Fertile loam (5)
I believe the answer is:
loess
'fertile loam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for loess that I've seen before include "desposit" , "geological feature" , "Wind-deposited silt" , "Compacted wind-blown sediment (geology)" , "Wind-blown loamy deposit" .)