Is Comfrey High in Calcium? Understanding Its Nutritional Value

October 28, 2025 •

3 min read

While often praised by gardeners for its nutrient-accumulating properties, analysis of Russian comfrey (Bocking 14) showed it contains approximately 2.77% calcium on a dry matter basis. Comfrey's reputation as a mineral powerhouse stems from its deep root system, which draws up nutrients from inaccessible subsoil. It is indeed high in calcium and many other minerals, though this is primarily relevant for its use as a fertilizer rather than a human supplement due to significant safety concerns.

Quick Summary

Comfrey is rich in calcium and other minerals, making it a valuable organic fertilizer for gardens. However, due to the presence of hepatotoxic pyrrolizidine alkaloids, internal consumption is banned or restricted in many countries. It is best used topically or as a nutrient-rich mulch and compost activator for plants.

Key Points

High in Calcium: Comfrey is rich in calcium and other minerals, but its accumulation varies by species and growing conditions.
For Gardening, Not People: The high mineral content is primarily leveraged in gardening as an organic fertilizer, not for human consumption due to toxicity.
Dynamic Accumulator: The plant's deep taproots draw up minerals like calcium from inaccessible subsoil layers.
Severe Toxicity Risk: All species of comfrey contain liver-damaging pyrrolizidine alkaloids (PAs) that pose a serious risk if ingested.
Internal Use Is Dangerous: Oral consumption of comfrey is banned or heavily restricted in many countries due to the potential for severe liver failure and cancer.
Limited Topical Use: Comfrey ointments are restricted to short-term use on unbroken skin, as PAs can be absorbed through the skin.

Understanding Comfrey's High Calcium Content

Yes, comfrey is recognized for its high mineral content, including significant levels of calcium, which it accumulates from deep within the soil. The plant, often called "knitbone" in folk medicine, possesses an extensive root system that can extend up to 10 feet deep, drawing up valuable minerals that other plants cannot reach. For gardeners, this makes it an excellent organic resource for creating nutrient-rich liquid feeds and mulches. Analyses of the leaves of Russian comfrey, a common garden variety, have confirmed its high calcium levels, along with potassium, nitrogen, and other trace elements.

The Role of Comfrey as a Dynamic Accumulator

Comfrey's ability to draw minerals from the subsoil is a key feature of its reputation. This process, known as dynamic accumulation, is what makes it so valuable in organic gardening. The plant acts as a 'mineral pump,' enriching the topsoil when its leaves are cut and used as mulch or compost. This practice effectively recycles nutrients and makes them available to more shallow-rooted companion plants.

Deep roots: Its taproots access minerals far below the typical root zone of most vegetables and garden plants.
Nutrient cycling: When used as a 'chop and drop' mulch, the decaying leaves release stored minerals back into the soil's top layer.
Compost activator: Adding comfrey leaves to a compost pile accelerates decomposition due to their high nitrogen content and moisture.

Comfrey for Gardening vs. Human Consumption

It is crucial to differentiate between using comfrey as a soil amendment and consuming it as a supplement. While its high mineral content is beneficial for plants, it is not recommended for internal human use due to toxicity.

Comparison: Comfrey for Garden vs. Internal Use


Feature	Garden Use	Internal Human Use
Nutrient Benefit	Enriches soil with deep-mined calcium, potassium, and nitrogen.	Potential source of vitamins and minerals, but carries severe health risks.
Primary Purpose	Creating liquid feeds, mulch, and compost activator.	In traditional folklore, used for bone healing, but now widely condemned.
Safety Concerns	Minimal to none. PAs in the soil are not considered a risk for food crops.	Contains toxic pyrrolizidine alkaloids (PAs) that can cause severe, potentially fatal liver damage.
Regulatory Status	No restrictions on growing as a garden plant.	Banned for oral use in many countries; warnings against topical use on broken skin.
Risk Level	Very low	Very high; potentially lethal.

How Does Comfrey Compare to Other Herbs for Minerals?

While comfrey is a notable accumulator, other herbs also provide significant mineral content, especially nettle. A comparison of comfrey and nettle as liquid feeds shows both have strong nutritional profiles, though their primary mineral concentrations differ.

Comfrey: Higher in potassium, making it an excellent fertilizer for flowering and fruiting plants like tomatoes.
Nettle: Higher in nitrogen, making it a better feed for promoting leafy growth in vegetables like cabbage and kale.

This difference highlights why many organic gardeners value having both plants in their systems, creating specialized fertilizers for different crops.

The Serious Health Risks of Comfrey

For humans, the primary concern is the presence of pyrrolizidine alkaloids (PAs), toxic compounds found in all species of comfrey, particularly in higher concentrations in the roots. When ingested, PAs can cause hepatic veno-occlusive disease (VOD), leading to liver failure, and have been shown to be carcinogenic in animal studies. Regulatory bodies like the US Food and Drug Administration have issued strong warnings against oral comfrey products and advised their removal from the market. Topical use on broken skin is also discouraged due to absorption risks.

Conclusion: A Gardener's Gold, Not a Human Supplement

In summary, comfrey's reputation as a calcium-rich plant is well-deserved, but its high mineral content is most safely and beneficially harnessed in the garden. By acting as a dynamic accumulator, it enhances soil fertility and provides an exceptional organic fertilizer for crops. However, due to its toxicity from pyrrolizidine alkaloids, comfrey should never be taken internally by humans. Its use is best restricted to external applications on unbroken skin or, more safely, as a valuable, nutrient-recycling asset for the garden.

Authoritative link: This detailed review provides an extensive scientific overview of the toxicity and metabolism of pyrrolizidine alkaloids found in comfrey.

Frequently Asked Questions

Comfrey is a 'dynamic accumulator' with deep roots that pull minerals, including calcium, from lower soil layers that are typically out of reach for most plants. These nutrients are then stored in its leaves.

No, it is extremely dangerous to consume comfrey. All species contain pyrrolizidine alkaloids (PAs) that are hepatotoxic and can cause severe liver damage and failure, and they have been shown to be carcinogenic in lab animals.

Gardeners use comfrey primarily as a compost activator, a nutrient-rich mulch, and to create 'comfrey tea'—a liquid fertilizer high in potassium that is excellent for fruiting plants.

The concentration of toxic pyrrolizidine alkaloids (PAs) is generally higher in the roots, but all parts of the comfrey plant contain PAs. Therefore, no part of the comfrey plant should be consumed orally.

Comfrey tea is higher in potassium, which is beneficial for flowering and fruiting crops. Nettle tea is higher in nitrogen, making it better for leafy green vegetables.

No, experts advise against applying comfrey to broken or damaged skin, as the toxic pyrrolizidine alkaloids can be absorbed into the bloodstream. It should only be used topically on unbroken skin for short durations.

The nickname 'knitbone' comes from its historical folk use in poultices to help treat broken bones and promote wound healing, a tradition based on its active compound allantoin.