Hi svengali, a few ways that can change your soil/sand,
ask you local council if they recycle there green waste as compost, this can be used for digging into the soil or as a mulch. some councils give it away,or charge £1.00 -- £2.00 per bag or around £70.00 by the 3 ton skip full.
shredded paper/cardboard can also dug into botton of a trench (this will hold moisture) and mulches around the plants, adding nutrients by fertilizers,
standard stuff- can you get your hands on any grass cuttings as this can be placed around plants any time of the season, or leafmould, both can be dug in or used as mulches.
the more you turn your compost heap, the sooner it makes into a useable medium, that can be dug into the soil or used as a mulch.
Hope this helps
windy