Are you worried about adding clay soil to the potting mix, as many gardeners claim it makes the substrate too 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.
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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.
It has to do with the leaching of organic acid (from microorganisms such as fungi, yeast, and bacteria) that makes it denser and sterile.
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 Type | Specification |
|---|---|
| Silty clay | It contains over 50-80% silt contents. |
| Sandy clay | It contains over 50% sand content. |
| Clay loam | It is a mixture of clay, sand, and silt with additional organic matter. |
| Silty clay loam | It boasts a mixture of clay, sand, and significant amount of silt. |
| Sandy clay loam | It 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.
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.
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.
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.
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.
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.
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.
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.
| Chemicals | Specifications |
|---|---|
| Elemental Sulfur | When mixing elemental sulfur with clay soil, the naturally occurring soil bacteria transform it into sulfuric acid. |
| Aluminum Sulfate | The metal salt contains a deficient pH level, usually 1.8-2.2, providing an immediate acid boost to the soil. |
| Iron Sulfate | This method relies on a chemical reaction to create acid, requiring an ideal soil conditions. |
| Ammonium Sulfate | The 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.
