Manure

Manure was organic waste used for agricultural fertiliser. The Third Doctor, while visiting West Yorkshire, negotiated a trade agreement between Martin Chisom and a metal bug. Chisom would allow the bug to live in his forcing shed and provide it with manure. In exchange, the bug would supply "the tenderest rhubarb in the galaxy, grown at a rate unheard of on Earth." (AUDIO: Lost in the Wakefield Triangle)