I used to shoot in broadhead leagues; and practiced in SAND. I took eigth bales of hay; and put two in the back; two on each side; and put two on top.
The yard of sand goes inside. I backed up the hay ( which is heavier than straw) with a piece of plywood; and in time laid a piece of plywood under the two top bales- to hold them up.
Warning danger warning! ALWAYS check behind the target! I used to shoot everyday; and one day I hit the top of the sand pile; and the broadhead went through the plywood. I looked behind to see how to get the broadhead out; and it was sticking out next to my sons head ! He and his friend made a fort behind the target... argh!
Good thing about sand is that it last forever. You might have to replace the bales; but the sand will not wear out!
Now I have several acres of sand to shoot into; uphill; and downhill; and straight; and I practice with broadheads all the time. I even missed a local 3d shoot because I didn't have any field points.
Sand will really dull the broadheads; to the point that I check my hunting arrows for a few days- and then sharpen them; and practice with other heads that are butterknife dull.
I have told some people- we used to shoot in competition with broadheads; and they didn't believe me.. but we did !
Here is another idea - make two of these targets; so you shoot into one; pull your arrows; and shoot into the other... and don't do all that walking back to the line to shoot.
SAND works way better- than dirt.