I wanted to use as much precut wood as possible. I chose 1" thick white wood (cheap, untreated soft-woods like Pine) for construction. My calculations showed that at 4' width, the shelves would sag with heavy loads. So I decided to add edge strips to increase the rigidity.
I used the sag calculator at http://www.woodbin.com/calcs/sagulator.htm
To make sure I wasn't missing anything, I made a sketch for the shelf unit. The bottom shelf is mounted at 9.5" so that I can use the space underneath as well. The sketch shows the spacing between (lower surfaces of) individual shelves. I wanted 16 inches to accommodate the fertilizer bags. The top shelf is mounted closest to the top. I decided to have edge-support strips on both edges for the 1st, 2nd, and the top shelf. All other shelves are supported only on the front. The little rectangles you see below each end of a shelf are end supports. This will become clear in step 2. Note that because of the edge supports, frontal spacing between shelves is less by 1.5". The back is open as I have installed it flush to the wall.