True, Postgres 9.1 does not have json / jsonb data types. But there is the additional module hstore providing the data type of the same name.
hstore has been in the core release since Postgres 8.3 (it's very mature). For just text data and no nesting it's just as good as json, if not better. (jsonb introduced some new candy.)
If you only have a few dozen of sparsely populated columns I would also consider wide rows with many columns just being NULL. NULL storage is very cheap in Postgres.
You could also go with an EAV (Entity-attribute-value) design. Related answer discussing pitfalls of related :
But consider upgrading to a current version of Postgres first. Postgres 9.1 reaches EOL in Sept 2016.