Orc and other evil creature toxicity

Question

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?

Answer 1

The reverse is also the case (from The Silmarillion, Of the Fifth Battle: Nirnaeth Arnoediad):

Haudh-en-Ndengin the Elves named it, the Hill of Slain, and Haudh-en-Nirnaeth, the Hill of Tears. But grass came there and grew again long and green upon that hill, alone in all the desert that Morgoth made; and no creature of Morgoth trod thereafter upon the earth beneath which the swords of the Eldar and the Edain crumbled into rust.

We shouldn't believe that the bodies of Men and Elves have special restorative powers for blighted wastelands, so it also seems that we shouldn't believe that the bodies of evil creatures are toxic.

Instead this is just a somewhat common mythical trope used by Tolkien.

I'm not aware of any in-universe explanation for it, and looking for one is perhaps over-thinking things. It may be magical, it may be the will of Eru or the Valar, or it may be just embellishment on the part of the storyteller within the feigned framework of the legendarium.

Answer 2

Scientifically speaking, dead bodies make excellent compost material; and provide much-needed nutrients to plants in semi-arid grasslands such as Rohan, northern Gondor and Ard-galen. Since the Orcs are documented as having black blood, it is apparent that their hydrostatic chemistry is different from that of the Elves and Men from whom they are ostensibly bred "in mockery".

Red-blood chemistry is based on a combination of hemoglobin (as an iron-based oxygen carrier, hence the red color) and the molecular composition of sea-water, in which red-blooded animals evolved (primitive animals such as sponges used open circulatory systems to absorb nutrients directly through sea-water.) There are other, rarer methods of circulation found in nature besides hemoglobin.

A black-blooded family of species would presumably have a different blood pH and a much different salt balance in the tissues of the organism. It follows that the olvar, created by the Vala Yavanna, were not adapted to take advantage of the nutrients found in dead Orcs, which might be poisonous to them.

This does raise the question of whether Morgoth bred evil plant-life or fungi capable of decomposing Orcs and Trolls. Such would be necessary to overcome a resultant toxic-waste problem if long-term sustenance of (edible) plant life was a concern. Per Tolkien's Letters (in reference to Sauron's motives), "evil" in Sauron's realm is not deliberately wasteful, merely hostile to the natural ecosystem, which Sauron saw as inefficient use of resources (and a haven for Elves whom he hated and envied; Sauron would have greatly prized defoliants in warfare).

Morgoth however was often deliberately wasteful and nihilistic, so presumably he got a kick out of the steady nutrient drain from the ecosystem that would result in piling up millions of orc bodies on the battlefield that could not be decomposed except over very long periods of time (under high heat and pressure) into oil and plastics using abiotic chemical processes.

The fact that grass and moss would easily grow on a hill of decomposing bodies of elves and men in the desert or grassland, in comparison, easy to explain. The existence of flowers like Simbelmÿne which only grow on the mounds of tombs can be seen as a form of specialization, since those mounds provide turned soil, better aeration, deeper topsoil depth and rich nutrients.

Rocks, on the other hand, as used by the Huorns to bury the Uruk-hai, are equivalent to using a gravel layer in construction as a form of rip-rap; water passes right through them, preventing vegetation from laying down a nutrient-rich sediment layer which would then be subject to natural processes of erosion and plant cover.

There's also some illusion in HoME and Silmarillion to the order in which various plants were awakened by Yavanna, for instance flowers didn't "evolve" until the birth of the Sun. So it's unclear what sort of grasses existed in the Third Age of Middle Earth.

In our place and time, grasses aren't often the best suited to take advantage of decomposition, they rely on a thin layer of topsoil and often rely on worms and other beneficial organisms to do so. It is quite possible that red-blooded worms don't like to eat black-blooded Orcs, and so the cycle of decomposition is broken, preventing them from being composted.

As noted (and attested in the case of bog men), their bodies would be preserved as seen in the Dead Marshes, and gradually degrade under heat and pressure into carboniferous fossil fuels, creating phenomena such as marsh lights.

(note, this is simply a hard science justification, your mileage may vary.)