A soil modifier that includes: base particles that are particles of SiO.sub.2 having a particle size of 100 nm or more, and the central particle size of the base particles is 120 nm to 800 nm in a particle size distribution of the SiO.sub.2 particles. A culture soil includes soil and the base particles.

A soil modifier that includes: base particles that are particles of SiO.sub.2 having a particle size of 100 nm or more, and the central particle size of the base particles is 120 nm to 800 nm in a particle size distribution of the SiO.sub.2 particles. A culture soil includes soil and the base particles.

A floatable island includes at least one layer having a structure containing grains, the grains including an expanded mineral material.

A floatable island includes at least one layer having a structure containing grains, the grains including an expanded mineral material.