Actually, not all soils have a weight to volume ratio of 1.4. Among other things, it depends on the density of the various minerals that comprise the soil, the particle sizes, the moisture content and the compaction.
There are a few charts online that will give you an
approximate weight to volume conversion and vice-versa.
CWS/IMAC -
http://www.imac.ca/technofocus/weights.htm#MINERALS%20AND%20EARTHEN. Nice as it has loose and bank weights/volumes.
Cat also has a chart in their Caterpillar Handbooks, all editions.
Readme has a pcf and specific gravity chart here:
http://www.reade.com/Particle_Briefings/spec_gra2.html
George