Soil additive shifting colour of grass? (9)
I believe the answer is:
quicklime
'soil' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for quicklime that I've seen before include "Field dressing to reduce acidity" , "Calcium oxide" .)