The sales tax issue interested me, so I searched and found what the laws are in California.
http://www.boe.ca.gov/pdf/pub61.pdf
on the 5th page of the document, which is numbered page 1, it lists as exempt :
ANIMAL LIFE, FEED, SEEDS, PLANTS AND FERTILIZER, DRUGS AND MEDICINES—The sale and use of animal life or feed for animal life, seeds, and plants the products of which normally constitute food for human consumption are exempt from tax.The sale and use of fertilizer to be applied to land the products of which are to be used for human consumption are also exempt. In addition, the sale and use of drugs and medicines including oxygen that are administered to food animals, the primary purpose of which is the prevention or control of disease, are exempt from tax. This is an expansion of the general food exemption.
I find it interesting to learn that fertilizer for food plants is also exempt. I'm thinking the stores do charge tax anyway though.