As methods of manufacturing such plant cultivating substrate, there are known a method of manufacturing it by mixing and reacting water-retentive filling material such as bark compost, chaff compost with aqueous solution or solvent dispersion of a small amount of urethane prepolymer and then allowing the mixture to cure, and a method of manufacturing by reacting and curing a suspension containing water-retentive filling material, water and urethane prepolymer (see, e.g. Patent Documents 1 and 2).
There is also known a method by pre-mixing water-retentive filling material and urethane prepolymer and then adding a small amount water to the resultant mixture for its reaction and curing (see e.g. Patent Document 3).    Patent Document 1: Japanese Patent Application Published No. 56-18165    Patent Document 2: Japanese Patent Application “Kokai” No. 2-20221.    Patent Document 3: Japanese Patent Application “Kokai” No.