It is "normal" to separate modifiers with commas, but not to have a comma between the last modifier and the noun.
So, based on this view of normality I would write:
... light-grayish, moderately-loamy, cultivated forest soil.
I have one objection to this: I would lose the hyphen between moderately and loamy. But I know that hyphens are more popular in British English than in American English.
Some more comments:
I think I would just say either light gray (in which case I'd also lose the hyphen) or grayish. Light-grayish strikes me as too much gray-modifying, but if you need that degree of precision then by all means go with it.
This may have been obvious to you, Panj, but I'll explain it for the benefit of the greater WRF population. It is true that when we have a list of adjectives preceding a noun, we do not place a comma between the last adjective and the noun. But note that in this case there is no comma between cultivated and forest, either. That's because forest is so closely attached to soil that the two form a unit that could be treated as a single noun for punctuation purposes. We may as well spell it forest-soil or forestsoil.
a beautiful, expansive, state-of-the-art auditorium
a stately, well-maintained grand piano (no comma between well-maintained and grand)
The term hierarchical scared me a bit, but I see what they are getting at.
We have soil that has the following attributes:
Tell me about the soil?
It is cultivated and light-grayish and moderately-loamy soil.
It seems really bizarre to say that it is forest and cultivated and light-grayish and moderately-loamy soil.
The attribute "forest" cannot be detached from the soil so it is, in the terms of the link AngelEyes gave, an hierarchical adjective. This is the same "feeling" that Elroy explained earlier when he said this stuff was "forestsoil" or at least "forest-soil".
As a result, we have ... light-grayish, moderately-loamy, cultivated forest soil.