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.
Step 1: Materials and Tools
8" X 1" X 6' planks - 2
8" X 1" X 4' planks - 6
2" X 1" X 4' planks - 9
Some 2" X 3" material I had (about 8' needed)
(I found that the Phillips II outdoor screws are much cheaper than the zinc coated ones - these come in 3 different colors and are almost impossible to strip. I chose the ones suitable for use with Pine/Cedar)
Some left-over deck stain
Step 2: I Started With the Side Panels
I marked-up the side panels and mounted the end supports with glue at required heights. The bottom-end of side panels is seen on the left. The side panels are notched to accommodate a concrete ledge at the bottom of the wall. I wanted the shelf unit to stand flush with the wall. All notches were cut with the jig saw.
The second picture shows a close-up of an end support glued to the left side panel. This end support is notched at both ends in such a way that after a 2" X 1" edge-support strip is placed in the notch, it's top surface will be level with the top surface of the end support. End supports for shelves numbered 1, 2, and, 6 are notched at both ends. End supports for shelves numbered 3, 4, and, 5 are notched only on the front.
After the glue set, I turned the panels over and used screws to secure the end supports. The 3rd picture shows details for one of the end supports.
Step 3: Assembly
For shelf numbers 1, 2, and, 6, I screwed the edge strips to the end supports. For the remaining shelves, I fastened them from the sides. So far all screws used are 2" long. I kept checking the squareness of the frame at various stages before I tightened anything. Now the shelf frame could stand on its own. The depth being only 7.25" as compared to the height of 72", the frame was easy to tip over.
Next, I added the shelves. I had to cut these planks as well to matching lengths. I fastened the bottom, second, and, the top shelves using 2 screws on each side through the side panels. For the remaining shelves I used just one screw from top at each end - anchoring them to the end supports. These are the only 1.5" screws in the assembly. With the box like construction of shelves at 3 levels and the variety of ways I had fastened various parts, I didn't feel the need for adding more cross bracing (and I was going to anchor the shelf to the wall)
Step 4: Finishing and Installation
(My cheap camera doesn't give good results under low light. Sorry about the picture quality)