What nutrients are in spent coffee grounds?

November 26, 2025 •

4 min read

Annually, an estimated 6–8 million tonnes of coffee waste, primarily spent coffee grounds (SCG), are produced globally. While often discarded, these spent coffee grounds contain a surprising array of valuable nutrients and organic compounds that can be beneficially repurposed, especially for gardening and soil health.

Quick Summary

Spent coffee grounds are a source of valuable organic matter, including nitrogen, potassium, and phosphorus. They can improve soil structure and fertilize plants when composted correctly, offering a sustainable way to reuse a common waste product.

Key Points

Rich in Macronutrients: Spent coffee grounds contain important macronutrients like nitrogen, phosphorus, and high levels of potassium, which benefit plant growth.
Contains Micronutrients: Essential micronutrients such as iron, manganese, and copper are also present, contributing to overall plant health.
Improves Soil Structure: The high organic matter and fiber content improve soil aeration, drainage, and water retention.
Best When Composted: Composting spent coffee grounds is the safest and most effective method to neutralize their acidity and high C:N ratio, which can otherwise inhibit plant growth.
A Natural Pest Repellent: The texture and trace caffeine in coffee grounds can help deter pests like slugs and snails.
Attracts Earthworms: Coffee grounds are a favorite of earthworms, which help to aerate soil and break down organic matter.

The Chemical and Nutrient Profile of Spent Coffee Grounds

When coffee is brewed, many water-soluble compounds, including most of the caffeine and some minerals, are extracted into the beverage. However, the remaining solid mass—spent coffee grounds (SCG)—still holds a rich and complex nutrient profile. This profile makes them a valuable organic resource, particularly in agriculture and soil amendment. The composition varies depending on factors like the coffee bean species, roast level, and brewing method, but general characteristics hold true across most grounds.

Key Macronutrients

Spent coffee grounds contain several key macronutrients essential for plant growth. While the brewing process removes some nutrients, the solid matter retains significant quantities, which are released as the grounds decompose.

Nitrogen (N): Spent coffee grounds are relatively high in nitrogen, particularly after brewing removes easily soluble compounds. This nitrogen is initially in a slow-release, organic form that requires decomposition to become available to plants. As a result, adding large amounts of un-composted grounds directly to soil can lead to temporary nitrogen depletion as microbes consume it to break down the organic matter.
Phosphorus (P): An important nutrient for root development and flowering, phosphorus is also present in spent coffee grounds. Levels can vary, but it contributes to the overall fertilizing effect when incorporated into the soil. Some studies report approximately 1.8 grams of phosphorus per kilogram of dry spent coffee grounds.
Potassium (K): Potassium is one of the most abundant macronutrients remaining in spent grounds, crucial for plant immunity and water regulation. It is a major macroelement, with one study reporting levels as high as 11,700 mg/kg.
Calcium (Ca) and Magnesium (Mg): These minerals are important for overall plant health and are also found in spent coffee grounds. Magnesium is vital for chlorophyll production, and calcium contributes to cell wall strength.

Essential Micronutrients

Beyond the primary N-P-K elements, spent coffee grounds also provide a range of micronutrients that are important for plant function and health.

Iron (Fe): Present in spent grounds, iron is a necessary component for many plant processes, including photosynthesis.
Manganese (Mn): This micronutrient assists with photosynthesis and other metabolic processes in plants.
Copper (Cu): Although present in smaller amounts, copper is an essential trace mineral for plant enzymes.
Zinc (Zn): Zinc is a component of many plant enzymes and proteins.

Organic Compounds and Their Role

In addition to minerals, spent coffee grounds are composed of significant organic compounds that contribute to soil health and act as bioactive agents.

Dietary Fiber: The largest component of spent coffee grounds is dietary fiber, including hemicellulose, cellulose, and lignin. This fiber is crucial for adding bulk and improving the physical structure of the soil, enhancing aeration and moisture retention.
Proteins: While some protein content is lost during brewing, a notable amount remains in the grounds. These proteins decompose slowly, contributing to the long-term nitrogen supply.
Lipids: The brewing process does not extract much of the oil, leaving behind significant levels of lipids, which provide valuable organic matter.
Bioactive Compounds: Trace amounts of bioactive compounds like polyphenols (e.g., chlorogenic acids) and a small amount of residual caffeine persist in the grounds. These compounds have complex effects, with some acting as pest deterrents. However, fresh grounds' acidity and remaining caffeine can inhibit the growth of certain plants and seedlings.

