Carpentry and Woodworking - how much weight can a short 2x4 support?

i really know little about wood itself when it comes to its strengths and types.

but, i'm planning on building a terrarium out of wood and plexiglass, the part that will be holding dirt in it will be a 1ft by 2.5ft by 8 foot bin which will be 1 foot off the floor and lined in plywood and silicon epoxied plexiglass for waterproofing. in my plans i have it supported every 2 feet along the 8ft length with cross beams and supports holding it up figureing the 20 cubic feet of dirt would be quite heavy.

but, i'm wondering if i could save some money and use 2x4's lying flat instead of the 4x4s or would the weight be too heavy for them?

now that i think of it, i'm also wondering if the plywood could hold up the weight of the 1 foot of dirt on top of it in the 2 foot by 2.5 foot space between the supports.

You could use the 2x4's, but stand them on edge and support them as you stated you would every 2 feet along the 8 foot length. Use at least 1/2" plywood for the flooring, and use a marine grade if you can find it. It will help, since your sand will probably be moist most of the time.

ty for the info.