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.
South Brent
South Brent (population 2,822) is a large town on the southern side of Dartmoor, England, in the valley of the River Avon. The parish consists of the tiny hamlets of Aish, Harbourneford, Lutton, Brent Mill, and also several scattered farmhouses. It is five miles (8 km) north-east of Ivybridge and 14 miles (22 kilometres) east-northeast of Plymouth. On the high moorlands are several hut circles, enclosures, and barrows, all dating from the Bronze Age. The manor of Brent came from Buckfast Abbey from the moment of the foundation of the abbey in the very early 11th century. It was purchased the Dissolution by Sir William Petre, a large receiver of monastic spoils in South Devon. South Brent was initially a woollen and also market centre with 2 annual fairs. Brent Hill is the steep hill just outside the town from which it takes its name (Old English brant-- steep). On it are the damages of an old structure, meant to have been a church, and of a windmill built regarding 1790.