Cover crop for comfrey patch

Hello all,

Ok, I have to admit, I did ask this question in another thread, but it seems more applicable here (hope this doesn't violate any rules).  At present I have a total of 6 comfrey plants, about 6 months old.  They are doing nicely and I expect to get good green manure next spring.  4 of the 6 are in an area I am thinking about dedicating to a comfrey patch--I could comfortably add 4-6 additional plants here and have plenty of comfrey for just about any reason I want.  However, upon reading more I have found (and this does make sense) that comfrey does not really fix nitrogen but only uses that already in the soil, requiring future N2 inputs.  With that in mind I was thinking about planting a companion plant for the comfrey, preferably a legume.  I was thinking Dutch White Clover as it fixes N2, is easy to control and would not shade out the Comfrey which is my "Crop" for the patch.

It has been suggested to me that DWC might be too small and actually get crowded out by the comfrey and that red clover or alfalfa might be better.  The other two options (especially alfalfa) seem like a difficult plant to control.  With all this in mind, this all boils down to two questions


1.  Is a companion plant to fertilize comfrey a reasonable route?

2.  What would that companion plant be?



     If question 1 is correct, the characteristics I am looking for are
   
         Compatibility with comfrey
       
         ability to fix nitrogen

         will not crowd out or be crowded out by comfrey



I know this is a lot, but thanks in advance,

Eric

I think you will be hard pressed to find a permanent ground cover  that won't be crowded out by the comfrry.
A seasonally reapplied legume or a dappled shade giving legumous tree might work better.
Alternatively, just mulch. Comfrey is said to accelerate decomposition in compost piles.
Plant directly into compost,the comfrey acts kind of like an animal, speeding the conversion into accessible nutrients.
Which reminds me: pee on them.
Yeah,comfrey needs nitrogen, so give it your pee.

I think the DWC idea would work as long as you have a pretty good sized area.  If you did a comfrey plant every 8 feet with clover filling all the rest of the area, I think it would work really well.  I would be more concerned about alfalfa or very tall clover affecting the growth rate of the comfrey after you cut it.  That being said, I've never tried it.  You really have nothing to lose except the cost of the clover seed.  You aren't going to kill your comfrey with it.

I have given my comfrey pee.  Actually I gave them a fair amount and they grew nicely while I was fertilizing them this way.  We have cat litter containers that are a nice, sturdy, slightly transparent plastic.  The containers are 2 1/2 gallons and have a nice screw on cap.  I would pee in them throughout a morning and then fill the rest with water and go out and fertilize.  Worked great.  I was hoping that since at present there is more space than comfrey that the space could be taken up by a companion plant that could add something in return. How reasonable does it sound to add Dutch White Clover and let it grow until they get crowded out by the comfrey?  I would think I would get nitrogen and some nice fertile soil.  On the other hand, Red Clover might grow upwards, live out a bit longer and actually provide some decent cuttings to go along with the comfrey.  I am concerned that alfalfa would get out of control quickly.

Eric

How about broad beans for winter cover? Comfrey stays dormant winter long while broad beans pump nitrogen and leaves substantial residue.

I don't dilute urine but "apply" it directly.  It doesn't burn it or anything -- comfrey can take straight-up pee.

I love your idea of a perennial n-fixing crop interplanted with your comfrey.  Please update this thread and keep us appraised on what you figure out.

Nitrogen fixers do not add nitrogen to the soil.
This is a long term misconception I see all the time.
Almost all nitrogen fixing plants store nitrogen in nodules that are home to N fixing bacteria, these nodules will decompose once the plant has died thus releasing that stored nitrogen back to the soil.
However, most nitrogen found in soil is in the form of ammonium compounds and needs bacteria to break it down to plant usable forms.
The N found in nodules is also stored as ammonium compounds, so if the soil doesn't have those bacteria that break down ammonium compounds to feed on the N, and the corresponding fungi that eat those bacteria aren't present,  there won't be any benefit to new plants looking for nitrogen.

