We sell it in bags, boxes, or in bulk bins. Buy 1 ounce to 100 poundss, whatever amount you need. No thumbs - honest weight.
Fertilizer is a material which provides nutrients to plant roots in a form that the roots can use.
N, P and K are required most heavily and used up more quickly. Other nutrients needed are usually present in soil. Or use compost made up of several different materials. It's better NOT to apply fertilizer than to add too much. Too much fertilizer can kill plants.