The Role of Composting

To maximize the benefits and mitigate the drawbacks of using spent coffee grounds, composting is often recommended. Composting neutralizes residual acidity and breaks down potentially phytotoxic compounds like caffeine and phenols.

Balances Nutrients: The high carbon-to-nitrogen (C:N) ratio of raw grounds can cause nitrogen immobilization in soil. Mixing spent coffee grounds with 'brown' materials, like dry leaves, helps balance this ratio and accelerates the decomposition process.
Eliminates Phytotoxins: Decomposition helps break down compounds that can be harmful to seedlings or sensitive plants.
Creates Humus: The end product of composting is humus, a dark, rich organic material that improves soil fertility and water retention far more effectively than fresh grounds.

Spent vs. Fresh Coffee Grounds: A Comparison


Feature	Fresh Coffee Grounds	Spent Coffee Grounds
pH Level	Moderately Acidic	Closer to Neutral
Nitrogen Content	Higher Initial Nitrogen (organic form)	Slower Release Nitrogen (organic form)
Caffeine Level	Higher Concentration	Significantly Lower (most extracted during brewing)
Tannins & Phenols	Higher Concentration	Significantly Lower (most extracted during brewing)
Immediate Plant Impact	Can inhibit growth of sensitive plants; may cause nitrogen depletion if not composted properly	Generally safer for direct soil application, especially after aging or composting
Best Application	Better for composting to break down compounds and balance nutrients.	Excellent as a compost additive or used sparingly as a top-dressing for established plants

Conclusion

Spent coffee grounds are a valuable, nutrient-rich byproduct of coffee consumption. Their composition includes a wide range of organic compounds and minerals, notably nitrogen, phosphorus, potassium, calcium, and magnesium. While fresh grounds can present challenges due to potential phytotoxicity and nitrogen immobilization, composting them effectively transforms this waste into a potent, slow-release fertilizer and soil conditioner. By understanding what nutrients are in spent coffee grounds and using them appropriately, home gardeners and commercial cultivators alike can contribute to sustainable waste management and cultivate healthier, more robust plants.

For further reading on how to effectively integrate coffee grounds into your gardening practices, consider resources like the Oregon State University Extension, which offers science-based advice(https://extension.oregonstate.edu/news/used-appropriately-coffee-grounds-improve-soil-kill-slugs).

Frequently Asked Questions

Brewed or spent coffee grounds are surprisingly close to neutral pH, not highly acidic. While fresh grounds are acidic, the brewing process washes out most of the acid. When added to soil in moderation or composted, they do not significantly alter soil pH.

Yes, but in moderation. Applying a thin layer or raking it into the topsoil prevents the grounds from clumping and forming a barrier that repels water. For seedlings or sensitive plants, composting is the safer option to avoid potential caffeine and acidity effects.

The safest and most beneficial method is to add spent coffee grounds to your compost pile. This balances the carbon-to-nitrogen ratio, accelerates decomposition, and creates a nutrient-rich, balanced fertilizer for all your plants.

Yes, research suggests that the texture and residual caffeine in spent coffee grounds can deter soft-bodied pests like slugs and snails. A light top-dressing can create a repellent barrier around vulnerable plants.

Spent coffee grounds are known to attract earthworms, which are beneficial for soil health. Worms consume the grounds and help incorporate organic matter into the soil, improving its structure and fertility.

Coffee grounds are considered a 'green' or nitrogen-rich material in composting. They help balance the high-carbon 'brown' materials and accelerate the decomposition process by providing food for the microbes that break down organic waste.

While generally safe for the garden, coffee grounds contain small amounts of caffeine that are toxic to pets, especially dogs, if ingested in large quantities. It is best to bury grounds in compost or rake them into the soil to prevent access.