“Have you heard about comfrey as being an organic fertilizer for gardens, and do you have any additional information on it?” Question from Sylvia of Belle Plaine, Minnesota
Answer: The claim is that the old-fashioned herb comfrey (Symphytum officinale) is a “dynamic accumulator,” or a plant that absorbs and retains higher amounts (10 x or more) of essential macronutrients, such as nitrogen, phosphorus, and potassium, as well as micronutrients. Plants that absorb and retain lots of nutrients make great natural fertilizers that boost compost and can be used as green manure crops or to make fertilizer teas.
Some research has been done on the subject, and despite the claims, comfrey does not fall into the category of a dynamic accumulator. It only has a Nitrogen, Phosphorus, and Potassium (NPK) ratio of between 3-1-5 and 1.8-0.5-5, while other common plant products, such as corn gluten meal (~9-1-0) and soybean meal (~7-1-2) are far more nutritious but still not considered dynamic accumulators. So, comfrey offers some fertility but is not an extraordinary natural fertilizer.
Dynamic Accumulators for Natural Fertilization
But, there are several true dynamic accumulators worth considering for natural garden fertility. Some of the best are common weeds, such as stinging nettle (Urtica dioica) with an estimated NPK of 12.5-8.5-16, lamb’s quarters (Chenopodium album), which is reported to accumulate very high amounts of Nitrogen, Potassium, Phosphorus, Calcium and Manganese, horsetail (Equisetum arvense), which accumulates high amounts of Potassium, Phosphorus, and tons of Silicon, amaranthus (Amaranthus spp.), which accumulates very high amounts of Calcium, and finally the humble dandelion, a high accumulator of Iron. Some of the annual weeds, such as amaranth and lamb’s quarters, can be planted as green manure crops and tilled under before they set seed.
Finally, don’t forget the power of garden legumes, such as beans, peas, alfalfa, and lupines, for naturally adding nitrogen to the soil. These are some of the finest plants for providing natural fertility to your garden soil. (Click here to read an article about nitrogen-fixing plants.)
I hope that this answer helps!
Happy gardening,
Jessie Keith
Black Gold horticulturist
