Alkaline Soil: Everything You Need to Know

We're looking at what alkaline soil is, how to identify it, work with it, and how to fix it if you've got soil that's a bit too basic.

What Is Alkaline Soil?

Objectively speaking, alkaline soil is any soil with a pH above 7.0.

Relatively speaking, alkaline soil is any soil with a pH higher than your desired pH for growing the type of plants you want to grow.

Alkaline vs. Acidic Soil

Alkaline soil, also know as "sweet soil" is the conterpart to acidic soil, colloquially called "sour soil".

Most veggies that you might grow in your garden prefer a pH in or around the 6.0 - 7.0 range.

Recall that 7.0 is neutral, and below that is acidic "sour soil" - so most plants you choose to cultivate in your garden liku it just a little acidic.

If you've got alkaline soil, this means your plants might need a liitle help - or you might have to choose different plants to grow which are more tolerant of the alkalinity.

Causes

Calcium-Rich Bedrock

If the bedrock in your region is rich in calcium, as is the case with limestone or dolomite, it will have weathered into the soil over eons and will turn soil alkaline.

Arid Climate

The low rainfall in arid climates means that less alkaline ions like calcium, magnesium, and sodium are leached away from the soil than it wetter climates.

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These accumulate near the surface where plant roots grow, creating alkaline soil environments which fewer plants are adapted for.

Your veggie garden won't display that level of resilience, so if your garden soil is very alkaline you'll want to work to lower the pH for the sake of your plants.

Issues

When soil is too alkaline for the plants you want to grow, nutrient lockout can rapidle become a problem.

Nutrients which are present in the soil can become insoluble or inaccessible (locked out) so your plants can't get enough of what they need.

Boron, Copper, Iron, Manganese, Phosphorus, and Zinc absorption all become reduced or cease enirely when the pH is too high.

Identification & Testing

Let's go over how to test your soil and identify if it's alkaline, acidic, or neutral.

Characteristics

While the best way to gauge your soil alkalinity is by doing a pH test (covered below), there are a few soil characteristics that can be dead giveaways that you're likely dealing with alkaline soil:

• Salty white crust of calcium carbonate and sodium
• Compacted soil with poor water infiltration rates
• Hard clods if the soil is clay

Testing Soil pH

The basics of soil pH testing are easy:

• Grab a digital pH meter, a small container, and some distilled water (which has a neutral 7.0 pH)
• Fill your container part way with distilled water
• Pull some soil samples with a trowel (or with a soil core sampler for a deeper, more representative sample)
• Mix the samples together for an average
• Plunk a small bit of the mixed sample into the water
• Swish it around
• Test with the meter (be patient as it takes a minute to lock in on the true pH).

To learn more about the specifics, check out our Guide to Soil pH.

Alkaline Soil pH

a common issue we have here in the Sonoran Desert It's soil that is highly alkaline. In Southern Arizona, our soil is often at a pH of 8.0, 8.5, or even higher.

When starting a new garden bed recently, I tested my soil at a 9.25 pH. Native desert plants grow fine in these highly alkaline soils, but most vegetables and herbs that you would want to grow for food prefer much lower pH levels - closer to the 6.0 - 7.5 range.

What Plants Like Alkaline Soil?

One thing to consider before you jump into endeavoring to reduce the alkalinity of your soil is whether you can use it to your advantage.

For any soil pH there are usually plants which are uniquely adapted to thrive in it.

Ask yourself whether there may be plants you'd want to grow which would love your alkaline soil. It's much easier to work around your soil than to change it.

Let's look at some common plants which tolerate alkaline soils and are worth your consideration:

Plants That Tolerate Mild Alkalinity (pH 7.0 - 8.0)

Vegetables:

• Asparagus
• Beets
• Broccoli
• Brussel Sprouts
• Cabbage
• Cauliflower
• Garlic
• Kale
• Peas
• Spinach
• Swiss Chard
• Turnips

Herbs:

• Thyme
• Oregano
• Sage
• Chives
• Rosemary
• Marjoram
• Fennel

Fruits:

• Figs
• Gooseberries
• Grapes
• Mulberries
• Pomegranates
• Serviceberries

Flowers:

• Bachelor Button
• Daylilies
• Lavender
• Lilacs
• Marigolds
• Peonies
• Russian Sage
• Shasta Daisy
• Sunflower
• Tidytips
• Yarrow
• Zinnias

Grains:

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• Barley
• Oats
• Rye
• Sorghum
• Wheat

Plants That Tolerate Strong Alkalinity (pH 8.0 - 9.0)

If your soil is this alkaline, consider native plants and expect mixed results from the following plants.

Strongly consider working to drop the pH over time, but you may find success in leaning on the following plants in the interim while the pH drops.

