This question surrounds Ultima all through the novel. So, is she a witch? If not, what makes her different from the brujas or witches that do come up in the novel? And why does it matter in the first place?
I had been afraid of the awful presence of the river, which was the soul of the river. (2.36-37)
Here's a note Anaya plays over and over again. With the supernatural, comes fear. The people accept the supernatural, but they don't necessarily understand it. This lack of understanding makes them afraid, and it often causes them to lash out against the people connected with the supernatural, like, say, Ultima.
Ultima's spirit bathed me with its strong resolution. (2.343-344)
At different times in the book, the power of Ultima and other supernatural occurrences are compared to whirlwinds, said to "bathe" people, and cause crazy powerful physical reactions. It's as if a supernatural act or gift bears both spiritual and physical properties (kind of like the way people see things in nature, sometimes).
For Ultima, even the plants have a spirit. (4.17)
For Ultima, the natural world and the supernatural world are no different. Natural items like plants and water bear supernatural properties. If one is willing to listen and learn, those properties will reveal themselves.
I looked at Ultima. Her magic. The magic of Jasón's Indian. They could not save me now. (6.95-96)
Not even the awesome supernatural powers of Ultima can save Antonio from the misery of the first day of school.
"The golden carp," I said to myself, "a new god?" (9.399)
Antonio's new knowledge of another supernatural being—one not linked to the God of his Church—causes Antonio to start to question everything he believes. His concept of reality has been shaken to the core.
"He drew near and saw that it was no natural fire he witnessed, but rather the dance of the witches." (10.135-137)
The reaction to this story is one of horror and fear, but not disbelief. As we see throughout stories with elements of magical realism, things like witches are totally accepted as being a part of the world. So, how is this different than say a fantasy movie or a science-fiction novel?
Green bile poured from his mouth, and finally he vomited a huge ball of hair. (10.630-631)
Okay, that's absolutely disgusting. But hey, who doesn't like a nice gross-out moment every once in a while? What's important to take away from this, though, is not the nausea it induces, but the fact that this is a physical manifestation of a supernatural curse. Again, we see the supernatural exist on more than one plane.
"The brujas who laid the curse knew this […] they simply awakened the ghosts of the three Indians and forced them to do wrong." (18.182-186)
Ghosts and witches in cahoots? It's like a Stephen King novel or something. Beyond the horror aspect of it, though, this quote marks just how powerful witches are in this novel. Not only can they control and curse men, they can actually gain power over the dead.
The owl was her soul! (22.439)
If there was ever a question about Ultima's link to the supernatural world, it ends here. The woman's soul rests in an owl. Her link to nature is complete and her link to powers beyond that of the natural world holds strong. Now, we're not saying she's a witch (we're not Tenorio, after all), but it might be worth doing a little digging on the power of a witch's familiar.