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Is Clay Soil Acidic? Everything You Need to Know!

Are you worried about adding clay soil to the potting mix, as many gardeners claim it makes the substrate too acidic?

Clay soil is the least acidic soil type as it boasts a pH level between 8-10, requiring adding organic matter, rich compost, or sulfur to neutralize or turn acidic.

Using clay soil may not be appropriate for most houseplants, veggies, and herbs that thrive in neutral or acidic soil.

Read on to find out whether all clay soil types are alkaline and ways to neutralize them.

What Type of Soil is Acidic?

When it comes to gardening, you can choose from four types of soil; sandy, silt, clay, or loamy, either a whole or in combination.

Similarly, each soil type has a different pH level, which determines how plant roots absorb nutrients, oxygen, and moisture.

Clay soil is known to be the most alkaline among all the soil types because it has the most sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3), which significantly raises the pH level.

It has to do with the leaching of organic acid (from microorganisms such as fungi, yeast, and bacteria) that makes it denser and sterile.

Clay soil with rocks can be used for household consturction
Clay soil with rocks can be used for household construction.

However, not all clay soil types are alkaline. The geographic location of the clay soil will significantly determine its organic content and pH level.

Here is a brief overview of different clay soil types and how much they vary in pH level.

Clay Soil TypeSpecification
Silty clayIt contains over 50-80% silt contents.
Sandy clayIt contains over 50% sand content.
Clay loamIt is a mixture of clay, sand, and silt with additional organic matter.
Silty clay loamIt boasts a mixture of clay, sand, and significant amount of silt.
Sandy clay loamIt contains 25-35% clay, over 25% silt, and 45% sand.

For instance, red clay soil has a pH of less than 5.0, resulting from the leaching of calcium and the accumulation of iron oxide (rust) that makes it acidic.

Similarly, Georgia clay soil has an acidic pH level between 5.0 and 5.9, resulting from heavy soil structure and good nutrient-holding capacity.

Besides, London clay soil is stiff bluish clay heavily oxidized with iron oxyhydroxide (FeOOH) and retains an alkaline pH level.

On the other hand, black clay soil is highly alkaline and denser, usually found in Southern US states like Texas.

How to Make Clay Soil Acidic?

If you have clay soil in your backyard, you can be assured that it is alkaline, except if you live in an area rich in red clay, London clay, or Georgia clay.

There are many different and affordable ways to turn clay soil acidic instantly. Beforehand, consider checking the pH level using a soil testing kit to determine alkalinity.

Remember, alkaline soil is not favorable for most houseplants, including vegetables, herbs, and flowering shrubs, requiring correcting the pH before planting.

Most houseplants, veggies, and herbs will thrive in soil with a pH between 5 and 7, which helps retain most nutrients from the ground.

Here are some proven ways to make your alkaline clay soil acidic!

1. Add Organic Compost

Adding organic compost is the best way to acidify clay soil and is less likely to create side effects.

It is made from decomposing organic matter like plant material, living or dead animals, manure, lichens, sewage sludge, sawdust, etc.

Amending the clay soil with organic compost will release macronutrients and healthy microorganisms into the soil and adds air pockets for better drainage.

With lesser water retention and thriving healthy bacteria, the clay will begin breaking down to promote better biological activity.

This process will gradually turn alkaline clay soil acidic and help retain more nutrients.

2. Use Acidifying Fertilizers

Soil-acidifying fertilizer contains ammonium nitrate, urea, and phosphate, directly injecting soil with acid-based components.

However, be wary about using fertilizers to amend the soil pH as it exposes clay soil to excess nitrogen and chemical salts.

Stick with organic fertilizers that have NPK with other organic matters which help break down the clay soil.

Moreover, always apply the solution in moderation or as diluted to offset the risk of over-fertilization.

3. Add Sphagnum Peat Moss

Peat moss is a popular choice to make any soil acidic because it has a pH of 3.0-4.5.

The organic matter is made from decomposed materials salvaged from peat bogs, making it highly acidic and rich in microbial organisms.

Loamy cay soil mixed
Loamy cay soil mixed with peat moss for houseplant potting.

Therefore, adding just 2 inches of peat moss and working it to a depth of 6-8 inches of clay soil will result in moderately acidic soil.

Melinda Myers says, “Peat moss helps enhance water drainage in clay soil and retain water in lean, sandy soil.” 

However, this process will likely work for potted or raised-bed clay soil but extensive gardens.

4. Use Coffee Grounds

Coffee is rich in NPK contents and micronutrients like calcium, magnesium, copper, iron, and zinc, which makes it a popular compost choice.

In addition, the coffee grounds naturally contain low pH levels (6.5-6.8), which helps release acid in the clay soil.

It helps improve tilth (soil surface) and even adds structure for sturdy plant growth.

However, be wary about adding coffee grounds in an already nitrogen-rich soil as it may stun the plant growth with excess nitrogen.

5. Use Leaf Molds and Pine Needles Mulch

Leaf mold is decayed leaves that are broken down by fungi instead of decomposing by bacteria.

Usually, oak and maple leaves make excellent leaf mold that adds acid and helps retain essential nutrients in the soil slowly.

Similarly, fallen pine needles make perfect mulch for soils and even protect garden beds.

Remember, pine needles have a low acidic value (usually 3.8 pH), requiring the use of them in moderation.

6. Add Cottonseed Meal

Like coffee grounds, cotton waste makes excellent mulch for soil and fertilizer for nitrogen-loving plants.

However, beware of using cottonseed meals that did not come from an organic farm, which may contain harmful pesticides.

7. Add Vinegar or Lemon Juice

Vinegar and lemons contain 2-3 pH levels and are popular kitchen items used for acidifying food.

Besides, these items are not only suitable for kitchen use but also for acidifying soil.

Yellow clay soil ry
Yellow clay soil is often used for ceramics and pottery.

Therefore, adding lemon juice or vinegar to the clay soil will release a significant amount of acid.

However, beware of using it in moderation to avoid acid-related problems.

Mix 1 cup of vinegar or lemon juice with 1 gallon of water (3.7 liters) and pour it onto the 5 gallons of clay soil to acidify it quickly.

8. Add Acidic Chemicals 

Another quicker way to lower the pH in clay soil is to use acidic substances.

However, excessively acidic soil (lower than 5.5) can stunt plant growth and damage the soil ecosystem, leading to infertile soil.

Therefore, you are strongly recommended to use these substances in sparse or moderate quantities to avoid repercussions.

Elemental SulfurWhen mixing elemental sulfur with clay soil, the naturally occurring soil bacteria transform it into sulfuric acid.
Aluminum SulfateThe metal salt contains a deficient pH level, usually 1.8-2.2, providing an immediate acid boost to the soil.
Iron SulfateThis method relies on a chemical reaction to create acid, requiring an ideal soil conditions.
Ammonium SulfateThe ammonium ions released will form small acid particles, which spread around the clay soil and quickly turn it acidic.

From Editorial Team

You can Still Grow a Few Houseplants in Clay Soil 

Houseplants, including Azaleas, Hydrangeas, Lavender plants, Peonies, Roses, and Turfgrass, thrive well in a yard or garden with clay soil. 

If you wish to grow these plants indoors, buy LECA clay balls that are fungus free and rot-free and contain a neutral pH value.