PROBLEM TO BE SOLVED: To provide culture soil for raising seedling, from which a root ball with high strength can be made, and allowing a user to smoothly carry out automatic transplantation using a transplanter. SOLUTION: This culture soil for raising seedling is obtained by formulating a culture soil base material with fusible fibers each having a fiber length of 0.5-2 mm, aspect ratio of 20-300, fiber moisture contant of <=10 wt.% based on the fiber mass, and the number of crimps of <=6/cm, and a method for hardening thus obtained culture soil by heating is provided.