Soil is improving containing mineral spring (9)
I believe the answer is:
bespatter
'soil' is the definition.
(thesaurus)
'improving containing mineral spring' is the wordplay.
'improving' becomes 'better'.
'containing' indicates putting letters inside.
'mineral spring' becomes 'spa' (I've seen this before).
'better' placed around 'spa' is 'BESPATTER'.
'is' is the link.
(Other definitions for bespatter that I've seen before include "Dirty" , "Spray with droplets" , "Make mess of" , "Defame" .)