In the British Isles there are a number of sayings which mention three plants each of which indicates the fertility of the soil in which it grows.  In England one species sometimes featured in such sayings is bracken (Pteridium aquilinum):  

Where there’s bracken there’s gold; where there’s gorse [Ulex europaeus] there’s silver; where there’s heather [Calluna vulgaris] there’s poverty [Newton Rigg, Cumbria, September 1988].

Whenever bracken is mentioned it is associated with gold, implying that it grows on fertile soils.  However, in their recently published ‘The ethnobotany of ferns and lycophytes’, in The Fern Gazette, 20: 1-13 (2015), H.A. Keller and G.T. Prance record:

‘In Misiones, Argentina, farmers identify compacted and degraded soils by means of observations on the presence of Pteridium aquilinum.’

This seems to suggest that in Misiones bracken is taken to indicate poor soil, the opposite to what it is thought to indicate in England.

Update.  Ghillean Prance writes:  ‘This bit of information came from my co-author, but I have heard this said by the local Guarani myself, so I think it is accurate.’

Image: Black Down, West Sussex; November 2014.

Updated 4 March 2015.