How to Increase Soil Acidity

Send the sample to a professional laboratory for the most accurate results. If you're serious about planting or want to increase soil acidity for any reason, you'll find that sending a soil sample to a professional lab for testing is far more accurate than doing it yourself at home. You might not think so, but the difference between a pH of 5.5 and 6.5 on the scale is quite significant!

• If you're in the U.S., contact the nearest county extension office. They will assist you with basic soil tests, including pH testing, which is either free or very low-cost.

Try using a home pH testing tool. If you prefer not to send your soil for professional testing, you can easily check the pH level of the soil at home, but keep in mind that the results may not be as accurate as a professional lab test. There are a few methods to get relatively accurate results at home:

• Use pH test strips. This method will indicate whether your soil is acidic or alkaline, and it can be a fun way to test soil for different types of plants like flowers, vegetables, and herbs.
• Use vinegar and baking soda to test the pH. This is a rudimentary method to check whether the soil is acidic or alkaline. Take about 1 cup of soil and divide it into two containers. Add vinegar to one, and baking soda and water to the other. Observe which side bubbles. If the vinegar side bubbles, the soil is alkaline; if the baking soda side bubbles, the soil is acidic.
• Purchase a home pH testing kit. A home pH kit will provide you with a numerical reading, giving more detailed information than just “acidic” or “alkaline” from the previous methods.

Don't forget to test the pH of water as well. The pH of groundwater that you use for irrigation typically ranges from 6.5 to 8.5, but it usually leans more alkaline, which prevents it from corroding pipes. If both your irrigation water and soil are alkaline, you will need to take measures to create the acidic environment your plants need.

• One solution is to use purified water. Purified water has a pH of 7, which is almost perfectly neutral. While this method is effective, you will soon realize that it can become quite expensive over time.

Understand how to read the pH results from the test kits you use. pH is a measure of how acidic or alkaline a substance is, with a scale from 0 to 14, where 0 is extremely acidic (like the acid in a battery) and 14 is highly alkaline (like drain cleaner). A pH of 7 is considered "neutral" on the pH scale.

• For example, if you get a pH of 8.5, it means your soil is slightly alkaline. You would need to add a small amount of acidic material to the soil to reduce the alkalinity. A pH of 6.5 shows that the soil is slightly acidic. If you want to increase the acidity, you should add more acidic materials to the soil.
• If you want more precise information, you can calculate the pH using the logarithmic scale, meaning each pH value increases tenfold. So, a pH of 8 is ten times more alkaline than a pH of 7, a pH of 8.5 is 15 times more alkaline, and so on.

Identify your soil type. This step is different from determining the pH level of the soil, but it is very important. The method for increasing soil acidity depends on the type of soil you are working with.

• Soil that drains well and is relatively loose will make it much easier to increase acidity. For this type of soil, you can use a large amount of organic compounds that naturally increase acidity as they decompose.
• Clay soil that is compacted and dense will make increasing acidity much harder. Adding organic matter to this type of soil will only increase alkalinity, not reduce it.

Add organic matter to well-drained, loose soil. Incorporating organic materials is the best way to increase acidity in this type of soil. Organic materials will lower the pH of the soil as they decompose, but a substantial amount is required to make a significant impact. Here are some excellent organic materials you might want to consider:

• Sphagnum peat moss
• Composted oak leaves
• Compost and manure

Apply elemental sulfur to compacted or clay-heavy soil. As mentioned earlier, adding organic matter to compacted soil may worsen the situation, as it will retain moisture, further raising the alkalinity. Therefore, the most reliable way to increase acidity in clay-heavy soil is to apply elemental sulfur or iron sulfate.

• Elemental sulfur helps increase soil acidity as bacteria convert it into sulfuric acid. You will need about 1 kg of elemental sulfur per 10 square meters of soil to reduce pH from 7 to 4.5.
• Since elemental sulfur acts slowly, it is best to apply it about a year before planting for optimal results.
• When applying elemental sulfur, bury it about 15 cm deep in the soil.

Introduce iron sulfate into compacted or clay-rich soil. Iron sulfate works through a chemical reaction to create acidity. As a result, it is less dependent on temperature than elemental sulfur, which relies on bacterial activity for the conversion.

• You may need up to 5 kg of iron sulfate per 10 square meters of soil to decrease pH by one unit.
• If you're adding more than 5 kg of iron sulfate per 10 square meters, it should be done in two applications, spaced 1 or 2 months apart, to give the soil time to absorb the iron sulfate.
• Iron sulfate works much faster than elemental sulfur. It can lower pH significantly within 3-4 weeks, unlike sulfur, which can take months. This means iron sulfate can be used immediately before planting season.
• Be cautious when handling iron sulfate, as it can stain clothing, sidewalks, and driveways. It's best to wash any clothes that come into contact with it separately to avoid staining other items.

Use fertilizers containing ammonia. In many cases, all you need is a fertilizer that contains ammonia. Many fertilizers designed for acid-loving plants contain ammonium sulfate or sulfur-coated urea.

• Do not use calcium nitrate or potassium nitrate as fertilizers, even if they do not contain ammonia. These fertilizers will actually increase the pH of the soil.

If you've already planted trees and flowers, consider using elemental sulfur. This chemical works slowly, so you don't need to worry about getting the dosage wrong. Apply the elemental sulfur to moist soil as much as possible, but try not to disturb the plant's root system. Continue monitoring the soil's pH over the next few months.

Don't impulsively add vinegar to the soil. Vinegar will lower the soil's pH, but in this case, that can be harmful. The change happens too abruptly and disappears too quickly, potentially killing the beneficial microorganisms in the soil. Stay away from vinegar unless you're willing to risk harming your plants.

Use cottonseed meal as a fertilizer to help acidify the soil over the course of a year. For instance, if you've treated the soil with iron sulfate and just planted blueberries, you can maintain a low pH by adding a significant amount of natural fertilizer like cottonseed meal. Cottonseed meal, a byproduct of cotton processing, is especially beneficial for acid-loving plants like azaleas and camellias.

Check the pH at least once a year. It's recommended to check the pH near the plant's base and add fertilizers like aluminum sulfate (especially for hydrangeas), while being cautious not to damage the roots. For the best results, use a pH testing kit available on the market or send a soil sample for professional testing.

• Most vegetables and ornamental plants prefer a slightly acidic environment, with a pH between 6.5 and 6.8.
• Hydrangeas, azaleas, and blueberries thrive in more acidic conditions, around 5-5.5.

If necessary, raise the pH in the soil with lime. In some cases, your efforts to increase the soil's acidity may have been too effective, making the environment too acidic for vegetables and other plants. When this happens, you'll need to increase the soil's alkalinity by adding lime. There are three main types of lime—limestone, quicklime/ hydrated lime—and the amount you use depends on the soil type and the lime variety you choose. You can read the instructions on the packaging or consult with other gardeners for more details.