How to Lower Soil pH (to make soil more acidic)

When garden plants are struggling to grow, producing little fruit, or appear to be diseased, the problem may be with the pH level of your soil, not a pest. While most plants are perfectly happy in neutral pH soil, others require acidic soil (low pH) to thrive. Here we’ll examine how to lower soil pH (make your garden soil more acidic) for plants such as blueberries, azaleas, rhododendrons, and mountain laurels.

What is soil pH?

In chemistry, pH stands for “potential hydrogen”. It’s a scale used to rank the relative alkalinity (basicity) or acidity of substances to other substances, based on the amount of hydrogen ion activity in the tested substance. The pH scale ranges from 0 to 14, with 7 being neutral. A measure below 7 indicates acidity and above 7, alkalinity.

The pH scale is logarithmic, which means a change in 1 pH unit reflects a 10X change in acidity or alkalinity. For instance, a soil pH of 6 is 10 times more acidic than a soil pH of 7, and a pH of 5 is 100 times more acidic than a pH of 7. The optimum soil pH range for most plants is between 6.0 and 7.2, but some thrive in pH outside of this range or even require it.

Why soil pH is important

Soil pH controls the chemical forms of the soil’s nutrients and influences the chemical reactions they undergo. These chemical reactions affect a nutrient’s availability or toxicity to the plant. This is why some species won’t survive in pH ranges outside of their comfort zone.

In the case of blueberries, a soil which is too alkaline inhibits the ability of the plant to absorb enough iron. While iron exists in any soil, it’s only in acidic soil that it becomes available to plants through specific chemical reactions. Acidifying the soil to a level of 5 or lower allows the blueberry to utilize iron and other micronutrients efficiently. Iron chlorosis is one of the most common problems with blueberries struggling in an alkaline soil – its leaves become yellow. These symptoms are frequently confused with a nitrogen deficiency, but acidifying the soil cures the symptoms. Depending on the variety, blueberries will struggle to survive in alkaline soil, and this stress may result in a scraggly looking shrub which produces little if any fruit, and whose leaves may turn brown. The plant appears to be diseased, but it simply may be a pH problem.

How to lower pH in soil (make soil acidic)

You’ve probably seen bags of soil amendments and fertilizers at garden centers that advertise their use for “acid-loving plants”: azaleas, rhododendrons, blueberries, hydrangeas, and similar species. The soil acidifiers in these products are usually forms of sulfur, aluminum sulfate, or ammonium sulfate.

• Elemental sulfur has been used for thousands of years in agriculture to lower pH in soil and to control certain diseases. It lowers soil pH over a period of months, as soil bacteria must convert it into sulfuric acid. Sulfur is acceptable for organic gardeners. See this study on using sulfur to lower the soil pH for blueberries.
• Ammonium sulfate or aluminum sulfate are used to acidify soil fast and are commonly used in agriculture. For organic gardeners, ammonium sulfate is not acceptable. Aluminum sulfate is rated as “acceptable” for organic agriculture, but many organic gardeners steer clear of it. These chemicals must be added seasonally as they eventually degrade.
• Sphagnum peat moss works wonders for lowering soil pH, but it can take 1 or more seasons to achieve the desired pH. Use 1-2″ of peat moss and work it into the top layer of soil before planting. For established garden beds, use peat moss as an annual mulch around the acid-loving plant’s root zone. In my organic garden, I use peat moss generously to achieve a lower pH, along with sulfur for plants like blueberries. Peat is also an excellent source of organic matter and helps to break up alkaline clay soil. There are also some environmental concerns with using peat moss.

Note: When using sulfur to lower pH in soil, don’t exceed 2 pounds per 100 square feet per application. Using more than this may damage the plant. Before making a second application, wait at least 3 months and only after testing the soil pH.

Hydrangeas are a great example of plants that adapt to pH – they’ll survive in soil between 4.0 and 7.5. Their flowers are a sort of litmus test – the color turns blue in low pH and pink in neutral to high pH.

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Test your garden soil pH before adding any amendments

Your region’s native soil will tend toward a certain pH, but so many local factors play into this that you should never assume it’s of a certain alkalinity or acidity. The only way to know for sure is to test it. This can be done with a soil pH testing kit or pH probe you can buy online or at a garden center. Once you know the pH of your garden soil, then add the appropriate soil amendments to achieve the target pH for the plant.

Note: If the pH of your garden soil is higher than 7.5, it may contain a lot of calcium carbonate, common in areas with an abundance of limestone. Calcium carbonate is remarkable for resisting attempts to lower pH and acidifying soils may be difficult to impossible. Stick with native plants or build raised garden beds and import topsoil from a reputable garden center.