Redhawk

Bryant RedHawk wrote:Nitrogen fixers do not add nitrogen to the soil.
This is a long term misconception I see all the time.
Almost all nitrogen fixing plants store nitrogen in nodules that are home to N fixing bacteria, these nodules will decompose once the plant has died thus releasing that stored nitrogen back to the soil.
However, most nitrogen found in soil is in the form of ammonium compounds and needs bacteria to break it down to plant usable forms.
The N found in nodules is also stored as ammonium compounds, so if the soil doesn't have those bacteria that break down ammonium compounds to feed on the N, and the corresponding fungi that eat those bacteria aren't present,  there won't be any benefit to new plants looking for nitrogen.

Redhawk

That being said, if a person chopped their comfrey and then mowed the clover, wouldn't it release nitrogen to the soil for the comfrey to be used as it grows back?  I don't know if this is true, but if you grow the two together for a period of time, it seems like the bacteria and fungi that you need will show up sooner or later.

However, most nitrogen found in soil is in the form of ammonium compounds and needs bacteria to break it down to plant usable forms.
The N found in nodules is also stored as ammonium compounds, so if the soil doesn't have those bacteria that break down ammonium compounds to feed on the N, and the corresponding fungi that eat those bacteria aren't present,  there won't be any benefit to new plants looking for nitrogen.

Redhawk

 So, is that where some manure would be beneficial to add  the bacteria needed to release the nitrogen? I'm trying to grasp this...

Redhawk, I also have a question regarding ammonium eating bacteria.

What happens to the ammonium after the bacteria eats it?

My observation is that comfrey crowns get larger in diameter and the leaf and flower stalks get larger as organic material breaks down around the crown.  Like many crowning plants it fertilises itself with the decomposing litter from previous seasons.  The problem then stems from taking away what the comfrey produces to fertilise something else. If you make up the loss with some other mulch as Redhawk pointed out the bacteria and other soil life will supply the nitrogen.
I have patches of all the things mentioned in the original post growing around my comfrey plus rhubarb and vetch  Alfalfa and vetch can be a problem of forming a big bubble if they are not cut regularly. The rhubarb and comfrey arm wrestle with neither winning.  White clover only gets shaded out if the comfrey leaves are not cut and will persist if there is space between the crowns. Red clover that I have blooms at the same height as the comfrey.  The bloom stalks of red clover will stand and hold their seed in fall and early winter to be dispersed when they finally fall.  So it can define a patch after the comfrey has gone dormant.

Kamaar Taliaferro wrote:Redhawk, I also have a question regarding ammonium eating bacteria.

What happens to the ammonium after the bacteria eats it?

The ammonium compounds (H4N+, on the + side can be carbonate, nitrate, nitrite, phosphate, etc.) are broken down into the component parts so the bacteria can use those portions they need, all excess is now free N, H and which ever of those mentioned.
What the bacteria don't use becomes food for other organisms and plant roots pick up what they need now, leaving the rest and then the bacteria and other organisms take those leftovers to eat and the circle continues.

Redhawk

Todd Parr wrote:

Bryant RedHawk wrote:Nitrogen fixers do not add nitrogen to the soil.
This is a long term misconception I see all the time.
Almost all nitrogen fixing plants store nitrogen in nodules that are home to N fixing bacteria, these nodules will decompose once the plant has died thus releasing that stored nitrogen back to the soil.
However, most nitrogen found in soil is in the form of ammonium compounds and needs bacteria to break it down to plant usable forms.
The N found in nodules is also stored as ammonium compounds, so if the soil doesn't have those bacteria that break down ammonium compounds to feed on the N, and the corresponding fungi that eat those bacteria aren't present,  there won't be any benefit to new plants looking for nitrogen.

Redhawk

That being said, if a person chopped their comfrey and then mowed the clover, wouldn't it release nitrogen to the soil for the comfrey to be used as it grows back?  I don't know if this is true, but if you grow the two together for a period of time, it seems like the bacteria and fungi that you need will show up sooner or later.

