i live in the south,clay is what you get,best way to break it up is with sand and mushroom compost,doesnt happen over night,but will in a few years as with all gardens more compost more soil life,i think it takes all 3 structures of texture to even start,once that dialed in then it easier to break it down,my ground floor is straight out white clay and sedement,i tried all the stuff imaginable to break the shit up,gypsum put a can of whoop ass on it,but only as deep as you can apply it,does wonders,but again only as deep as you apply it,so if you apply it with any fertilizers as a side dress you will over time break it up,i shoot for at least a 12in depth of fine rich soil,i dont even have a tiller,i turn mine the old way with a shovel,timing rain storms and dry period after the rain is the best way to bust the clay up,after a rain the stuff is hard as hell to turn but you can at least stick it in shovel deep and then apply gypsum,let dry couple days turn again and so forth,thats the only true way of breaking the stuff up i wouldnt ever think there could be a fantasy gimmick or trick to break it down,if there was id be afraid to use it,i been digging in the stuff for 59 yrs now,started young and my mentors tools were a shovel rake and hoe,all feed was coming right out the animals,are feilds were spread with the manure on the quarters,clay spots was sand and leaf material,rotation of crops were the key,beans and such were grown there not for the harvest but for seed stock and nitrogen added natrual back to the soil,lot of sweat broke it right up,AMERICA wasnt fat in those days,you worked all day for a pack of crackers can of potted meat and maybe a coke,that it,it wasnt a time were folk looked for the magic bullet or lazy in other words,if you hungry work for it days.
comfrey is a magic plant as is the flowers and meant to stay that way, bagged soil and such dont even have any soil texture in my opinion,just loaded with all kind of amends that poop out come magic time,im by no means disagree with the thread,just the statment of comfrey breaking down soil,if you are saying it will over a course of years,i do agree with that but while doing that the comfrey is sure enough the best feed shred,ferment