Modern mental health professionals consider geophagy, the practice of consuming soil, to be a mental disorder.

Others consider it to be art: via Edible Geography comes tell of artist Laura Parker’s Taste of Place project, where individuals are invited to taste soil as a way of understanding its relation to food production.

…is about to get technical classification courtesy of Homegrown Evolution.  Erik Knutzen’s blog recommends The U. Mass (not that U. Mass) Soil and Plant Tissue Testing Service for inexpensive quantitative soil testing.

While the service is intended for those that want to grow stuff, the pH ($5) and Soil Texture Classification Tests ($60) should be well suited to those dabbling in mud construction who need more sophisticated soil analysis than is offered by the jar test.  The Textural classification used by the U. Mass service is anecdotally less exact than chemical analysis, but $60 is a bargain compared to sending soil samples to a California geotechnical lab for similar testing.

Edit:
Wallace Engineering in El Segundo offers the same tests for less bucks.   For example, here are two that might be of interest to mud construction enthusiasts:

• Testing for pH, salinity and presence of lime.  $20/sample.

• Soil texture testing – sand, silt and clay percentage and gravel.  $25/sample