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, seeds 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 utilized as a fertilizer, these include:
Comfrey for potatoes – freshly cut comfrey should be wilted for a day or two, then laid along potato trenches about 2 inches deep. Avoid using flowering stems as these can root. The leaves will rapidly break down and supply potassium rich fertilizer for the developing potato plants.
Comfrey as a compost activator– include 2-3 inch deep layers of comfrey in the compost heap to encourage bacterial activity 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– a 2 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 tomatoes, and also fruit bushes like gooseberries and currants.