marking outdoors

Thank you in advance for all the hard work you do for us JRT owners.

I have three dogs--Otis, a 7 year old male hound mix (since he was 1), Shiner, a 4 year old female Jack (since she was 2) and Rex, a 7 year old Jack (for about a year) All three are rescues. They get along just fine and behave very well in the house. They have a large backyard that is well secured with a six foot fence. I have conceded the back yard to them to run and dig and bark and do their various doggy stuff. Shiner and Rex and I are working with a trainer so they won't jump up on people so much and so I can walk all three of them at the same time in a more or less orderly fashion. They get a fair amount of exercise--at least I don't have any exercise related problems.
Here is the problem--the place where I sit to throw the ball to Rex is on a patch of bare dirt with a bit of a slant to it, so I built a small deck with benches to sit on. Now I can throw the ball without slowly sinking into the ground, and it stays somewhat less dirty. The litle pepperhead has decided that this pristine new deck is the perfect place for marking; I suspect because it is new to the 'hood, so to speak. He has never marked inside. All the animals have been fixed. Every day I have to go out and hose off my new deck. Any suggestions on how to stop Rexie from marking outside?

Thanks Again,