Fertiliser that's good for minute valley (5)

I believe the answer is:

gulch

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'valley' is the definition.
(I know that gulch is a type of gorge)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gulch that I've seen before include "American ravine" , "Narrow ravine (US)" , "Narrow gorge with a stream in North America" , "Ravine - canyon" , "A narrow gorge with a stream running through it" .)

I've seen this clue in The Telegraph.

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