Comfrey

Comfrey (also comphrey) is an important herb in organic gardening. It is used as a fertilizer and as an herbal medicine. 

Comfrey is a particularly valuable source of fertility to the organic gardener. It is very deep rooted and acts as a dynamic accumulator, mining a host of nutrients from the soil. These are then made available through its fast-growing leaves (up to 4-5 pounds per plant per cut) which, lacking fibre, quickly break down to a thick black liquid. There is also no risk of nitrogen robbery when comfrey is dug into the soil as the C:N ratio of the leaves is lower than that of well-rotted compost. Comfrey is an excellent source of potassium, an essential plant nutrient needed for flower, seed and fruit production. Its leaves contain 2-3 times more potassium than farmyard manure, mined from deep in the subsoil, tapping into reserves that would not normally be available to plants.

There are various ways in which comfrey can be used as a fertilier. These include:

• Comfrey as a compost activator - include comfrey in the compost heap to add nitrogen and help to heat the heap. Comfrey should not be added in quantity as it will quickly break down into a dark sludgy liquid that needs to be balanced with more fibrous, carbon-rich material.
• Comfrey liquid fertilizer - can be produced by either rotting leaves down in rainwater for 4–5 weeks to produce a ready-to-use 'comfrey tea', or by stacking dry leaves under a weight in a container with a hole in the base. When the leaves decompose a thick black comfrey concentrate is collected. This must be diluted at 15:1 before use.
• Comfrey as a mulch or side dressing - a two-inch layer of comfrey leaves placed around a crop will slowly break down and release plant nutrients; it is especially useful for crops that need extra potassium, such as fruit bearers but also reported to do well for potatoes. Comfrey can be slightly wilted before application optionally but either way, avoid using flowering stems as these can root.
• Comfrey potting mixture - originally devised to utilize peat, now environmental awareness has led to a leaf mold-based alternative being adopted instead; two year old, well decayed leaf mold should be used, this will absorb the nutrient-rich liquid released by the decaying comfrey. In a black plastic sack alternate 3-4 inch layers of leaf mold and chopped comfrey leaves. Add a littledolomitic limestone to slightly raise pH. Leave for between 2–5 months depending on the season, checking that it does not dry out or become too wet. The mixture is ready when the comfrey leaves have rotted and are no longer visible. Use as a general potting compost, although it is too strong for seedlings.

In the school garden we prepared liquid comfrey fertilizer. We cut the leaves of the comfrey and stuffed them into a 'tea bag' that we left inside a container for two weeks. After those two weeks we took our the 'tea bag' and let the comfrey liquid in the container for future use.