Chop and drop works great but it is not releasing nutrients from the nodules since the comfrey crown remains alive, it does release the nutrients that are in the cut down portion of the plant. The reason this works so well is that all the nutrients remain in the same spot (so to speak) and are recycled through the system.

Redhawk

Hans Quistorff wrote:My observation is that comfrey crowns get larger in diameter and the leaf and flower stalks get larger as organic material breaks down around the crown.  Like many crowning plants it fertilises itself with the decomposing litter from previous seasons.  The problem then stems from taking away what the comfrey produces to fertilise something else. If you make up the loss with some other mulch as Redhawk pointed out the bacteria and other soil life will supply the nitrogen.
I have patches of all the things mentioned in the original post growing around my comfrey plus rhubarb and vetch  Alfalfa and vetch can be a problem of forming a big bubble if they are not cut regularly. The rhubarb and comfrey arm wrestle with neither winning.  White clover only gets shaded out if the comfrey leaves are not cut and will persist if there is space between the crowns. Red clover that I have blooms at the same height as the comfrey.  The bloom stalks of red clover will stand and hold their seed in fall and early winter to be dispersed when they finally fall.  So it can define a patch after the comfrey has gone dormant.

Great observations Hans, exactly right. It sounds to me like you have devised an extremely good set up for continual renewal.

Redhawk

Bryant RedHawk wrote:

Todd Parr wrote:

Bryant RedHawk wrote:Nitrogen fixers do not add nitrogen to the soil.
This is a long term misconception I see all the time.
Almost all nitrogen fixing plants store nitrogen in nodules that are home to N fixing bacteria, these nodules will decompose once the plant has died thus releasing that stored nitrogen back to the soil.
However, most nitrogen found in soil is in the form of ammonium compounds and needs bacteria to break it down to plant usable forms.
The N found in nodules is also stored as ammonium compounds, so if the soil doesn't have those bacteria that break down ammonium compounds to feed on the N, and the corresponding fungi that eat those bacteria aren't present,  there won't be any benefit to new plants looking for nitrogen.

Redhawk

That being said, if a person chopped their comfrey and then mowed the clover, wouldn't it release nitrogen to the soil for the comfrey to be used as it grows back?  I don't know if this is true, but if you grow the two together for a period of time, it seems like the bacteria and fungi that you need will show up sooner or later.

Chop and drop works great but it is not releasing nutrients from the nodules since the comfrey crown remains alive, it does release the nutrients that are in the cut down portion of the plant. The reason this works so well is that all the nutrients remain in the same spot (so to speak) and are recycled through the system.

Redhawk

Redhawk, I didn't explain what I meant very well  What I mean is, supposed I cut my comfrey down for use somewhere.  I also mow the clover.  When I mow the clover, I would assume that some of the clover roots die, so the nodules would release nutrients into the soil to be broken down.  Then as the comfrey grows back, it could use the clover's nutrients (assuming enough time has elapsed for the bacteria and fungi needed).  Is this correct, or am I still missing something?

You aren't missing a thing Todd, that is exactly what will happen in that situation.  Most clovers are annual type plants so when you mow them they tend to die (this is especially true if you mow them almost to the soil).
Most of us that grow comfrey do use the cut comfrey elsewhere, I like to use chopped comfrey around our tomatoes and any leftovers go to the compost heap area.
I grow clovers for pure chop and drop or as erosion control (no cutting).

I went out to look at some areas. I have no alfalfa, so cannot comment on that. The red clover seems to be doing the best, it twines up above the comfrey. Crimson clover is not yet evident, at least I can't tell it from the red. White clover is around right next to this patch. I think the best would be white clover if you are cutting your comfrey a couple times per summer. Red clover does not reliably perennialize here due to crown rot, but if you can grow it in your climate sounds good. I also have sweetclover planted but none is doing anything so far. That would be another durable option in comfrey in my opinion.

