Question

Alternating of crops preserve the soil ________.

Answer 1

FERTILITY..............
HOPE THIS HELPS U

Answer 2

Hey there!! :)

The word in the blank will be ' fertility '.

So,
Your sentence will be as,
Alternating of crops preserve the soil fertility.

However,
Your sentence seems some wrong to me. You used the word ' Alternating ' but it should be ' Rotation '. That's the correct term for your sentence.

Hope it helps!