PURPOSE:To provide a method for improving the strength of soft soil by which an economical and sufficient strength is imparted to the soft soil to provide an optional strength as the strength. CONSTITUTION:This additive for improving the strength of soft soil comprises ferronickel slag, slaked lime or quick lime and calcined plaster and preferably an additive for improving the strength at (5-95):(2-95):(1-10) ratio of the ferronickel slag:slaked lime or quick lime:calcined plaster. The total amount thereof is 100%. Furthermore, this method for improving the strength of the soft soil comprises carrying out the mixing treatment of the additive for improving the strength comprising the ferronickel stag, slaked lime or quick lime and calcined plaster with the soft soil. The mixing treatment is preferably carried out by using the additive for improving the strength comprising the ferronickel slag, staked lime or quick lime and calcined plaster at (5-95):(2-95):(1-10) ratio of the ferronickel slag:slaked lime or quick lime:calcined plaster so as to provide 100% total amount thereof in an amount of 1-95wt.% based on the whole with the soft soil.
