How to Increase Soil Acidity

Some plants such as camellias, lupines, lilies and primroses are acid-loving. If your garden soil is not acidic enough or has been fertilized with too much lime, here are some methods to slightly increase soil acidity to help acid-loving plants grow well.

Table of Contents

Test soil and water pH

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Bring the sample to a professional testing agency for the most accurate results. If you are serious about growing plants or want to increase the acidity of your soil for some reason, you will find that taking a soil sample to a professional testing agency is more accurate than doing it yourself at home. You may not think so, but the difference between 5.5 and 6.5 on the pH scale is quite large!

If you are in the United States, contact the nearest rural development office in the county. They will help you with basic soil tests including pH measurements for free or for a very small fee.

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Try using a home pH meter . If you don’t want to have your soil tested by a professional, you can easily measure your soil’s pH at home, but be aware that the results won’t be as accurate as professional test results. There are several ways to get relatively accurate results at home:

Use a pH test strip. This method will only tell if the soil is acidic or alkaline, but it’s also a fun way to work with a variety of flowers, vegetables, and herbs.
Use vinegar and baking soda to test the pH. This method is another rudimentary way to test whether the soil is acidic or alkaline. You will take about 1 cup of soil and divide it into two containers, then add vinegar on one side, baking soda and water on the other. Observe which side is bubbling. If the side adds effervescent vinegar, it means the soil is alkaline; If the baking soda is bubbling, the soil is acidic.
Buy a home pH test kit. A home pH test kit will tell you the pH of your soil in numbers. This number is more informative than the “acidic” or “alkaline” results of the above methods.

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Be sure to test the pH of the water as well. The pH in groundwater that you can use for watering plants is usually between 6.5 and 8.5, but is usually more alkaline so it won’t corrode pipes. ^{[1] X Source of Research} If the water used to irrigate the plants is initially alkaline and so is the soil, you will need a bit of work to create the desired acidic effect on the plants.

One way to deal with this problem is to use purified water. Pure water has a pH of 7, which is almost absolutely neutral. Using pure filtered water is an effective way, but soon you will find it very expensive.

Increase the acidity in the soil

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Determine the soil type. This step is different from determining the pH of the soil, and is a very important step. The method of increasing soil acidity will depend on the type of soil to be treated.

Soil that is well drained and relatively loose will make it much easier to increase acidity. With this type of soil, you can use large quantities of organic compounds that increase acidity as they decompose.
Clumped and compacted clay will make the process of increasing acidity much more difficult. Adding organic matter to this soil will only increase alkalinity, not decrease it.

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Apply organic material to loose and well-drained soil. Adding organic material is the best way to increase the acidity of this soil. Organic materials will increase the acidity in the soil as they decompose, but you will need to use large amounts to lower the pH in the soil. ^{[3] X Research Sources} Here are some very good organic materials you should consider:

sphagnum peat moss
Aged oak leaves
Compost and manure

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Apply elemental sulfur to compacted or clay-rich soil. As noted above, the addition of organic material to compacted soil can worsen the condition as the soil will retain more moisture causing additional alkalinity. So, the surest way to increase the acidity of heavy clay soils is to add elemental sulfur or iron sulfate to the soil.

Elemental sulfur helps increase soil acidity as bacteria convert the chemical into sulfuric acid. ^{[4] X Research Source} You will need about 1kg of elemental sulfur per 10 m2 of soil to lower the soil pH from 7 to 4.5. ^{[5] X Research Sources}
Since elemental sulfur is slow acting, it’s best to add it to the soil about a year before planting for best results. ^{[6] X Research Sources}
Add elemental sulfur to the soil, dig down about 15 cm.

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Add iron sulfate to compacted or clay-rich soil. Iron sulfate relies on a chemical reaction to make acids. Therefore, the chemical is less dependent on temperature conditions than elemental sulfur, which relies on bacteria to generate biological reactions. ^{[7] X Research Sources}

You may need up to 5 kg of ferrous sulfate per 10 square meter of soil to reduce the pH by one unit. ^{[8] X Research Sources}
If you plan to add more than 5 kg of ferrous sulfate per 10 m2, you will have to divide it into two doses, 1 or 2 months apart to give the soil time to absorb the ferrous sulfate.
Iron sulfate acts much faster than elemental sulfur. This chemical can reduce the pH significantly within 3-4 weeks instead of months. ^{[9] X Research Source} This means that ferrous sulfate has the added advantage of being available during the pre-planting season.
Be careful when using ferrous sulfate. This chemical can stain clothes, sidewalks and yards. It is best to separate clothes with ferrous sulfate and wash them separately to avoid spreading to other items.

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Use a fertilizer containing ammonia. In many cases, you just need to use a fertilizer that contains ammonia. Many acid-loving plant-specific fertilizers contain ammonium sulfate or sulfur-coated urea.

Calcium nitrate and potassium nitrate should not be used as fertilizers, even if they do not contain ammonia. These fertilizers will actually increase the pH in the soil. ^{[10] X Research Source}

Maintain the right pH for plants

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If you’ve grown plants and flowers by mistake, use elemental sulfur. This chemical is slow acting, so you don’t have to worry about taking the wrong dose. Apply elemental sulfur to the moist soil as much as possible, trying not to disturb the roots of the plant. Continue to monitor the pH of the soil after several months.

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Don’t follow your gut feeling and put vinegar in the soil. Vinegar will lower the pH in the soil, but in this case that’s not good. The change is too sudden, disappears too quickly and this kills beneficial microorganisms in the soil. ^{[11] X Research Source} Stay away from vinegar, unless you accept the risk of plant death.

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Using cottonseed residue as a fertilizer helps increase acidity over a year. So, assuming you’ve treated your soil with ferrous sulfate and just planted blueberries, you can keep the pH low by adding large amounts of a natural fertilizer like cottonseed meal. Cottonseed residue, a by-product of cotton production, is particularly beneficial for acid-loving plants such as azaleas and camellias. ^{[12] X Research Source}

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Test the pH at least once a year. You should check the pH of the soil near the base of the plant, add a fertilizer like aluminum sulfate (especially for hydrangeas) and avoid damaging the roots. For best results, you should use a commercially available pH test kit or submit a soil sample for professional testing.

Most vegetables and ornamental plants prefer mildly acidic environments between 6.5 and 6.8.
Hydrangeas, rhododendrons and blueberries prefer a more acidic environment – around 5 -5.5.

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Increase the pH in the soil with lime if necessary. In some cases, your efforts to increase the acidity of the soil have proved too effective to make the acidity too high for vegetables and crops. You will then need to increase the alkalinity of the soil by adding lime. Lime comes in three basic types – limestone, quicklime/slaked lime, also known as hydrated lime – and the amount used will depend on the type of soil as well as the type of lime you choose. You can read the instructions on the package or talk to the gardener for more information.

Advice

Sulfur Flower is a fine and pure sulfur powder. You can find this chemical at gardening centers or order it online.
Iron salts are also useful; Soils that are too alkaline can “lock in” iron, preventing it from reaching plants that need it. You should also wait for the results of your first treatment before adding iron.

Steps

Test soil and water pH

Increase the acidity in the soil

Maintain the right pH for plants

Advice

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