Orc and other evil creature toxicity

In the Lord of the Rings, various creatures of evil are killed, and when buried or burnt, no grass grows there.

For example, the fell beast of the Nazgul

"Green and long grew the grass on Snowmane's Howe, but ever black and bare was the ground where the beast was burned"

also Orcs

"but in the morning they went out and were amazed; for the slain Orcs were gone, and the trees also. Far down into the valley of the Deep the grass was crushed and trampled brown, as if giant herdsmen had pastured great droves of cattle there; but a mile below the Dike a huge pit had been delved in the earth, and over it stones were piled into a hill. Men believed that the Orcs whom they had slain were buried there; but whether those who had fled into the wood were with them, none could say, for no man ever set foot upon that hill. The Death Down it was afterwards called, and no grass would grow there.

Is this due to some magical effect, or is it that these creatures are actually toxic?