To promote the raising of a plant by mixing a water purifying cake having a specific phosphoric acid absorption coefficient and an available phosphoric acid with a versicular-arbuscular mycorrhizal(VA) fungus and forming the culture soil.

This culture soil for raisin a plant is contained by carrying out the disintegration and particle size selection of a water purifying cake, prepared by treating a sedimented muddy soil produced in a water purifying treating process according to a chemical nonfeeding method and having ≤2000mg/100g phosphoric acid absorption coefficient, ≤10mg/100g available phosphoric acid content, about 0.1-2.0% total phosphoric acid content and about ≥0.3% total nitrogen content into about ≤10mm diameter, than adding and mixing the water purifying cake disinfected with chlorine sterilization, etc., with about 5-50vol.% composing material such as a rice straw and about 5-50vol.% soil conditioning material such as coal, subsequently adding and mixing the resultant mixture with a VA fungus belonging to the Gigaspora, etc., in an amount so as to provide ≥10 spores, preferably ≥100 spores in 1L culture soil for raising the plant. A plant in which the VA fungus can be symbiotic such as a leguminous plant is cultivated in the culture soil for raising the plant to efficiently proliferate the VA fungus and further well grow the plant.