Research indicates that depression is relieved by gardening. In a study some bipolar disorder patients found that gardening 6 hours per week improved their mental health and moods.
Another study suggests that gardening supports an increased happy mood. It was discovered that a harmless bacteria in the soil which we interact with when we garden, increases serotonin secretion and metabolism, which improved cognitive functions and mood same as an anti depressant drug would do.
Some argue that as part of what man evolved with, playing with soil, the deprecation of it could well be responsible for mood swings and mental imbalances some experience.