Yes, your shed does need a base. This is to give it a solid, level foundation. Open soil will not help with the longevity of the shed itself or the contents within. The best materials to use to make your shed base are concrete, natural stone or wood.
Builth Wells
Builth Wells is a market community and area in the county of Powys as well as historical area of Brecknockshire (Breconshire), mid Wales, lying at the convergence of rivers Wye and also Irfon, in the Welsh (or top) part of the Wye Valley. It has a population of 2,568. Much of the community's buildings, including the 1876 Market Hall, were developed from stone from Llanelwedd Quarry. Much of the facing as well as other dressed rock used in the building of the Elan Valley dams was likewise quarried here. The quarry produced the first occurrence of laumontite in Wales. The quarry is presently run by Hanson Aggregates. Builth is a longstanding anglicization of the Old Welsh Buellt/Buallt which integrates bu be, equivalent to some Middle English orthography), indicating "ox" with gellt (later gwellt), indicating "lea or leas"; the very same form is used no matter sex of the animal. The town included "Wells" in the 19th century when its springs were promoted as a visitor destination. Its modern Welsh name Llanfair-ym-Muallt means "Saint Mary in Ox Leas".