Does Comfrey Really Improve Soil?

Comfrey (Symphytum spp.) has a great reputation as a "dynamic accumulator" of soil nutrients, but there's precious little published evidence of this fact.  I decided to do my part to change that, with the help of some before-and-after soil tests.

Read the whole story with comments at Permaculture News, or just the article and my updates below.

All of us who have studied permaculture have heard some impressive claims about comfrey. It is a dynamic nutrient accumulator; it improves the soil; it is “a slow motion fountain” of nutrients, bringing them up from the subsoil to improve the topsoil. We’ve heard lots of anecdotal evidence, but where is the empirical data for these claims? Peter Harper’s article in The Land last summer, “Permaculture: the Big Rock Candy Mountain,” made me want to find the scientific basis behind some of the anecdotal claims I’d heard other permaculturalists make, that I’d previously absorbed without question.

I ran to my books. From Introduction to Permaculture by Mollison and Slay to Gaia’s Garden by Hemenway to Edible Forest Gardens (Volume I & Volume II) by Jacke and Toensmeier, it is difficult to tell where any specific piece of information comes from, since all these books lack footnotes or endnotes. The Wikipedia article is peppered with “[citation needed]” and has been for years. The online database that comes closest to citing a source is Plants for a Future, which says comfrey’s value in compost was established by “a small booklet” (no indication where to obtain it) and the New RHS Dictionary of Gardening, which “contains a number of silly mistakes.” Clearly some newer and more conclusive evidence is in order!

I was feeling pretty foolish about planting so much comfrey around my small urban farm (about 20 plants in all) without any hard evidence for its effectiveness, when I realized I could just do a soil test and compare it to my past years’ tests, and then I would have at least one data point to support or refute the claims. The test results are pictured here, and they will benefit from some explanation.

The first test is from February 2009, after our front yard had sat under sheet mulch all winter and our back yard was still mostly turf grass. The mulched front yard already shows a lower pH (we have alkaline clay soil here, so lower is better), more than double the organic matter, three times the nitrate, and about 50% higher phosphorus and potassium.

By the time of the second test in 2011, I had sheet mulched the back yard as well, but I distinguished between a “raised” bed where a sewer repair had exposed the heavy clay subsoil and a greenhouse area which had not been excavated. The greenhouse bed shows higher nutrient levels than either the front yard or the raised bed, with the exception of potassium, but even this is higher than it had been two years before.

So far so good, and I think I’ve made a good case for sheet mulching, but all these figures are blown away by the sample I took this year under the comfrey plants. After 5 years of comfrey, the topsoil in this sample shows a lower pH and higher percent organic matter than any of the previous samples, and the nutrient levels are practically off the charts – a 47 to 232% increase over the previously observed highs. I did not test for calcium or magnesium either before or after, but just on the basis of NPK the comfrey is completely vindicated.

It appears I owe comfrey and its promoters an apology, but still this is just one data point. To really make the case for comfrey, we will need lots of data from lots of soil types, and we need to publish the results in a reputable source that can be cited by Wikipedia, PFAF, and all the reference books we turn to for authoritative information. And we need to collect negative results (if any) as well as positive ones. Do you have any data on comfrey to share? Please let me know in the comments below!

[February 3, 2017] Follow-up to the original article: I just got a new set of soil test results, and they are discouraging. The sample taken from beneath comfrey plants has regressed toward the mean on all factors — that is, it the soil there now tests pretty much the same as the soil in nearby beds that do not have comfrey. Either one of tests was in error (I should repeat the test to figure out which is correct), or comfrey plants only improve soil for the first few years of growth and in their maturity use the nutrients they have brought up.

Here’s a more in-depth article about my soil tests which show the soil under comfrey plants regressing toward the mean over time: /content/soil_tests_after_9_years_no-spray_no-till