Yesterday, my kids were playing and were carrying a (King size) down comforter stuffed in an extra large canvas laundry bag with string cinch closed top over their shoulder/on their back -- up and down the stairs. They said they were playing soldier/sailor because it reminded them of the gear duffel they use.
Of course soil would be much heavier than a comforter, but maybe something similar would work? A heavy burlap sack like for coffee beans or even used/2nd hand actual sailor's duffle bag from whatever organization/system you have in your area that sells them.
You might even be able to lay a quantity of soil in a middle of a square tarp and pull up the four corners (tying opposite corners or bunching them together if you are strong enough to grip) then carry that over your shoulder/back.
(Squat. While leaving the bottom of the sack/middle of the tarp resting on the ground, pull up and hold the top of the sack/duffel/tarp corners from behind you, over your dominant shoulder, and lean forward/rest most of the the weight on your back. Then lift up by unsquatting/using your thigh muscles to stand up.)
...don't try to carry too much all at once. If you can't carry very much on your own, then definitely a two person job.
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