I have white clover right next to the comfrey and it does not appear to be able to get sunlight and maintain in the comfrey patch.

Those clovers will rebound when the comfrey is cut down. They are just being shaded out right now.

Could you add a nitrogen fixing tree or shrub? Comfrey is very shade tolerant and the tree/shrub would mulch the comfrey with fallen leaves... Also, I always leave the last 'cut' of comfrey on the plant - the last cut I take away would be around mid September here.

I did some plant identification, there is also Berseem clover mixed in. That is, I believe, related to sweet clover, so I anticipate the mix of sweet clover and comfrey to be durable.

My plan is to be able to get comfrey plus sweet/red clover tops for transplanted fertility cut as a unit (adding this to leaf and wood chips for compost). If you are looking for just a productive comfrey patch (for compost tea for instance), I would go with a durable white clover that will grow after harvesting the comfrey.

So it depends on your intended purpose. Some pretty great options out there now!

Could you add a nitrogen fixing tree or shrub?

Absolutely would be great in this climate. The problem is that most of my comfrey is around fruit trees. I am thinking of installing some goumi in the same area, but yes that would be a great strategy depending on what layers you have available. I am testing several right now for interplanting and the goumi/autumn olive seems to be the most prolific. Other areas I just started actually have comfrey under honeylocust for the same reason. Lots of ways to crack the nut.

WOW!  So much activity on this thread so recently!  Thanks to all who contributed.  Redhawk,  I have red numerous comments of yours on this and other threads and you sound like a wealth of knowledge.  I am curious as to your thoughts on my plan.  As I stated earlier, my future comfrey patch is at present 4 plants with room for 4-6 additional plants.  As I am trying to incorporate a legume, I will likely limit the patch to 4 additional plants to leave room for the legume.  My question now of course is which legume.  I am leaning towards Dutch White Clover on the grounds that it is long lived and densely covers the ground.  I would be regularly cutting the comfrey to use as green manure and in the process I would also cut the clover close to ground and leave it as green manure for the comfrey.  Do you think that this practice would also cause some root die-back thus causing a N2 release in the soil for the comfrey roots to feed upon?  I am not thinking to use clover as a quick N2 replacement.  Rather I am looking for a long-term source of N2 for the comfrey (and thus for the garden).  If I understand your reasoning correctly I essentially need to grow a legume in order to later kill part of it, forcing the legume to give up that N2.  I have considered red clover, but I don't want the clover to shade the comfrey (I know that comfrey is shade tolerant, but doesn't it do best in full sunlight?).  Also, since this patch is close to my gardens, I don't want my companion plant to grow out of control.  As I understand, red clover will grow larger faster (which would give good green manure) but spreads easier than DWC and only lives a couple of years.  If the comfrey plants get so large down the road that they can't compete and get completely shaded out, then I would consider another legume. However, if the DWC did die out I would think that this would present a nice N2 boost to the soil?  



To all others who have contributed, thanks so much for your thoughts, opinions and encouragement.  I truly appreciate it.  As I stated above, I am currently leaning towards DWC, but have not ruled out a small amount of Red Clover near the center of the patch where it will be contained or even plant some annual crimson clover.  Is there any problem with growing two types of clover together?  In this scenario I still see DWC as being the primary companion plant with either red or crimson clover there to add a little N2 boost.



Thanks to all,

Eric

hau Eric, pila'waye for the kind remark.  In your stated planting I love your idea of using Dutch White clover as the understory for the comfrey.
It will provide you with plenty of nitrogen just cut it at about 2.5 inches tall (remaining height) and let the cuttings decompose in place to add N for the comfrey.
Dutch White is a great clover because it is perennial in nature and it will self seed to create a nice thick ground cover.

Cutting any clover to short will tend to kill the plant thus making it a short term instead of long term addition. Cut the clover longer and the roots can live because there are still some leaves left to provide energy.
Comfrey does prefer full sun to mostly sun, but the large leaves do allow it to do well in part shade too.

