On Wed, 06 May 2015 20:11:45 +0100, Martin Brown
wrote:

snip

Compaction in heavily trafficked grassland can also weaken trees in the
likes of Kew gardens where they have taken to experimentally injecting
compressed air into the soil around particularly sensitive specimens.

An interesting article on this is at:
http://www.forestryjournal.co.uk/new...9/Iss27Kew.pdf