The company I work for sells fertilizer to landscapers in Alabama. We sell the actual fertilizer sometimes with top soil. We are not collecting sales tax, but I'm not sure if this is correct.

The fertilizer itself is exempt from sales tax. However, top soil is still subject to sales tax in Alabama. You need to start charging sales tax immediately for the sale of top soil. Your invoices should separately state that cost for fertilizer and top soil to delineate between the two items as one is subject to sales tax and the other is not.