Apparently only a tiny abount of soil is needed.Reminds me of story in BioChem text in 1940, about German blockade of Britain in WWI cutting of food shipments, with widespread hu;nger in UK. Scurvy began to be common. To the surprise of the authorities, as the blockade became more effective, incidence of scurvy decreased. Interpretation was that because people were hungry, they stopped peeling potatoes, and small amounts of vitamin C in peel pretected them against scurvy.