We’ve been doing sheet mulching or lasagna gardening to build topsoil in our forest garden. Over a period of three years we’ve gradually added layers of rice hulls, manure, straw, leaves, sugar cane compost, etc. to raise our forest garden beds about 12” above the paths. Sheet mulching has worked superbly for us in our tropical climate. Just don’t make the layers too thick. In colder climates, let sheet mulch layers break down over winter.