Soil-enriching material (5)
I believe the answer is:
mulch
'soil-enriching material' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mulch that I've seen before include "Treat with compost" , "Mixture of straw, leaves etc spread on ground to protect plants" , "It's laid around plants to retain moisture and keep down weeds" , "Weed suppressant" , "It holds moisture and checks weeds" .)