Comfrey's Core Mineral Profile
Comfrey, a plant historically used in herbal medicine, has gained significant recognition in gardening for its rich mineral content. Its impressive ability to access and concentrate nutrients is largely attributed to its deep and extensive root system, which can reach depths of up to six to ten feet, mining subsoil nutrients unavailable to many other plants. While this trait has earned it the nickname "dynamic accumulator," some debate the scientific basis of the term, suggesting it is more a testament to comfrey's rapid biomass production than its unique mineral-collecting ability. However, the result is the same: the plant's leaves become packed with a wide array of macro- and micronutrients.
Macronutrients: The Building Blocks
The mineral composition of comfrey includes several key macronutrients essential for robust plant growth, forming the basis of its use as a potent organic fertilizer.
- Potassium (K): Comfrey is notably high in potassium, with some analyses showing its dried leaves containing up to 7.09% potash. This makes it a fantastic nutrient source for fruiting and flowering plants like tomatoes and peppers, and it is reported to contain two to three times more potassium than farmyard manure.
- Phosphorus (P): The plant also provides significant levels of phosphorus, with dried leaves containing around 0.75% phosphoric acid. This nutrient is crucial for flower and fruit development.
- Nitrogen (N): As a leafy, fast-growing plant, comfrey contains a respectable amount of nitrogen, making it a valuable "green" material for compost heaps.
- Calcium (Ca): Rich in calcium, comfrey leaves can contain nearly 3% of this important mineral. Calcium supports strong plant cell walls and overall plant health.
Micronutrients and Trace Elements
Beyond the primary macronutrients, what minerals does comfrey contain in smaller, but still significant, quantities?
- Iron (Fe): Comfrey contains iron, a micronutrient vital for photosynthesis. A specific analysis found dried Bocking 14 comfrey to have 0.144% iron content.
- Magnesium (Mg): This mineral is essential for chlorophyll production, and comfrey is known to contain it.
- Sulfur (S) and Zinc (Zn): Trace amounts of sulphur and zinc are also present, contributing to the plant's overall nutritious profile.
Comfrey's Practical Uses in Gardening
Because of its rich mineral composition, comfrey is a prized resource for organic gardeners.
- Liquid Manure or 'Comfrey Tea': Gardeners ferment comfrey leaves in water for several weeks, creating a potent, concentrated liquid feed. This is particularly beneficial for heavy feeders.
- Nutrient-Rich Mulch: Chopped comfrey leaves can be spread around plants as a mulch. As they decompose, they release their minerals and add organic matter to the soil.
- Compost Activator: Adding comfrey leaves to a compost pile accelerates decomposition due to its high nitrogen content, helping to balance carbon-rich materials.
- Trench Fertilizer: A layer of comfrey leaves placed in the bottom of a trench or planting hole for crops like potatoes acts as a slow-release fertilizer.
Comparison of Comfrey to Other Organic Fertilizers
While comfrey is a remarkable source of minerals, it's useful to compare its content to other common organic materials used in gardening.
| Feature | Comfrey | Stinging Nettle | Kelp Meal | Farmyard Manure |
|---|---|---|---|---|
| Potassium Content | High (e.g., 7.09%) | Moderate | Moderate (e.g., 2.5%) | Lower |
| Nitrogen Content | High | Very High | Low (e.g., 1.0%) | Low to Moderate |
| Phosphorus Content | Moderate (e.g., 0.75%) | Moderate | Low (e.g., 0.5%) | Low to Moderate |
| Sourcing | Can be grown at home | Can be grown or foraged | Commercial product | Commercial product/farm |
| Nutrient Release | Medium to Fast | Fast | Slow | Medium |
Important Considerations: Comfrey Toxicity
It is crucial to remember that while comfrey is an exceptional garden resource, it is highly toxic when ingested. Comfrey contains liver-damaging chemicals called pyrrolizidine alkaloids (PAs), which are carcinogenic. For this reason, oral preparations of comfrey have been banned in the United States and other countries. Even topical use on broken skin is cautioned against, as the toxins can be absorbed. Gardeners should use comfrey responsibly, wearing gloves when handling the leaves and ensuring it is only used for horticultural purposes. The European Medicines Agency provides extensive information on the risks associated with comfrey.
Conclusion
What minerals does comfrey contain? A powerful blend of macro- and micronutrients, including high levels of potassium, calcium, and phosphorus. Thanks to its deep taproots, comfrey excels at gathering these minerals from the subsoil, transferring them into its lush foliage. This makes it an incredibly valuable asset in organic gardening for creating mineral-rich mulches, compost activators, and liquid fertilizers. However, its use requires caution and is strictly limited to external, horticultural applications due to the presence of toxic pyrrolizidine alkaloids. Understanding both its potent nutritional value and its inherent risks is essential for any gardener looking to harness the power of this remarkable plant.
For more detailed scientific information on comfrey, consult resources such as the National Center for Biotechnology Information at the NCBI Bookshelf.
