Baking soda is unfortunately not 'natural' in the sense that it was probably chemically synthesized in a factory rather than mined from the earth. It is a good abrasive, though. Perhaps you can use salt for this purpose instead? Vinegar is also useful, though you should be aware that the fermentation probably involved ethanol produced from genetically modified corn or other plants and took place in a giant aerated vat. See if you can find a source of the copper ore chalcanthite. The mineral contains large amounts of a substance called cupric sulfate (a.k.a. copper II sulfate), which fungus does not like at all. A more powerful toxin you can use is cinnamon bark oil. This is an excellent preservative against all manner of fungi and bacteria.