To try to help save Greenroot's corrupted herd, you must venture into the heart of the darkest glade in Fangorn's eaves.
You can do nothing for Greenroot by consoling him; you must venture into the shadows of the Black Heart. You must try to understand the place before you can hope to heal it.
Samples of soil can be found in the eastern parts of the Black Heart in Fangorn.
You should examine the soil to see if it contributes to the corruption of the Black Heart.
It is clear that the Black Heart is rotten from its very roots, with vile crawlers squirming out of every patch of mud.