Will coffee grounds lower pH? Debunking the acidity myth for gardeners

December 2, 2025 •

4 min read

Research from Oregon State University has shown that after brewing, coffee grounds are surprisingly close to pH neutral, not highly acidic. This debunks the long-held gardening myth that coffee grounds will lower pH and dramatically acidify your soil for acid-loving plants.

Quick Summary

Used coffee grounds are nearly pH neutral and will not significantly lower soil pH. They are, however, a beneficial organic soil amendment when used correctly.

Key Points

Used Grounds are Neutral: After brewing, coffee grounds are not highly acidic; their pH is close to neutral (6.5 to 6.8).
Acids are Water-Soluble: The acidic components are washed out into your coffee, leaving the grounds with a different chemical makeup.
Adds Valuable Nutrients: Coffee grounds are rich in nitrogen and other essential nutrients that act as a slow-release fertilizer.
Enhances Soil Structure: Incorporating grounds improves soil aeration, drainage, and water retention.
Use in Compost Safely: The best way to use coffee grounds is by adding them to compost, which neutralizes residual compounds and prevents clumping.
Apply Sparingly: Avoid applying thick layers directly to soil, as they can form a water-repellent crust that hinders plant growth.
Not for All Plants: While many plants benefit, those sensitive to caffeine or preferring alkaline soil may be negatively affected.

Understanding the pH of Coffee Grounds

Many home gardeners believe that because coffee itself is acidic, the leftover grounds must also be acidic. While fresh, unbrewed coffee grounds can have a pH around 5 to 5.5, the brewing process changes their chemical composition significantly. The water-soluble acidic compounds are extracted into your cup of coffee, leaving the used grounds with a pH much closer to neutral, typically ranging from 6.5 to 6.8. This slight acidity is not enough to make a meaningful, lasting impact on the pH of most garden soils.

Fresh vs. Used Coffee Grounds: Why pH Levels Differ

The fundamental difference lies in what happens during the brewing process. When you pour hot water over coffee grounds, the acidic components and caffeine are washed out. What remains is an organic material rich in nutrients but with a much higher, near-neutral pH. Applying fresh, unbrewed grounds to your garden is not recommended, as the higher caffeine content can harm earthworms and stunt the growth of young plants.

The Real Benefits of Adding Coffee Grounds to Soil

If not for altering pH, then why do so many gardeners swear by coffee grounds? The benefits come from their value as a nutrient-rich organic matter. Coffee grounds improve soil health in several key ways:

Adds Nutrients: Used grounds contain essential nutrients like nitrogen, potassium, and phosphorus, which are released slowly as they decompose. They also provide micronutrients such as calcium, magnesium, and copper.
Improves Soil Structure: Incorporating grounds into the soil enhances its texture. This helps improve drainage and aeration, which is vital for healthy root growth.
Attracts Earthworms: Earthworms are drawn to coffee grounds and help to aerate the soil and break down organic matter, further enriching it.
Suppresses Weeds: When used as a mulch, coffee grounds can help suppress weed growth, although their effectiveness is debated and best when mixed with other mulch materials.
Deters Pests: Some studies suggest that the caffeine in grounds can help repel slugs and snails, though results vary.

Comparison Table: Fresh vs. Used Coffee Grounds


Feature	Fresh Coffee Grounds	Used Coffee Grounds
pH Level	Acidic (pH 5.0–5.5)	Nearly neutral (pH 6.5–6.8)
Caffeine Content	High	Low (most washed out)
Recommended Use	Avoid direct application to soil; best for very specific composting mixes.	Safe for compost, can be mixed into soil or used as mulch sparingly.
Effect on Plants	Can inhibit seed germination and stunt growth.	Acts as a slow-release fertilizer and soil amendment.
Risk of Compaction	High if applied thickly, forming a water-resistant crust.	High if applied thickly; mixing with other materials is best.

How to Properly Use Coffee Grounds in Your Garden

To reap the benefits without the risks, proper application is key. Follow these methods for success:

Compost Them: This is the safest and most effective method. Add used grounds to your compost pile, mixing them with carbon-rich materials (browns) like shredded leaves or cardboard. This helps balance the nitrogen-rich coffee grounds and prevents potential mold growth.
Mix into Soil: If you choose to add grounds directly to your garden beds, use them sparingly. Work a thin, half-inch layer of dried grounds into the top few inches of soil to avoid clumping and water resistance.
Create a Liquid Fertilizer: Mix 2 cups of used coffee grounds with a 5-gallon bucket of water and let it steep for 24-48 hours. This “coffee ground tea” can be used to water plants, providing a gentle nutrient boost.

Plants that May or May Not Like Coffee Grounds

While coffee grounds are a great amendment for many gardens, it’s important to consider your plants' specific needs. Some plants thrive on the nutrients they provide, while others are less tolerant.

Plants that can benefit from coffee grounds:

Azaleas
Rhododendrons
Camellias
Blueberries
Hydrangeas
Carrots
Roses
Leafy greens (lettuce, spinach)

Plants that are sensitive to coffee grounds (due to caffeine or alkalinity preference):

Lavender
Rosemary
Sunflower
Leeks
Geraniums
Asparagus ferns

Conclusion: The Myth and the Reality

The myth that coffee grounds will significantly lower soil pH is a misconception. Used coffee grounds are practically neutral in pH, and any acidifying effect on soil is minimal and temporary. However, this doesn't mean they are useless in the garden. By adding them to your compost or mixing them sparingly into the soil, you can improve its structure, provide valuable nutrients, and promote a healthier ecosystem that benefits many plants. Always use moderation and consider your plants' specific needs to ensure they thrive.

For more information on soil science and proper soil amendments, consider consulting your local university extension service or a reliable resource such as the Soil Quality Knowledge Base.

Frequently Asked Questions

Yes, fresh coffee grounds are acidic (pH 5.0–5.5) and can slightly lower soil pH temporarily. However, applying them directly is not recommended due to high caffeine content, which can harm plants and soil life.

Used coffee grounds are close to neutral, with a pH typically between 6.5 and 6.8. This is because the acidic compounds are extracted into the brewed coffee.

It is generally safe to mix small amounts of dried, used grounds into the topsoil. Applying a thick layer on the surface can cause clumping and prevent water from reaching roots, which can be harmful.

Plants that thrive on the nutrients in coffee grounds include azaleas, blueberries, rhododendrons, and leafy greens. Acid-loving plants may benefit from the nutrient boost, though the pH change is minimal.

Some plants prefer alkaline conditions or are sensitive to caffeine. These include lavender, rosemary, geraniums, and asparagus ferns.

Yes, used coffee grounds are an excellent compost ingredient. They are considered a 'green' material, adding nitrogen and attracting earthworms to your compost pile.

Some gardeners report that the caffeine in coffee grounds can deter slugs and snails, but the results are not always consistent. It's often recommended to use them alongside other pest control methods.