I have my own little organic fertilizer factory, cranking out free mulch, compost activator, and a potent plant food.
The fuel for this factory is Russian comfrey (Symphytum X uplandicum). It has 6-foot-long roots that harvest nutrients from deep in the soil, making comfrey leaves a fantastic natural source of nitrogen, phosphorus, and potassium.
Researchers in British Columbia analyzed the NPK (nitrogen, phosphorus, and potassium) ratio of comfrey leaves by air-drying them and analyzing the powdered leaf tissues. They found that the leaves have an impressive proportion of 1.8-0.5-5.3. To compare, kelp meal has an NPK ratio of 1.0-0.5-2.5, and homemade compost ranges from 0.5-0.5-0.5 to 4-4-4 (depending on what ingredients you use). Comfrey is also rich in calcium and many other valuable plant nutrients it mines from deep in the subsoil.
Harnessing the Power
Mulch. Freshly cut comfrey leaves make good mulch because they're high in nitrogen, so they don't pull nitrogen from the soil while decomposing, as high-carbon mulches like straw and leaves do. And comfrey's high potassium content makes it especially beneficial for flowers, vegetables (such as tomatoes, peppers, and cucumbers), berries, and fruit trees. Michelle DeFord, owner of Crimson Sage Nursery, in Colton, Oregon, has mulched her stock herb plants with comfrey for 25 years because it boosts seed yields. For home gardeners, this means increased flower and fruit production. But using comfrey to mulch root crops (like carrots) or leafy greens (like lettuce and spinach) may encourage them to go to seed prematurely.