Red clovers grow tall when in full sun, not what it sounds like you desire.
Crimson, sweet (yellow) and other clovers grow the same as red clover.

Redhawk

Redhawk,

Thanks so much for the quick reply.  So I suppose Dutch White Clover it is!  It there any reason I could not plant it now, or more likely in 2 weeks as I will have to order the seed.  I live in zone 6 and right now it is in the mid-50's outside.  I have never planted clover before so I am not entirely certain what is involved.

Thanks,

Eric

Check this out, I got most of my species of clover at low prices.  Good time to be shopping.

Sometimes they will not emerge until spring, but frost setting is fine with clovers.

White clover should be available at any feed and seed if you have one near, although they may only have durana or medium white this time of year up there. I do recommend inoculated unless you have true clover growing near there, it made a huge difference.

TJ,

Thanks for both this specific info and your other very informative posts.  I plan on ordering some inoculated seed in the next day or two and put some in the ground immediately.

Eric

I’ve had bocking Comfrey for years and I’ve never found any sort of cover helpful or necessary as the stuf has explosive growth after the first year and the plants literally crowd out anything near the base.

I also have quite a few Comfrey plants and the only thing that seems to stunt them is thick grass. I have some under my Black Locust trees and they seem to get huge leaves in the shade and don't seem to flower as easily/ quickly. I chop and drop the Comfrey 3 times per season and I try to have 2 Comfrey plants beside each tree or shrub. I "do" have lots of WDC around each guild, but that is for the main plant in each guild. I also chop and drop the clover when needed. Comfrey is pretty easy to grow. I had about 200 cuttings in pots sitting in the garden ready to sell at the local farmers market. Some of the small roots had already grow out the bottom of the pots and into the grass below. After i moved the last half of the containers, we ended up with an "unexpected" Comfrey patch because a lot of those small root pieces grew into plants.................

s. ayalp wrote:How about broad beans for winter cover? Comfrey stays dormant winter long while broad beans pump nitrogen and leaves substantial residue.

Broad beans can be sown succssionaly every few weeks of the year if you so choose . Mine are able to stand minus 5 oC and up to 33 oC quite happily .
The added bonus is that the stalkscan be dried for chopping as browns or simply chopped green for greens once the beans have been harvested

TIP
The shelled beans are thought by many to be at their best when slightly smaller than your thumb nail and picked an hour before cooking / eating raw .
I tend to use pruners to snip the whole stalks off , to  leave the roots undisturbed in the ground for the brassica or salad crop that follows.

I would say that the best companions for comfrey would easily be the N-fixing shrubs or trees.  If you already have a use for alfalfa, that could totally work.  If not, choose something that has some use to you.  

Comfrey seems to get along nicely with anything taller than it that doesn't mind having it's roots shaded out (most plants/trees seem to enjoy this, but raspberries for example need more air circulation around their base so do not make good companions).  It quickly out-competes any other groundcover near it as it expands.  My entire yard has comfrey growing everywhere, just as a weed.  You seriously cannot kill it.  There are certain places where they have formed mass patches of maybe 50-100 plants, it's hard to say.  The only thing that manages to survive among them is the odd blackberry vine that escapes my attention until winter when the comfrey dies back.  

As mentioned before comfrey is very shade tolerant, so I would be more worried about the success of the companion rather than the comfrey itself.

Jared France wrote:I would say that the best companions for comfrey would easily be the N-fixing shrubs or trees.  If you already have a use for alfalfa, that could totally work.  If not, choose something that has some use to you.  

Comfrey seems to get along nicely with anything taller than it that doesn't mind having it's roots shaded out (most plants/trees seem to enjoy this, but raspberries for example need more air circulation around their base so do not make good companions).  It quickly out-competes any other groundcover near it as it expands.  My entire yard has comfrey growing everywhere, just as a weed.  You seriously cannot kill it.  There are certain places where they have formed mass patches of maybe 50-100 plants, it's hard to say.  The only thing that manages to survive among them is the odd blackberry vine that escapes my attention until winter when the comfrey dies back.  

