Nitrogenous fertilizers such as ammonium sulphate and urea undergo nitrification in the soil, producing hydrogen ions (\(H^+\)), which lower the soil pH.
Thus, excessive use of these fertilizers results in soil acidification.
Hence,
\[
\boxed{\text{Soil acidification}}
\]
is the correct answer.
Therefore,
\[
\boxed{(C)}
\]