I'm a geologist with some experience in soil science. A loam is a soil with roughly equal proportions of sand, silt, and clay. If you can find a natural soil that can be squeezed into a lump that mostly holds together, but is moderately crumbly, and that feels gritty, that is a good approximation of loam.
To make it an acidic, organic loam, mix that loam well with roughly an equal volume of peat moss.