As mentioned before comfrey is very shade tolerant, so I would be more worried about the success of the companion rather than the comfrey itself.

Chickens help keep comfrey down & it's supposed to beneficial to them & their eggs .

I have true comfrey plants in my small food forest, their purpose is mostly to provide mulch and attract pollinators. The ground cover is composed of white clover, alpine berries and strawberries. It all grows in a shade - from partial to deep, and is doing fine.

Eric,
An issue with integrating clover, or other secondary cover crops to the comfrey is the distinct potential of chocking out one or the other. Try integrating peas, beans and/or possibly a small legume tree to the mix. They'll grow taller and die out. Even if you've no interest in harvesting the byproducts, they'll increase the nitrogen upon their demise and will often self-seed.
Just rip off a few comfrey leaves to give access to the soil, drop in the seeds and before the comfrey can choke it out the beans will be well on their way.
Personally, I put comfrey throughout my food forest. This makes it easily accessible, without having to drag a bunch of it from one spot to the other. It's great under fruit trees, especially if you have a dog that likes to dig and play under them. It helps to increase the amount of individual comfrey plants and continually releases minerals with a natural chop & drop process.
I also randomly toss old beans and legume around to do what they will. Later, I might harvest some, or just chop and drop them.
On a side note, comfrey makes a great weed barrier & border to paths and around beds. Kneeling on it to work in an area only increases the productivity of the soil. Comfrey is hard to hurt in the long run, so crushing a few leaves isn't a negative.
Hope this helps.

So I understand that comfrey has a really spectacular growth rate.  What I don't understand is where does it get all of its nitrogen?  I know that its leaves have a very high N rating, and not being a legume, cannot fix nitrogen from air.  As far as I know, that leaves the soil.  I would think that the soil would eventually run low on N, but many suggest otherwise.  So just how does comfrey get its N?  

Eric Hanson wrote:

So I understand that comfrey has a really spectacular growth rate.  What I don't understand is where does it get all of its nitrogen?  I know that its leaves have a very high N rating, and not being a legume, cannot fix nitrogen from air.  As far as I know, that leaves the soil.  I would think that the soil would eventually run low on N, but many suggest otherwise.  So just how does comfrey get its N?  

Comfrey is a nutrient and mineral accumulator.  It can have taproots up to 20 ft deep.  So in poor soils it may be a little slow to get started as it puts it's roots in, but with some care should take off once established.  In rich, moist soils it seems to take off immediately.

I see it as a scavenger plant, soaking up nutrients that have soaked deeper into the soil than most plants can reach and recycling it back up to the soil surface.  This is why people use it to border garden beds or poultry runs, because it can help to slow runoff and recycle some of the lost nutrients.  Pretty incredible plant, and in no way limited to just a Nitrogen plant!  Also Phosphorous, Potassium, and a whole spectrum of minerals.

Eric Hanson wrote:
I know that its leaves have a very high N rating

Except for "your" post, I've never heard anybody say that Comfrey is high in nitrogen. ........ It is noted for being a "dynamic accumulator". (it accumulates a lot of minerals and trace elements in it's leaves). The leaves have very few fibrous parts and decompose "very" quickly.  If "you" provide Nitrogen (compost, clover, etc.) that "will" help it grow fast and healthy though.

kirk dillon wrote:

Eric Hanson wrote:
I know that its leaves have a very high N rating

Except for "your" post, I've never heard anybody say that Comfrey is high in nitrogen. ........ It is noted for being a "dynamic accumulator". (it accumulates a lot of minerals and trace elements in it's leaves). The leaves have very few fibrous parts and decompose "very" quickly.  If "you" provide Nitrogen (compost, clover, etc.) that "will" help it grow fast and healthy though.

