NOVELTY - Producing ecofriendly water retentive nutritious soil via lake sludge, comprises adding ecofriendly water retention nutritious soil semi-finished product into a twin-shaft compulsory mixer, adding a biogas slurry water retention agent, uniformly blending and mixing, transferring mixture into an hard particle extruding and pelletizing machine, extruding into cylindrical particle, and introducing cylindrical particle into an integrated insecticidal sterilization plant for carrying out insecticidal sterilization, where the particle is ecofriendly water retention nutritious soil finished product.    USE - The water retentive nutritious soil is useful for cultivating bonsai flowers in 45 days without watering.    ADVANTAGE - The method is simple, economical and is suitable for cultivating bonsai flowers without watering in 45 days; ensures high production efficiency, and can process the piled or waste sludge at the dredging field of the port, channel, lake or river into ecofriendly water retention nutritious soil with various formulations. The ecofriendly water retention nutritious soil is non-toxic, harmless and odorless, and rich in nutritional components; and has high fertilizing efficiency.    DETAILED DESCRIPTION - Producing ecofriendly water retentive nutritious soil via lake sludge, comprises adding ecofriendly water retention nutritious soil semi-finished product into a twin-shaft compulsory mixer, adding a biogas slurry water retention agent, uniformly blending and mixing, transferring the mixture into an hard particle extruding and pelletizing machine, extruding the mixture into a cylindrical particle, and introducing the cylindrical particle into an integrated insecticidal sterilization plant of soil science society for carrying out insecticidal sterilization, where the particle is the ecofriendly water retention nutritious soil finished product. The ecofriendly water retention nutritious soil comprises (in wt.%) ecofriendly water retention nutritious soil semi-finished product (45-85) and biogas slurry water retention agent (15-55). The biogas slurry water retention agent comprises (in wt.%) biogas slurry (95-99.5) and high water absorption resin (0.5-5). The biogas slurry water retention agent is prepared by adding high water absorption resin and biogas slurry into a container, mixing for 10-60 minutes, and curing the biogas slurry into the biogas slurry water retention agent. The ecofriendly water retention nutritious soil semi-finished product comprises (in wt.%) lake sludge (35-80), fermented poultry excrement (2-25), edible fungus mushroom residue (2-20), biogas residue (2-20), carbonized edible fungus mushroom residue (1-10), high viscous attapulgite clay powder (1-10), bentonite powder (1-10) and trace elements mixture (0.01-2). The trace element mixture comprises (in wt.%) ferrous sulfate (0.01-25), zinc sulfate (0.01-25), ammonium molybdate (0.01-20), copper sulfate (0.01-20), borax (0.01-20) and manganese sulfate (0.01-20).