
Most local extension offices offer simple soil testing services. If you plan on growing plants for a hobby business, or if you are battling difficult soil conditions, ask your local agent how to get a soil analysis.
You will likely need to take your own soil samples from multiples locations on your property, and send them into a local university for testing. This analysis will give you an in-depth report on your soil, including pH, available nutrients, organic matter, and structure.
How Do I Lower My Soil’s pH?
Alkaline soil will have many negative characteristics, but the solution for all of them is the addition of organic matter.
Compost doesn’t technically lower pH, but it does act as a buffer so soil can operate as if it were more acidic. Over time, compost will act as a conditioner that lessens the effects of alkaline soil, and improves nutrient availability and enzyme activity.
The downside is that you will need a lot of compost. A soil with a pH over 8 will need a 4” layer of compost consistently mixed in each year. A large influx of organic matter can actually lower the availability of nitrogen in the soil, so while soil conditions will improve, you may see signs of nitrogen deficiency for a few months after application.
Compost also provides the ideal habitat for soil microorganisms, which help improve all aspects of soil life. The texture of compost improves the structure, and the soil’s ability to hold or drain water.
Elemental sulfur and ammonium sulfate can also lower pH, but this will have a drastic effect on pH values. If you are battling extremely alkaline soils, sulfuric amendments will dramatically lower the pH value over the course of a year. (source)
However, with this drastic change comes the potential for doing serious damage. Without the proper application rate, too much sulfur will cause extremely acidic soils that are in worse condition than the original soil profile. Sulfuric amendments are best used in conjunction with a local expert who can read a soil analysis and recommend a specific application rate for your soil.
Both compost and sulfuric amendments can be used together to improve alkaline soils. Compost will act as a buffer that helps create a healthier overall soil structure, while amendments can help to make your soil immediately able to grow plants that would suffer in alkaline conditions.
Pine needles, coffee grounds, and peat moss can also lower pH, but are best suited for mulching small areas at the base of established plants. These materials are effective at lowering pH, but difficult to use on a large scale. There aren’t many companies that sell used coffee grounds by the truckload.
Instead, use these materials sparingly and as they become available. Dump your used morning coffee grounds around the base of a tree, rake up pine needles and mix them into your compost pile, and incorporate peat moss into your mulch for a mild pH reduction.
What Plants Tolerate Alkaline Soil?
Most plants prefer slightly acidic soil, so while few thrive in alkaline conditions, many will tolerate them.
Trees are incredibly resilient, and many tolerate a wide variety of soil conditions. However, if you are installing a new landscape on alkaline soil, there are some trees that will perform better than others:
- Katsura tree
- Silver Maple
There are many shrubs that do well in alkaline soils, and many of the ones listed below have beautiful blooms and foliage to add interest to your landscape:
- Beauty Bush
- Smoke Bush
Perennials can be more picky about soil conditions, but there are still a wide variety that tolerate alkaline soils:
- New England Aster
- Bigroot Geranium
- Shasta Daisy
There aren’t many fruits that tolerate alkaline conditions. Most prefer acidic soils, but there are still a few that will do well:
Many vegetables tolerate alkaline soils, so, if in doubt, plant them and adjust as needed:
- Sweet Potato
There are many other alkaline-tolerant plants, but it is best to visit a local nursery to see which plants will do well in your climate zone as well as soil conditions.
Alkaline soil is a difficult challenge, but it can be managed with consistent applications of compost and by installing plants that tolerate a high pH value.
Soils with a high pH also have issues with compaction, overwatering, and nutrient deficiencies. Read our articles on soil management for more advice on managing difficult soil profiles.
How do I fix acidic soil?
Almost all soil problems can be fixed with the addition of compost. Organic matter improves all soil properties and acts as a conditioner that buffers the pH value of a soil. Add 2”-4” of compost each year until pH values are close to 6.5.
How accurate are pH tests?
Home tests are only accurate for determining whether your soil is acidic or alkaline. Test strips are accurate, but can be difficult to interpret. The pH meters are generally accurate as long as they are calibrated properly, and third-party analysis provides the best overview of the health of your soil.