I really like this question!
As "mejwestut" explained, yonic symbols pertain to femininity and can be shown in literature as containers of some kind. This symbolism is first seen when the story opens: "The high gray-flannel fog of winter closed off the Salinas Valley from the sky and from all the rest of the world. On every side it sat like a lid on the mountains and made of the great valley a closed pot." By comparing the valley where Elisa lives to a "closed pot", it suggests that Elisa's life is one of confinement. Elisa must exist within the narrow limits of her world on the ranch.
Another example of this type of symbolism would be the flower pot that Elisa uses for the chrysanthemums she gives the tinker. Elisa notices later that the tinker kept the pot but threw away the flowers. The flower pot represents the practical view the men have versus the aesthetic view Elisa has. Beauty is important to Elisa, so the flowers would have been what she valued. The flower pot is what the tinker values (representing practicality), so he tosses the flowers and keeps the pot so he can sell it to someone else. Elisa knows she has no one in her life who shares her feelings, making her feel even lonelier.