Hi John,
Just going back to the original question;
“It’s back to the original quote from Nick Pawson’s experimental plots – ‘Can a given level of food production be maintained, or even increased, with systems that are, as far as possible, self-contained?’”
This question needs clarifying before a really definitive answer can be arrived at.
For instance, what does he mean by as “far as possible”? Does he not imply/acknowledge in that short phrase that not every system can be totally self-contained. And what exactly does he mean by self-contained? Within your Plot, Allotment Group, the actual field the Allotments are in, or the general area.
The point you make about the carbon footprint created by sourcing materials from off the plot is not really valid or, in my case at least, significant as anything I put on my heap comes from places I pass by everyday so no additional carbon footprint there.
BTW. If you are going to plant Russian Comfrey plant it somewhere you are not likely to need for the next 20 years!! Russian Comfrey is sterile so wont spread all over your plot but it has a vigorous taproot which can reach up to 8m deep very quickly and you wont get that lot out very easily. Also it will quickly regrow from any piece of root left in the ground.
I would encourage you to get a few root cuttings and grow it. I use the “Comfrey Tea” on everything during the growing season and you can really see the difference it makes in particular to Tomatoes and Potatoes but also around the flower garden.
Don.