PROBLEM TO BE SOLVED: To produce an organic substance-degrading bacterium and nitrifying bacterium by individually controlling them in a method for producing a nutrient solution for plant cultivation in which the reaction of decomposing organic matter in water to obtain nitrate ion is carried out in the same system (tank). Reduce the possibility that harmful bacteria will be mixed in the nutrient solution. SOLUTION: As a microbial source, a microbial agent 3 of organic matter decomposing bacteria and a microbial agent 4 of nitrifying bacteria are separately prepared. To the tank 1 containing water, fish liquid fertilizer as an organic fertilizer 2 is added in an amount of more than 0.5 g/L, and two kinds of microbial agents 3 and 4 are simultaneously added in the same amount. The amount of each of the microbial agents 3 and 4 added is 0.01% to 1.5% per 1 L of water. After the addition of the fish liquid fertilizer 2 and the microbial agents 3 and 4, there is provided a period in which the decomposition of organic substances and the nitrification in the water in the tank 1 proceed. The liquid in the tank 1 is diluted during or after nitrification, and additional fertilization is performed simultaneously with or after the dilution. A period during which decomposition of organic matter and nitrification are advanced is provided for the liquid after dilution and topdressing. [Selection diagram] Figure 2【課題】水中で有機物を分解させて硝酸イオンを得る反応を同一の系（槽）で行う植物栽培用養液の製造方法において、有機物分解菌と硝化細菌の添加を個別に制御でき、製造される養液に有害菌が混入する可能性を低くする。【解決手段】微生物源として、有機物分解菌の微生物剤３と、硝化細菌の微生物剤４とを別個に準備する。水が入れられた槽１に、有機質肥料２として魚液肥を０．５ｇ／Ｌより多い量添加するとともに、２種類の微生物剤３、４を同時に同量添加する。各微生物剤３、４の添加量は水１Ｌ当たり０．０１％～１．５％の量とする。魚液肥２及び微生物剤３、４の添加後、槽１の水中で有機物の分解及び硝化を進行させる期間を設ける。硝化の途中又は完了後に槽１の液を希釈し、希釈と同時又は希釈後に追肥を行う。希釈及び追肥後の液に対して、有機物の分解及び硝化を進行させる期間を設ける。【選択図】図２