Regarding nitrogen, see the link: https://www.allotment-garden.org/comfrey/comfrey-compost-feed-tea/

kirk dillon wrote:

Eric Hanson wrote:
I know that its leaves have a very high N rating

Except for "your" post, I've never heard anybody say that Comfrey is high in nitrogen. ........ It is noted for being a "dynamic accumulator". (it accumulates a lot of minerals and trace elements in it's leaves). The leaves have very few fibrous parts and decompose "very" quickly.  If "you" provide Nitrogen (compost, clover, etc.) that "will" help it grow fast and healthy though.

What the heck is up with all those quotes?

Ok, I will clarify a couple of statements,

I have read from numerous sources that comfrey is great for the compost pile, particularly for heating up that pile.  I assume that means nitrogen.  In addition, whenever I have seen a chemical analysis of comfrey leaves, nitrogen is always one of (if not THE) top nutrients in the comfrey leaves.

I totally understand that comfrey is a great dynamic accumulator--most soil is loaded with nutrients like phosphorus and potassium (P&K) but sometimes the P&K is not always in a useable form for most plants.  Comfrey, with its long roots reaches sources of P&K, (and many other nutrients) that would otherwise be unobtainable and can last for years from all of this banked supply.  Theoretically, if there was some barrier around the comfrey plant, then I would expect the comfrey to eventually show signs of P&K starvation, but in a real-world setting, this is not likely or practical.

Which leaves me to the nitrogen.  Since comfrey does not get its nitrogen from the air, this leaves the ground.  If I chop & drop, I would expect to eventually exhaust my soil's N supply.  Does comfrey have some microbiota that provides N?  I would think that a plant that grows such copious amounts of dark green foliage must have a good supply to come from.  I am just curious as to where it originates.  A fungal source that ultimately gets its N from the air would make sense but I have no evidence of such a relationship.

I hope I clarified my question and thanks in advance,

Eric

Eric Hanson wrote:Ok, I will clarify a couple of statements,

I have read from numerous sources that comfrey is great for the compost pile, particularly for heating up that pile.  I assume that means nitrogen.  In addition, whenever I have seen a chemical analysis of comfrey leaves, nitrogen is always one of (if not THE) top nutrients in the comfrey leaves.

I totally understand that comfrey is a great dynamic accumulator--most soil is loaded with nutrients like phosphorus and potassium (P&K) but sometimes the P&K is not always in a useable form for most plants.  Comfrey, with its long roots reaches sources of P&K, (and many other nutrients) that would otherwise be unobtainable and can last for years from all of this banked supply.  Theoretically, if there was some barrier around the comfrey plant, then I would expect the comfrey to eventually show signs of P&K starvation, but in a real-world setting, this is not likely or practical.

Which leaves me to the nitrogen.  Since comfrey does not get its nitrogen from the air, this leaves the ground.  If I chop & drop, I would expect to eventually exhaust my soil's N supply.  Does comfrey have some microbiota that provides N?  I would think that a plant that grows such copious amounts of dark green foliage must have a good supply to come from.  I am just curious as to where it originates.  A fungal source that ultimately gets its N from the air would make sense but I have no evidence of such a relationship.

I hope I clarified my question and thanks in advance,

Eric

Everything I have read about comfrey talks about adding manure in large quantities to get the best growth from it.  I pee on mine.  If you don't add nitrogen somehow, I would say yes, it would slowly use up the nitrogen in the area if it isn't returned by nitrogen fixing plants or the addition of some outside source of nitrogen.  Comfrey left to it's own devices will live, so it has to be getting enough nitrogen from leaf litter or the plants around it that die off annually.  Plants are also connected by vast networks underground, so I would think there are nitrogen fixing plants somewhere that other plants have access to.

Opinions vary as to whether comfrey is actually a dynamic accumulator or not, and whether those deep roots truly pull nutrients up from the deep layers.  See this article for instance: comfrey