I recently heard a little tidbit on the radio, but I don't rememeber exactly where I heard.  It said something to the effect that if you work with your hands in the soil, there is some positive effect.  I think it has something to do with increasing the seratonin levels in your body, and that makes a person healthier (?), or calmer (?) or ??? 

and it may not be even seratonin.  I just remember that working with soil in your hands is supposed to be healthy and good for you.