Step 1: Analyse the Assertion.
Improved crop varieties are widely used because they provide
• Higher yield,
• Better quality,
• Disease resistance,
• Early maturity.
Hence, the Assertion is correct.
Step 2: Analyse the Reason.
Improved crop varieties still require essential soil nutrients such as
\[
\text{Nitrogen, Phosphorus and Potassium (NPK)}.
\]
Therefore, the statement that they grow without soil nutrients is incorrect.
Step 3: Final conclusion.
Thus,
\[
\boxed{\text{Assertion is correct, but Reason is incorrect}.}
\]
Hence, the correct option is
\[
\boxed{\text{(C)}}.
\]