This was freshly deposited alluvial material from gully erosion perhaps a few days or weeks old, the density was about 1 gram per cc. That is the lowest I remember in my work. It later increased in density by adjusting in place. Possibly not what you were looking for, but the material was extremely light. Also I remember a few instances of light soils with highly miceaceous content of about 35%. Soils with alot of organics may also weigh out to be light, and I have seen pumice soils but never weighed them, but when you can pick up a small pumice boulder, that has to be pretty light material.
Thanks for responding my question. So, the lowest dry density is 1g/cm3 from your experience. I take your point on soils with miceaceous content or organic content and pumice soils which are lighter. I learnt something new from you.