Dog bathroom in yard.

I have made a corner in my yard for my dog to use as her bathroom area. she has been trained to go there when she needs to go. I dispose of her poop, but her urine just sinks into the sand that is in there. it tends to get smelly. And I think the sand may irratate here sometimes. Can anyone recomend a good matieral to use as a base and a good odor control or something. I was wondering if i could mix some baking soda or non clumping cat litter with the sand to help control the odor. or maybe i should get rid of the sand and use gravel instead.
Any suggestions?

Feel free to email me with help.
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In our kennel we had we used small stones. It worked really well, we'd take the hose and hose it down once a week to keep the stones clean.

was it congrete under the stones or dirt?

It was lined with no weed stuff, and it was six inches deep, then filled pea gravel.