Comfrey, scientifically known as Symphytum officinale or the sterile hybrid S. x uplandicum, is a robust perennial plant often found thriving in gardens. Its vigorous growth and nutrient-rich leaves have long made it a subject of fascination among gardeners and soil scientists. People frequently question whether this perennial herb can fix nitrogen from the air, an ability found only in specific plant groups. This discussion clarifies Comfrey’s function in soil fertility, distinguishing it from true nitrogen fixers, and explains its celebrated role as a dynamic accumulator.
Understanding Nitrogen Fixation
Nitrogen fixation is a specialized biological process that converts atmospheric nitrogen gas (N₂) into forms usable by plants, such as ammonia (NH₃). Gaseous nitrogen makes up about 78% of the air, but its strong triple bond makes it inaccessible to most life forms, including Comfrey. This conversion is carried out by certain microorganisms, known as diazotrophs, which possess the unique nitrogenase enzyme.
The most common example involves a symbiotic relationship between these bacteria, such as Rhizobium, and the roots of plants in the legume family (e.g., beans, peas, and clover). The bacteria live inside specialized structures on the roots called nodules, where they perform the chemical conversion. The plant provides the bacteria with energy and a protected environment, while receiving fixed nitrogen in return, directly enriching the soil.
Comfrey’s Role in Soil Biology
Comfrey is not a nitrogen fixer, despite its reputation for producing large amounts of nitrogen-rich foliage. It does not engage in the symbiotic relationship with diazotrophic bacteria that is characteristic of true nitrogen-fixing plants. The Comfrey root system completely lacks the specialized nodules required to house the microorganisms that perform this atmospheric conversion.
The common confusion arises from the plant’s extremely fast and lush growth, which suggests it is generating its own nitrogen supply. However, Comfrey derives its nitrogen entirely from existing soil sources, just like most other plants. Its high nitrogen content in the leaves is a result of its effective nutrient uptake and rapid transportation of these compounds to its above-ground biomass.
The Concept of Dynamic Accumulation
Comfrey functions as a dynamic accumulator, a term used to describe plants with the ability to draw up minerals from the subsoil. This accumulation is possible due to its powerful, deep taproot system, which can penetrate far beyond the reach of most garden plants. Comfrey’s roots can extend several feet down, often reaching mineral-rich layers that are unavailable to shallow-rooted annuals.
The plant acts as a biological pump, drawing up trace minerals and nutrients, including Calcium, Phosphorus, and notable quantities of Potassium, from the subsoil. These accumulated minerals are then stored in the large leaves and stems of the plant. By storing these compounds, Comfrey effectively cycles locked-up nutrients from deeper soil layers, making them available near the surface when the leaves decompose.
Practical Applications for Soil Health
Gardeners utilize Comfrey’s dynamic accumulation by harvesting its nutrient-dense leaves multiple times throughout the growing season. These methods complete the nutrient cycle, transforming deep-mined subsoil minerals into a bioavailable food source for the garden.
Chop and Drop
The simplest application is the “chop and drop” method, where the leaves are cut and placed directly on the soil surface as a green mulch. As this mulch decomposes, the stored minerals are released into the topsoil, becoming readily available for surrounding plants.
Liquid Feed (Comfrey Tea)
Another popular method is creating a liquid feed, often referred to as Comfrey tea or “com-fert.” This involves steeping the cut leaves in water for several weeks, resulting in a concentrated, mineral-rich liquid fertilizer known for its high Potassium content.
Compost Activator
The leaves can also be added directly to compost piles. Their rapid decomposition and high nitrogen content act as an activator, speeding up the overall composting process.