Vegetables:

• Beets
• Purslane
• Salsify
• Swiss chard

Herbs:

• Hyssop
• Oregano
• Sage
• Thyme

Fruits:

• Figs
• Jujube

Flowers:

• California Poppy
• Coreopsis
• Cowpen Daisy
• Gazania
• Lavender
• Yarrow

Grains:

• Barley
• Millet
• Quinoa
• Sorghum

How To Fix Alkaline Soil

Tap water in southern Arizona is often very alkaline as well - the drip lines in my local community garden tested above an 8.5 pH.

It's worth noting that most city water tends to drift in pH over time, so re-testing every few months is necessary if you intent to adjust the water.

That means that even once you get your soil to the proper acidity level, your water may be constantly pulling your pH back up to the alkaline side.

To counteract this, you'll need to do a lot to find and maintain the proper pH level for your alkaline soil.

With reservoir-based irrigation systems, you have the luxury of being able to pH your water before it reaches your plants. In this case you can add white vinegar to your water to correct the pH, therefore avoiding the issue of your soil constantly becoming alkaline again.

Unfortunately this is not an option for irrigation systems without reservoirs, and both city and well water are likely to be affected by the alkaline soils before being pumped up above ground. If you harvest rainwater, however, that should have the more neutral pH which won't pull your soil alkalinity up.

That's important to keep in mind that soil pH is change slowly over time, and is very difficult to adjust rapidly.

Therefore, it's prudent to begin adjusting your soil pH at least 6 months prior to planting if you have the time. It can often take years to acidify desert soil, so bear that in mind when prepping your soil.

While you can spray white vinegar on your soil to immediately decrease pH, it's rapidly broken down and not a lasting solution. The best way I've identified of adjusting your soil over time is with Elemental Sulfur.

Understand though that elemental sulfur can take months or years as it breaks down to acidify your soil. Organics are often slow but completely worth it - get ahead of things and be patient with slow-working organic acidifiers and amendments.

Note - I do not recommend ammonium sulfate nor ammonium phosphate, neither of which are rated for organic gardens. It's Important to understand that these ammonium forms of nitrogen can seep into groundwater and contaminate it. This is especially true if applied to unplanted soil, or prior to a rainy season.

In Southern Arizona with our extremely alkaline soil, a common approach is to apply elemental sulfur to garden beds every 6 months until the microbiome has stabilized and can regulate its own pH.

Package directions and common application instructions on these products will likely suggest doses that are for too low for our heavily alkaline soil. I know of gardeners here who are having good success applying 5 lbs elemental sulfur in 60 ft² beds, every 6 months.

The best approach to applying elemental sulfur is to mix it into your soil, however this may not be an option for established shrubs, trees, or living soil beds where no-till is being practiced.

In these cases, it's okay to apply it to the soil surface, watering them in and allowing them to sink down over time as they are broken down by bacteria.

One thing to note with surface applications is that the top couple inches of soil will acidify first, and you may still notice signs of pH issues on parts of your plants where the roots are deeper than that.

Best Fertilizer for Alkaline Soil

While elemental sulfur can be applied in the fall to prepare for a spring planting, it's often unadvisable to apply the ammonium forms of nitrogen to follow soil before winter rains.

Alkaline Soil Treatment

Over time, this will correct with repeated applications and patience.

keep in mind that it could take two or more years for these applications to adjust your soil pH, so it's important to plan ahead where possible.

Buffering Soil Alkalinity

Ultimately, nothing will buffer your soil pH better than building high quality soil over time.

A thriving microbiome will regulate your soil pH and greatly reduce to pH lockout affects of alkaline soil.

Related Issues

Saline Soil

Soil salinity is a tangential topic to soil alkalinity, but alkaline soils often also come with salinity issues, especially in arid environments. This salinity stems from high rates of evaporation which leaves behind salt accumulations in the soil.

If you're also dealing with saline soils, is a great amendment to utilize. Also known as calcium sulfate, gypsum has a natural affinity for displacing sodium through the interaction between sulfate ions and the sodium present in the soil.

In the Sonoran desert where I grow, gardeners generally apply both elemental sulfur and gypsum to their soil each year to help bring it within reasonable bounds for growing veggies.

After realizing that gypsum contains sulfur (calcium sulfate dihydrate), one may assume that the need for elemental sulfur in acidifying alkaline soils might be reduced if gypsum is applied for salinity and soil structure improvement.

Unfortunately this is not the case, as the sulfur in gypsum is already oxidized in a stable state. Therefore it does not go through the same bacterial weathering process as elemental sulfur does and does not produce sulfuric acid over time to acidify soil in the same way as applying elemental sulfur.

If you're applying gypsum for soil salinity, be sure to also apply elemental sulfur if you're dealing with alkalinity as well and don't assume that the sulfur in the gypsum will perform the same function.

Compacted Soil

Check out our article on cover crops for dealing with compacted soil