Not sure about detailed composition, but I have to assume that any herbivore poop is fine for composting. Any such manure can be contaminated with salmonella, but I don't see why that from turtles is any more of a risk. I believe that turtles combine liquid and solid excrement in the same stuff. So I'd have to suppose that turtle poop could be especially urea (nitrogen) rich, which would benefit composting.

In general, though, application of fresh manure directly to gardens isn't a particularly good idea, at least because the high nitrogen content can "burn" the plants. Best to compost it.

Be sure to provide a safe place he can hide from predators. He's in captivity now and can't provide for himself. The kindest thing would be to keep him a little while and then set him free to live wild and make more turtles. We had a very lame turtle from the vet who rescued him and I always cleaned his home into the garden with no problems.

Also, don't feed too much lettuce. Turtles and tortoises need all kinds of variety to be well. Please post pictures of Cecil if you can.

I'm feeding him lettuce right now. There is also a few sprigs of weeds in there as well as some chocolate mint. I have a thin layer of pine straw covering the soil over the winter. I'm in zone 9, so it doesn't get very cold here. I should have had something planted there, but I don't right now. Maybe he will take care of those weeds,. lol I don't know.

I did lay some fencing over the bed to keep out raccoons or whatever might want to eat turtle.

It sounds like you're talking about "in situ" composting, which is where you just throw the compostable on the ground in the garden bed, before anything is planted there, and just let it decay. That'll work. Once your plants are coming up, however, might be smart to move the turtle somewhere else to prevent fresh poop from being deposited around the plants.

Red eared sliders are almost 100% aquatic, except for their basking. They need to have a pond big enough to COOL OFF as well as sun to warm up in ...in zone 9 that means a big one. A vegetable bed is probably lacking enough shade.

"In the wild, they feed on aquatic vegetation, small fish and decaying material such as dead fish and frogs,"

No, he won't eat your weeds.

Your lettuce diet is lethally lacking in protein and carbohydrates and he'll be malnourished and dead in a month or so. Of starvation or heat stroke.

Give him a nice meal of commercial turtle pellets and take him back where you found him and turn him loose.

Wow. Slidiers are carnivores? Never knew that. Yeah, best keep that poop out of compost piles.

I want to thank everyone for their concern for Cecil's health, happiness, and well-being. Rest assured he is safe and doing fine. Let me apologize for any trauma and sleeplessness I may have caused anyone due to the incomplete reporting on his short term reptilian habitat. The spacious raised bed, that served as his turtle condo get-away vacation spot was well equipped with a pool, all-you-can eat buffet, and a various assortment of amenities that would remind him of home. I mentioned the treat of lettuce that sent a few folks into schoolmarm mode. The ice cream my uncle occasionally used to sneak me before dinner didn't ruin my apatite as my soccer-mom mother feared, neither did the turtle salad bar seem to prevent Cecil from feasting on worms, crickets, and an assortment of other creeping things during his short stay.

I don't know exactly where Cecil is now, probably happily swimming in the marsh that my property backs up to. Whatever the case, he is probably doing better than his cousins who made their way out of the busy road-side ditch, that I rescued him from, and into oncoming traffic.

For the record, the original question was more of a curiosity lark than anything else. I've seen threads related to dog, cat, chicken, goat, lama, lamb, rabbit, horse, cow, bat, worm, and even human manure, but never turtle. Knowing some of the folks on this forum, I figured they would get a kick out of the subject line "Turtle Manure," if nothing else.