How does Turtle impact Taylor and challenge her to become a good mother? How does she change and grow through this relationship?

Turtle and Taylor have an unusual relationship in Barbara Kingsolver's novel The Bean Trees. Taylor is a free-spirited young woman who's off on a solo adventure. Turtle is the three-year-old that's handed, unexpectedly, to Taylor in a roadside restaurant. For Taylor, the restaurant was just a quick stop, a place to have something to eat, in a town where she knows no one.
How does Turtle impact Taylor and how does Taylor grow though their relationship? Let's go over a few main points.
Turtle compels Taylor to become a mother.
Taylor didn't choose to be a mother. It's only because Turtle has been forced onto her, and she doesn't know what to do, that Taylor becomes a caretaker for the child. Turtle's real mother has abandoned her, and Taylor feels obliged to step in, even though she doesn't know anything about taking care of a child. Here, Taylor first feels the attachment to Turtle:

The most amazing thing was the way that child held on. From the first moment I picked it up out of its nest of wet blanket, it attached itself to me by its little hands like roots sucking on dry dirt. I think it would have been easier to separate me from my hair.

Turtle makes Taylor seek out (and embrace) a community.
Taylor, before meeting Turtle, didn't need a community (or at least she didn't think she did.) She left her own mother and took off on a solo road trip. But when she's forced into a position where she feels like she has to take care of another person, she has to get practical and think about the logistics. She needs an affordable place to live, so she replies to an ad from another young woman who's looking for a housemate. That's how she meets Lou Ann, her friend and confidante who will become like a surrogate family for Taylor and Turtle. 
Turtle helps Taylor see the beauty of the environment.
As a little girl, Turtle loves to play in the dirt, to sing songs about vegetables, and to look at a seed catalogue. Taylor sees the land they're on in a different way after spending so much time with Turtle. Here, Turtle points out the bean trees to Taylor:

Turtle shook her head.
“Bean trees,” she said, as plainly as if she had been thinking about it all day. We looked where she was pointing. Some of the wisteria flowers had gone to seed, and all these wonderful long green pods hung down from the branches. They looked as much like beans as anything you’d ever care to eat.
“Will you look at that,” I said. It was another miracle. The flower trees were turning into bean trees.

Turtle helps Taylor see the importance of female friendship and support.
Taylor needs Lou Ann to help her with childcare. It's because of Turtle that she lets Lou Ann into a more intimate family space; as a result, she reaps the benefits of a family life with her friend and with their respective children. Here, Lou Ann speaks to Taylor:

"Taylor, remember that time you were mad at me because you didn't want us to act like a family? That all we needed was a little dog named Spot? Well, don't get mad, but I told somebody that you and Turtle and Dwayne Ray were my family. Somebody at work said, 'Do you have family at home?' And I said, 'Sure,' without even thinking. I meant you all. Mainly I guess because we've been through hell and high water together. We know each other's good and bad sides, stuff nobody else knows."

After establishing an unconventional family life with Lou Ann, Taylor doesn't need a man to complete the picture, and on top of that, she helps Lou Ann find her own independence after Lou Ann's been abandoned by her husband.