To solve problems of perlite, zeolite and vermiculite which have been conventionally used as soil improving materials, and to provide a soil improving material which has excellent water retaining properties and is well-drained to retain air in a soil to thus make a soil suitable for plant growth, and a culture soil containing the soil improving material. The soil improving material of the invention includes porous ceramic particles having a moisture content of 5 vol% or greater at a pF value of not more than 2.7. The particles preferably have a water retaining ratio of 15% or greater. The particles preferably have a saturated hydraulic conductivity of 0.1 cm/s or greater.
