Ultrabasic rocks are so different from crustal rocks. So alien to continental earth surface, which is mostly granitic and not peridotitic.
The peridotite are with few exemptions, mantle rocks (deep under surface, 30 km under continents and about 8 under the oceans).
They have the crust squeezed out of them to form the continental crust.
So they lack of crustal elements.
They have Mg but also Fe, nickel and chrome.
These rocks usually occur in Alpine type Orogens. Like in Franciscan in California. Or Alps to Hellenides. Or Venezuelan Cordilleras. Ronda in Spain or Lherz in Pyrenees mountains.
Usually they high up in mountains from 1000-1500m and up.
But pastures are so green even in September....
Still plants are growing there. Some special, endemic, growing only on this type of soils.
Forest even on northern dipping slopes.
I haven't read BBB but must be in chapter 8 under difficult soils
Mg in the water. Mg is trace element in drinking waters and soils but still very necessary part of chlorophylle molecule and ?muscle¿ cells.
Concentrations of Mg in ultrabasic massif's spring's water are highest you can get, right? .....
Water on/in limestone/ dolomite is basic [Ph7 and higher]
Mg in Chlorophylle molecule
One tiny metal ion in the huge framework of C H and ?N¿.
Ultrabasic soil is geochemical term, I remember the term way back, 20 years, from my studies.
Limsotones (CaCO3 mostly built out of calcite) are producing basic soils which are more difficult for life to occur then acidic. The condition of living is tough there is not enough water since it dissolves carbonate rock and runs deep and forming caves...
Ultrabasic soils form mostly on ultrabasic rocks, like dunites and serptentinites or so called peridotites. Their mineralogy is completely different from limestone. But the soil is more basic (in Ph) then limstones.
Of course in the C, H, N have a role. I like one lecture video from Geoff Lawton going into details. I am far from being praxis. I was talking about Chlorophylle formula, I will google it. Sorry for confusion.
I hope I get some time to research this a bit more. I from my previous occupation know more about rocks then about soils.
But this soil don´t have sand, it has mostly serpentine minerals and very little clays. I see that my point of view is a bit screwed. I wish I didn´t started the topic