I might approach this by saying "It is not the case that many farmers will not grow green crops unless they abandon chemical fertilizers."
However, we can actually simplify (E) a bit before we negate it as it is conditional. We could rewrite it as If they do not abandon chemical fertilizers, many farmers will not grow green manure crops.
Or its contrapositive: If many farmers grow green manure crops, they must have abandoned chemical fertilizers.
This final statement is much easier to negate- we could say that many farmers will grow green crops even if they have not abandoned chemical fertilizers.
That said, I dont always negate all five answer choices. If you can fully understand an answer choice but are struggling to negate it, you can almost treat it as a must be true- that is, must it be true the author agrees with this statement? If so, then it is likely a necessary assumption.
