Dynamic Accumulators? This gobbledegook does nothing for the reputation of permaculture.

I have been using comfrey, both Symphytum officinale and Symphytum x uplandicum, as a green manure and liquid fertiliser for about forty years now without knowing it is a 'dynamic accumulator.'  Henry Doubleday and Lawrence Hills both researched and promoted the use of comfrey as a fertiliser.

Comfrey leaves in the butt.

The comfrey butt is a large water bin, which contains lots of comfrey leaves rotting down and producing a dark thick liquid.  I also add Urtica dioica and Myrrhis odorata to the mix to add a little variety.  These leaves rot down in the butt to a liquid that can be used as a fertiliser.

Comfrey liquid being collected from the butt.

Myrrhis odorata added to the comfrey butt

Comfrey growing and green comfrey butt.

I find the term 'dynamic accumulator' confusing. I cannot find any scientific research papers that refer to dynamic accumulators.

There is a lot of information on various different permaculture sites with long lists of dynamic accumulator plants. However, there is no referencing to scientific research or an indication of whether all these suggested plants are of any use at all.  Ken Thompson's book (2006) "Ear to the Ground" says that comfrey is rich in the big three mineral nutrients, nitrogen, phosphorous and potassium. However, comfrey is not unique in this and some plants may be a little better including Stellaria media, Urtica dioica, Myrrhis odorata, Chenopodium album, Gallium aparine and Alliaria petiolata.   

One of the advantages of using comfrey is that it all rots down very quickly producing a very humus looking liquid which is easily diluted and applied to the soil with a watering can.  However as White (2006) says in his book "Principals and practice of Soil Science"; 'The long term effect of green manuring on soil organic matter is minimal since the succulent residues are rapidly decomposed and contribute little to the soil humus.  Similarly, beneficial effects on soil structure through the stimulation of microbial activity are ephemeral.'

It is suggested in permaculture that dynamic accumulators are deep rooted and obtain their nutrients from deep in the soil.  I can find no research that indicates that deep rooted plants have a higher proportion of nutrients than shallow rooted ones or where the nutrients have come from in the soil profile.  Indeed,  Urtica dioica is not deep rooted but it is a valuable addition to the comfrey butt.

I do not confine my additions to the compost heap to those plants with deeply penetrating tap roots. I put all sorts of plants on the heap.  They all accumulate nutrients - they have to to live.
I am told that grass mowings are high in nitrogen and you have to do all sorts of complicated things to compost them as a result; and grass has very short adventitious roots.

Why is turf so fertile and why does grass seems to accumulate relatively 'large' amounts of nitrogen? Many monocotyledons start off with a normal 'root' as they germinate but this soon withers away and is replaced by adventitious roots that form from buds from nodes at the crown. These adventitious roots are very fibrous and form a dense sward full of roots. This denseness of root compounded by stolons, rhizomes and ramets seems to be a highly efficient Hoover of nitrogen. One could call it a dynamic accumulation; however I would not use the word dynamic in relation to plants without taking a lot of advice. The point is; short, thin, fibrous roots accumulate relatively lots of nitrogen compared to long, thick tap rooted ones.

The term may have come from the study of plant nutrition and the storage of heavy metals in plant cell vacuoles.  Some plants can accumulate high levels of certain heavy metals and these plants are called metal hyperaccumulators.  Some hyperaccumulators can live in highly polluted soil and can be used to remove heavy metal contaminants.
See Prasad  (2003) "Metal hyperaccumulation in plants- Biodiversity prospecting for phytoremediation technology"

There are other plants that are tolerant of heavy metal contamination but these do not store large quantities of heavy metals within their leaves.  They regulate the uptake of these metals through control measures in their roots.

Whether they have deep tap roots or adventitious roots seems to be irrelevant.   McCutcheon & Schnoor 2003 in , "Phytoremediation." New Jersey, John Wiley & Sons pg 19 say that mustard is a hyperaccumulator of strontium, cadmium, caesium, nickle and zinc - and mustard roots seldom penetrate subsoil.

There is evidence that elements such as potassium are lifted up the soil profile by the action of some pine trees in Argentina.  Also the lifting of manganese by eucalyptus trees.    These elements are taken to leaves and stems and deposited on the soil surface when plants loose leaves, loose branches or die.

Jobbagy and Jackson (2004) "The uplift of soil nutrients by plants; biogeochemical consequences across scales."

However, all plants have some heavy metals within their structures and will deposit them on the surface of the soil when they die or shed leaves. That is why chop and drop is an effective way of adding nutrients to the top soil.  To say some plants do not accumulate nutrients seems to be illogical.  They must do to grow.  We are also talking about tiny, tiny amounts even of the major nutrients like nitrogen, phosphorus and potassium so the relative accumulation of nutrients is a comparison of milligrams.  What are we comparing anyway the amount of nutrients accumulated by an oak tree compared to comfrey?  I think that I know which would be the dynamic accumulator in this comparison.

I know there has been research done on Symphytum officinale and Symphytum x uplandicum but has there been any research on other plants?
Hills L.D. (2008) "Comfrey:Past, Present and Future"
Hills L.D. (1955) "Russian Comfrey Report No1"

Hills did indicate that he thought there were other beneficial plants that could be used for fertiliser and at least tried to do this systematically.
See http://eap.mcgill.ca/publications/EAP67.htm

From the blog "One thing leads to another"
https://portageperennials.wordpress.com/2014/03/18/nutrient-accumulators/
It seems that people are just repeating gobbledegook and just like  a game of Chinese whispers adding plants to lists as their flights of fancy takes them.
This does no service to permaculture.

All plants accumulate nutrients but not necessarily in the same amounts, however the amounts are minute and differences may not be significant.  Comfrey is useful because it decomposes quickly and the resultant liquid can be used easily to fertilise the ground.  Comfrey does not necessarily have "more" nutrient than any other plant.