please share ideas for growing BIG sweet potatoes, several pounds per plant

We've grown them for years on three different pieces of land.
I've never done soil tests nor really added any amendments aimed at growing a great crop.
The oldest orange potatoes I clone are going on 30 years now and the purples at least five.
https://permies.com/t/17569/perennial-vegetables/sweet-potato-propagation-harvest
I've read where they can play out eventually but I was able to grow a twenty pound cluster (and also an 8#  and a 10#)from the oldest strain just a few years ago....and a few huge purples last year but not all of them and have not been able to duplicate that success.....some years are okay to good but not outstanding.
pictures below are of the twenty pound plant!



Here's what I've read recently that I see lacking in my methods ...

...choosing the potatoes for slips from plants producing at least four nice potatoes.

...feeding the soil, especially potassium.

...watering often enough they don't wilt.

...not letting the vines root along their length

...mulching, (we only do sometimes)

This years crop of purples were average size and only around a pound per plant except for three plants that produced the cluster of at least four good ones...they were still just around two pounds total.  I have an occasional single 3# potato from them though so I know they are capable.

This years orange, old variety, cut leaf produced a couple plants with one or two nice size but mostly small clusters of roots.

Some in tubs, some in holes I added sand to and some in clay that dried to a brick but still had similar size potatoes.

I suspect lack of nutrients rather than soil tilth.
The 20# plant was a left over slip, planted late in unworked ground in the front yard .. it also had a pile of pine logs near by from a recent tree removed from that yard...no amendments and we almost forgot about it until the rest of the garden crop was dug.

I hadn't thought about discouraging rooting along the vines' travel along the ground. Might have to give that a go this season...always looking for the cool ways to tweak the system. I didn't cover the surface with biochar last year before planting because I couldn't bear to have that much exposed soil. The plants established quickly in spite of the lack of extra heating so I will just plant directly into mulch this time as well.

I think allowing rooting along the vines in a longer season works and they will make more potatoes  along there also.  Here, I think it just takes energy away from the main plant.

Here's some photos of information from a book put out by our local extension service for this state...usually good info if we ignore the chemical solutions promoted and they do acknowledge organic growers now😊

As a long time grower of sweet potatoes in several regions, I can vouch for not letting the vines root along their length, unless perhaps you are in so tropical a climate as to have them basically become a perennial groundcover, and even there, finding the largest roots to harvest would still be a challenge.  Raised beds and mulching help with this, but in wet weather roots will grow down through mulch, and then going along the rows and lifting the vines and setting them back down helps, and/or stuffing coarse mulch, bundles of sticks, etc. up under there as well.  
   Not letting the leaves droop from drought is also important.  In fact this is good for just about every kind of vegetable.  When I was growing for high-value organic markets I was taught that if you see foliage droop on anything, even if it recovers overnight, you are losing yield and quality.  Although yes, in a homestead situation resilience is also important, and you will still get a yield in spite of this.  A long drought followed by heavy rain can also make roots split.
    Most important of all, though, is to work toward what I call a "fluffy" soil....this goes for carrots as well.  Neither likes a heavy clay and it will lead to small twisty roots on both.  Sandy soil is ideal, good loam is good too.  If you have clay the answer is finely divided organic matter incorporated in.  I used to make this by running a mower over dry leaves, grass, etc. in the pathways between those beds allocated to these crops, and then dig and/or till this into the beds themselves...up to an inch or two depth of "powder" per year.  A bit of urine helps counteract the nitrogen uptake issue with this kind of organic matter use, but fortunately neither sweets nor carrots like a lot of N anyway.
   

I haven't worried much about maximizing production as I've mostly been focused on breeding them for growing as seeded annuals. I do think that allowing them to root along the vines, especially if they have a trailing habit takes away from a nice harvest of larger roots and instead makes lots of little ones. Worse they are all spread out and hard to find and harvest but you might get by with it if you have a longer season and don't mind digging a crater to find them. I grow them mostly in pots and have selected for bushy, non-trailing vines and a clump of nice roots directly under the primary stem: I think those traits would also be nice to have even if growing in the ground.

I also haven't worried much about specific soil nutrients or anything else that would require me to test for or purchase anything, I just grow them in what I can scrounge up around the place. I do think though that a nice loose soil helps encourage larger roots and keeping them well-watered helps too. One year I saved a bunch of yard clippings and leaves and kept them dry in bags in the shed. In spring I mixed that up with some pretty decent compost and a little plain garden soil. That was the best ever as far as production, I got two to four pounds per 3.5-gallon pot. *If you grow in pots, especially small ones it is important that the drain holes be buried in the ground, so they don't dry out so fast and so feeder roots can exit into the soil below and never move the pots once those roots have established.

I saw a fellow on YouTube that grew them in large Rubbermaid tubs with a very specific soil recipe of purchased potting mixes. He had a fantastic harvest, many pounds per tub. It looked like most of the space in the tub was filled with just sweet potatoes. It was interesting but not something I would do because it violates my buy nothing policy, plus I don't need hundreds of pounds of sweet potatoes as there are only two of us here to eat them.  

Next year though, I am going to revisit the issue of maximizing production and have already stockpiled several large bags of leaves, weeds and grass to fill my pots. As I move to this new phase of my breeding project, I'm setting a goal of averaging three pounds per 3.5-gallon pot.