And just where would you think those nutrients come from? Circle of life and all of that. And not that this is conclusive evidence, but note the correlation between elven deaths and the size of forests. Wasn't the Old Forest much larger in the past, back after the days of Nirnaeth Arnoediad? By the end of the Third Age the elven losses were slight, and the size of Mirkwood and Fangorn significantly smaller. Hmmm...just what was in the box Sam got from Galadriel?
P.S. It's even more probable. When the Fell Beast dies, doesn't it leave a 'stain' where nothing grows? Aren't there other examples where evil creatures die and their rot becomes a herbicide? Now, take something that spent some time in the Blessed Realm. How much more likely would its 'dust' impart some good effect on whatever was grown from it?
2+2=22