Fuente: WIPO "tomato" A soil conditioner including a lignin-based compound as an active ingredient and capable of efficiently increasing the amount of microorganisms and inorganic components in soil. The soil conditioner includes a lignin sulfonic acid having a phenolic hydroxyl group content of 0.1% to 5.0% by weight, a methoxyl group content of 1.0% to 15.0% by weight, and a sulfone group-derived sulfur atom content of 2.0% by weight or higher. An improved soil composition, including the soil conditioner and soil; a method for preparing improved soil, including adding the soil conditioner to soil; and a plant production method in which a plant is produced with the improved soil composition are also described.