Growing plants in alkaline soil can be challenging. This type of soil doesn’t accommodate as many species as neutral or acidic soil, and because it’s less soluble, it can limit nutrient availability for your plants. But that doesn’t mean that it doesn’t have its uses. This article will explain the benefits of this type of soil, list the best alkaline plants and vegetables to grow, and provide some valuable tips on making the most of this soil type.
What Is Alkaline Soil?
Alkaline soil is any soil that has a pH of 7.4 and higher. It can occur naturally, through the presence of chalk, limestone, or wood ashes, or as a result of man-made activities, such as pollution or incorrect disposal of construction materials.
Chalky soils are the best example of alkaline soil, but you will also find clay, sandy, and silty soils with a pH veering towards alkaline.
Alkaline soil has several grades, depending on the pH:
- Neutral soil: 6.6 – 7.3
- Slightly alkaline: 7.4 – 7.8
- Moderately alkaline: 7.9 – 8.4
- Strongly alkaline: 8.5 – 9.0
- Very strongly alkaline: > 9.0
The Pros and Cons of Alkaline Soil
Gardeners often refer to alkaline soil as “sweet soil” instead of the “sour,” acidic soil. To the sweet-toothed gardener, finding out that your garden has sweet soil sounds like excellent news. And while alkaline soil has many uses, too much of a good thing can be detrimental to your garden.
Let’s take a closer look at the pros and cons of alkaline soil.
Pros of Alkaline Soil
- Alkaline soil has higher calcium, sodium, molybdenum, and magnesium levels, which promote healthy plant growth and make your crops more resistant to diseases.
- Chalky soil has good drainage and is an excellent pick for plants that don’t like having “wet feet” or roots sitting in stagnant water.
- The clubroot disease, which can decimate brassica crops, cannot survive in alkaline soil, so keeping the soil pH above 7.3 is often used to control this disease.
- Some plants benefit from growing in alkaline soil. This includes vegetables such as asparagus, cucumbers, squash, and brassicas, or flowering shrubs such as forsythia, lavender, and hydrangea.
Fun fact: The color of hydrangea flowers will change depending on the pH of the soil. Hydrangeas growing in acidic soil will have blue flowers, while those growing in alkaline soil will have pink blooms.
Cons of Alkaline Soil
- Most alkaline soils are chalky soils with poor water retention and are not suitable for drought-sensitive plants.
- Chalky soils are “hungry soils”. They break down organic matter quickly, leach nutrients during heavy rainfalls, and will need frequent applications of compost or manure to compensate for the poor nutrient retention.
- Alkaline soils that are not chalky, such as clay-heavy soils, tend to have poor aeration and drainage, which subjects plants to water stress and the inability to absorb nutrients correctly.
- If the soil pH is 7.8 or higher, it will limit the availability of copper, manganese, zinc, phosphorus, and iron, which means that plants will struggle with vital processes such as photosynthesis. Iron deficiency (PDF) is a real problem for plants growing in alkaline soil with a pH of 8 or higher and will cause yellowing of the leaves (chlorosis), wilting, stunted growth, and death of the entire plant.
Best Outdoor Alkaline Plants
There’s a wealth of alkaline-loving plants you can grow in your garden. From trees, shrubs, and flowering vines, to medicinal plants and annual flowers, many plants will tolerate growing in a pH of up to 7.8.
Check out our list of over 50 plant species you can grow in alkaline soil below.
Growing trees can be a lifelong commitment. They take years to become established, they need regular maintenance, and changing the garden soil to meet their requirements can be time-consuming and labor-intensive. Luckily, many trees will have no problem growing in alkaline soils.
Here are some examples of the best trees for alkaline soil:
- Common oak
- Common beech
- Crab apple tree
- Flowering plum
- Cherry tree
- Montezuma pine*
- Mugo pine*
- Field maple
- Korean mountain ash (Sorbus alnifolia)
- European ash
- Holm oak
- Horse chestnut
*A note on conifers: The vast majority of coniferous species grow best in acidic soil, although some can also tolerate slightly alkaline soil. This includes trees such as larch and cedar, and shrubs such as yew, juniper, and thuja. To give them a head start and ensure that they thrive, add some ericaceous compost to your garden soil before planting them.
Great or small, flowering or evergreen, shrubs are a great way to add interest, color, and texture to your garden.
Here are some of the best shrubs that like high pH soil:
- Spindle tree (Euonymus)
- Strawberry tree
- Shasta daisy
- California lilac (Ceanothus)
- Devilwood (Osmanthus)
- Lavender cotton (Santolina chamaecyparissus)
- Coral bells (Heucheras)
Some of the best-known flowering vines are, surprisingly, suitable for gardens with alkaline soil.
The list of popular species includes:
- Star jasmine
- Sweet peas
- Black-eyed Susan
- Firecracker vine
- Trumpet vine
- Passionflower (Passiflora)
- Morning glory
- Periwinkles (Vinca)
Annual and Perennial Flowers
- Lily of the valley
- California poppy
- Hardy geranium
- Ornamental clover
- Jacob’s ladder (Polemonium)
Many medicinal plants have evolved to survive in harsh conditions and, as a result, will have no problem growing in alkaline soil.
