Recognition of earth as one of the four classic elements in the context of garden design would seem almost obvious in the fact that all gardens are inherently outside and already "of the earth". Hence, all gardens are earthly by the nature of their physical proximity to the ground.
Earth can be seen as the home of all the aspects of nature, the ground, the soil, the plants, water, fire and air. Earth is our home into which we grow roots and plant seeds of aspirations and desires. We are anchored and nurtured by its solid foundation.
Mother Earth is a term derived from ancient cultures and religions including Native American and Pagan beliefs. Father Sky and Mother Earth are seen as entities that are revered because of what they provide all living things whether that be warmth, shelter, or food. The earth as a mother figure is the feminine energy of the planet -- nurturing and supportive.
So in regards to designing with the four elements, earth can be represented in a number of ways while its manifestation in a garden may be obvious or symbolic.
One must have an appreciation of what earth means to them and revere its inclusion in the physical design of a garden for it to not lose its recognition.
Its manifestation may be so subtle that one may overlook its use as in looking upon a typical Zen style dry garden without using your "symbolic eyes".
Our physical bodies are born into the lifecycle of a three dimensional existence that we call planet earth. Hence, life on earth is part of a larger cycle of birth, growth and death.
The earth as a physical form is grounding. Its surface contains an abundance of electrons that heal and stabilize life forms. When we lose that sense of being grounded, we feel disconnected with nature and yearn to renew that connection. By simply standing barefoot on the ground, we can rejuvenate the healing forces between our bodies and the earth.