GardenHacker:
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Comfrey: Sustainable Gardening's Deep Rooted Friend

comfrey Fertilizer: "...CORRECT THE NUTRIENT BALANCE Although the high potassium content in this liquid fertilizer promotes flowers and fruit, it may be considered too high for general use. It can however be adapted by mixing with other garden-made fertilizers. To make a more evenly balanced fertilizer may I suggest mixing 1 part Comfrey liquid with about 19 parts worm tea for an estimated N.P.K ratio of 2.5 : 2.2 : 2.5 % - then dilute. (This assumes that worm tea has the same composition as the worm casts and of course worm casts will vary in composition according to how the worms are fed.) ..." Link
"... Comfrey fertilizer: With its' high levels of potash comfrey tea can be used as an excellent fertilizer for tomato, pepper, cucumber and potato plants. The smell while it is "cooking" is strong. Pick a good sized handful of leaves. Place them in a container with enough water to cover the leaves. Cover and let this cook for 4 weeks in cool weather or 2 weeks in hot weather. Then squeeze the leaves to extract as much juice as possible Strain and use at a rate of 1/3 cup 0f comfrey juice to one gallon of water Use as a foliar feed and soil drench around the plants. Put the solid wastes into the compost pile. ..." Link
"...Lastly, comfrey is used for fertilizer. The leaves decompose easily, thus are used to accelerate compost. Comfrey leaves are used as mulch, as well as to line potato, sweet pea and bean trenches for nutrients. A fertilizer can be made by taking a water-tight container and putting a bunch of ripped up comfrey leaves in it, then water to cover them. Seal and leave alone for 2 weeks in hot weather and 4 weeks in cold weather. Drain off the foul smelling liquid, as it is full of potash, which tomatoes, potatoes, cucumbers and peppers thrive on. Mix 1/3 of the comfrey liquid with one gallon of water as fertilizer to water plants with. ..." Link
Comfrey. (herb plant) In my garden my main source of fertilizer is the herb plant comfrey as a liquid. At the same time it is a foliage spray and insect deterrent. It is also a compost activator. The leaves will break down the compost heap in half the usual time (in two to three months). Or just chop up the leaves and place around the garden. You can also try trench digging. Dig a trench 150mm wide 150mm deep as long as you require, then half fill the trench with chopped up comfrey leaves, replace the soil, then sow your seeds on top. As the leaves decompose they will feed your plants the nutrients they require Comfrey has nitrogen, calcium, potash and phosphorus, greater than any animal manures, and it is also a good foliage spray. The average garden will require around seven plants for a continual supply of leaves all year around. Link