Christine Noguere recommended www.gardenweb.com
for answers about fertilizers, etc.
Here is a list of some interesting web sites from Debbie D., one of our Master Gardeners, that might answer some of your questions:

www.gardenlist.com Cyndi's Catalog of Garden Catalogs, the home gardener's one stop to find all the mail-order catalogs in the US and Canada

www.finegardening.com  Fine Gardening Magazine provides inspiration, practical information and resources to help you create garden spaces

www.vegetablemdonline.ppath.cornell.edu Cornell University Vegetable MD Online was developed to provide access to the many vegetable diseases

www.usna.usda.gov  United States National Arboretum, information on invasive plants, a variety of gardening tips and ideas

www.wildflower.org Lady Bird Johnson Wildflower Center, search for information in their extensive native plant database

www.gardening.about.com About.com, a variety of articles on gardening topics as well as a gardening forum where you can find answers to your questions