PROBLEM TO BE SOLVED: To provide plant growth promoting methods in which filamentous fungus/actinomycetes are bred with an organic acid aqueous solution, components effective for plants, such as a potassium, are extracted from the soil, and plants are raised with the soil to increase the number of tillers or to enlarge the plant bodies.SOLUTION: A plant growth promoting method utilizing organic acid and filamentous/actinomycetous biofilms comprises impregnating a 0.1 to 50.0% or preferably 0.1 to 10.0% aqueous solution of lactic acid, citric acid, a malic acid, tartaric acid, acetic acid, a gluconic acid, an oxalic acid, a fumaric acid, succinic acid, mugineic acid, and a glycolic acid into the soil, forming a biofilm mainly composed of filamentous fungi and actinomycetes on the soil surface, extracting an effective component for plants such as potassium from the soil, and raising the plants with the soil to increase the number of tillers or enlarge the plant bodies after calming the fermentation activity by microorganisms in the soil or after filling water.SELECTED DRAWING: Figure 1COPYRIGHT: (C)2017,JPO&INPIT【課題】有機酸水溶液で糸状菌・放線菌類を繁殖させ、カリウム等植物に有効な成分を土壌から抽出させて、前記土壌で植物を育成して分蘖（ぶんげつ）数を増加させたり植物体を大型化させたりする植物生長促進方法を提供する。【解決手段】有機酸と糸状菌・放線菌類バイオフィルムを活用した植物生長促進方法は、土壌を０．１～５０．０％、好ましくは０．１～１０．０％の乳酸やクエン酸、リンゴ酸、酒石酸、酢酸、グルコン酸、シュウ酸、フマル酸、コハク酸、ムギネ酸、グリコール酸の水溶液を浸漬させ、土壌表面に糸状菌や放線菌類を主体としたバイオフィルムを形成させ、カリウム等植物に有効な成分を土壌から抽出させて、前記土壌の微生物による発酵活動の鎮静化後に、或いは水を張ってから、前記土壌で植物を育成して分蘖（ぶんげつ）数を増加させたり植物体を大型化させたりする。【選択図】図１