Here are some you should try growing in your garden:
- Pot marigold (Calendula officinalis)
- Lemon balm
- Tulsi (holy basil)
- Great burdock
Vegetables That Like Alkaline Soil
The goal of any vegetable garden is to aid in self-sufficiency. Ideally, you should strive to grow vegetables that fit alkaline soil to avoid expending too many resources. The downside is that most vegetables grow best in either neutral or slightly acidic soil.
This means that if your garden soil pH is too high, you risk limiting nutrient availability for your vegetables and having a smaller harvest as a result.
The good news is that some vegetables like growing in alkaline soil. Leeks, for example, can tolerate a pH as high as 8. Brassicas, such as cabbage and cauliflower grow better in alkaline soil and are less susceptible to the dreaded clubroot disease. And legumes such as beans and peas help fix nitrogen into the soil, improving your alkaline substrate for future crops.
Here are some vegetables you can grow in alkaline soil with a pH of up to 7.8:
- Brussels sprouts
- Mustard and mustard greens
- Pole beans
- Chinese cabbage (including napa cabbage and bok choy)
- Herbs such as oregano, marjoram, oregano, cilantro, sage, and tarragon.
Indoor Plants for Alkaline Soil
The best thing about growing plants indoors is that you don’t have to worry about soil pH. Store-bought soil is usually neutral or slightly acidic, making it ideal for growing most houseplants.
It’s best to avoid using garden soil for indoor plants since it can increase the risk of pests, diseases, and fungal problems.
If you do want to use soil from your garden for indoor alkaline plants, keep in mind that most shade-tolerant tropical plants such as pothos, philodendrons, and calatheas prefer acidic soil. Alkaline soil is also more likely to kill cacti and other succulents and is unsuitable for epiphytic plants such as Rhipsalis and orchids.
Here are a few houseplants you can grow in slightly alkaline soil:
- Maidenhair fern
- Asparagus fern
- Some Hoya varieties
- Parlor palm
- Snake plant
- Potted herbs such as oregano, thyme, sage, and peppermint.
Tips for Growing Alkaline Soil Plants
If your garden has alkaline soil, adding organic matter and soil amendments to improve drainage and moisture retention is always a good call. But going as far as drastically altering the pH of your soil is rarely necessary.
Here are a few tips that will make working with alkaline soil much easier.
- Always test the soil pH in the autumn, or 5 to 6 months before you plan to grow anything. Add amendments such as compost, leaf mold, pine needles, and peat moss soon after testing. This will give the bacteria in the soil plenty of time to work with the amendments you provided to create an improved, organic substrate.
- If using inorganic soil amendments to improve drainage, avoid using limestone, which will increase the soil pH, making it even more alkaline.
- Check the water profile in the area where you live. If your garden has chalky soil, there’s a good chance that your area has hard tap water, which is alkaline. In that case, use a water softener or collect and use rainwater for your garden.
- If your garden has chalky soil, plant grasses to improve topsoil structure and prevent nutrients from leaching. Planting clover, beans, peas, and other legumes is another great way to correct chalky soil and help fix nitrogen in the soil without the heavy use of fertilizers.
- Pay close attention to the plants growing in your area, and prioritize growing them in your garden. Native plants accustomed to chalky, alkaline soil require less maintenance, play an integral part in the local ecosystem, and are less likely to escape cultivation and become invasive.
- Use an iron foliar fertilizer to give your plants a mineral boost, especially if you notice early signs of iron deficiency, such as chlorosis.
It’s unlikely that if your garden has alkaline soil, the pH is high enough to have a severe negative effect on the health of your crops. However, the only way to be sure is to obtain a soil test. And if the pH is indeed higher than 7.8, look into ways to lower the soil pH and plant alkaline plants.
Check out our separate guide on how to make your garden soil more acidic.
Frequently Asked Questions
Should I Amend My Alkaline Soil?
Alkaline soil doesn’t always need to be amended. As long as the pH falls within the 7.4 to 7.8 range, there are plenty of alkaline soil plants you can grow in a garden.
Adding compost or well-rotted manure before you start planting is enough to balance the pH of the soil and improve moisture retention and nutrient availability.
You should make significant amendments to alkaline soil if the pH is higher than 7.8 or if you want to lower the soil pH for acid-loving plants.
Flowers such as azaleas and magnolias, vegetables such as potatoes and tomatoes, and most importantly, berries will need a soil pH between 5.5 and 6.5 to grow well.
Do roses like alkaline soil?
No, the ideal pH for roses is slightly acidic (6 to 7). If you have soil outside of that range, the roses might have difficulty taking up nutrients, which will often kill them.
How can I make my soil alkaline without lime?
You can make the soil more alkaline with lime. Check out our guide on how to make soil more alkaline.
Does lavender like alkaline soil?
Yes! Lavender is considered an alkaline plant and does well in soil with a pH of 6.5-8. Be sure to use well-draining soil (sandy is best). Lime can sometimes be used to increase the pH of the soil if your soil is too acidic.
Diagnosis and Improvement of Saline and Alkali Soils (PDF):www.ars.usda.gov/ARSUserFiles/20360500/hb60_pdf/hb60complete.pdf
SOIL ACIDITY, ALKALINITY, AND SALINITY (PDF): caenvirothon.com/wp-content/uploads/2018/02/Envirothon-07_Soil-Acidity-Alkalinity-and-Salinity-ver.-2017.pdf