In terms of setting, Kay Ryan doesn't give us a whole lot to go on: "Turtle" offers few details relating to place and time. And we can't rely much on general background knowledge about turtles, as many varieties of turtles exist in the real world. Most species spend all or part of their time in the water, but one type, the tortoise, lives solely on land.
In keeping with the allegorical nature of the poem (check out the discussion of turtle allegory in "Symbols, Imagery, and Wordplay"), there is little naturalistic description of the turtle or its environment. A mention of "grasses" (4) provides our only time-related clue. We can rule out a winter setting (unless our turtle is tropical). We're not told what the sun is doing, so we don't even know whether the action occurs at day or night.
The words "slope" and "ditch" suggest uneven terrain, but it's unclear whether the setting includes water. The poem's boat-related images ("four-oared […] rowing") could be purely metaphorical, meant to convey the turtle's lumbering movements, rather than to indicate the literal presence of water.
One might argue, though, that this minimalist sketch of a physical environment is not the only setting of the poem. In a way, the poem contains a kind of "shadow" setting, an alternate universe. (Note to self: Review Through the Wormhole series to see if Morgan Freeman ever mentions turtles.) Existing simultaneously with the objective outdoor setting of the poem, this second setting is a subjective vision of the world seen through the eyes of a turtle—a world full of looming dangers. From a turtle's perspective, a "slope" may resemble a treacherous mountain. A "ditch" might look like a perilous crevasse. And getting "stuck" in the mud could spell certain death.