Spartica

Hi there I'm looking at setting up a planted tank. Due to limited space I found a nice ice bucket to try it out in. I wish to try doing a walstad setup of potting mix and either gravel/sand layers. Through a video they suggested a potting mix with a nitrate level of 0.05%. However I cannot find an ingredients list for any of the main gardening suppliers (Yates, tui, Daltons). So was wandering what others have used for their setups? Many thanks!