Fertilizer is not regulated at the federal level, only on the State level. This leads to having different regulations for each state. AAPFCO oversees all of the states regulations of fertilizer. Fertton.com aims to simply this information with different spreadsheets containing information about all of the state's fertilizer laws and regulations.
This website contains up to date information, however for official laws and regulations, contact your state regulatory official or State Department of Agriculture.