Sok Chanty It could be happned when there is too much water in the soil. Firstly, make a proper drainage. Add compost in this soil and to fight the bad smell, effective microorganism (EM) is good. You can spray EM. If you don't have, alternatively you can spray fish amino acid which is also rich in microorganism.

Thein Soe how can I get the EM? I would like to use it too.

Use rice water and humic acid to enhance beneficial soil microorganisms.