The curse of Morgoth in CoH.

[Hookbill the Goomba]

In 'Morgoth's Ring' we're told that the whole ME is to Morgoth what the One ring is to Sauron. In his quest to be the undisputed Lord of ME, Morgoth has dispersed much of his vast powers into the very 'hroa' of the earth, which had the downside of making his own physical persona more earthbound and less potent.

Do you have quotes? I don't recall this from MR (then again, it's been about a year since I read it). The reason, as far as I could gather, for Morgoth's decrease in power was due to his putting his power into his servants, not the earth itself. His influence seems to go with the creatures that do his bidding. Weather or not this is more prominent in some, like Sauron, or less in others, like orcs, I don't know.
We come again to the 'seeds' of Morgoth, as mentioned most predominately in the Fall of Numenor. We know he was the spreader of lies and the account of his 'rise to power' before the slaying of the trees is a good example, I think, of how he worked.
As for Turin, we have an interesting problem. I do often wonder about his 'bad luck' as it were and exactly how involved Morgoth was in it. This raises further questions about what exactly the power of the Valar even is. Can they influence creatures that are not their servants. Certainly the moving and to an extent, controlling of things tends to be something they do, although I think Morgoth is the only one to do it extensively with living things. (Depending on how you view Manwe's Eagles). It always seemed to me that, in order for a Valar to have an effective influence on someone there had to be some willingness on the part of the one being influenced. Given Turin's hatred (albeit, of Morgoth), it may have been the source of Melkor's hand in him, if indeed this is true.
Then again, it could just be that Turin was dreadfully unlucky.