The present invention is a plant growth promoter, containing:a lignocellulosic biomass (A)； andat least one base (B) selected from the following (B1) to (B3),wherein the plant growth promoter has a contact angle with water of 80° or less,<；Base (B)>；(B1) a resin acid,(B2) a compound represented by formula (1),R1—；Z1—；Z2 ； ；(1)whereinR1 represents a hydrocarbon group with 9 or more carbons,Z1 represents a single bond or (OR2)p,R2 represents an alkanediyl group with 2 or more and 3 or less carbons,p represents a number that is on average more than 0 and 30 or less,Z2 represents a group selected from a carboxy group, a hydroxyl group, a sulfate group, and NR3R4,R3 and R4 each independently represent a hydrogen atom, a methyl group, an ethyl group, or (R5O)qH,R5 represents an alkanediyl group with 2 or more and 3 or less carbons, andq represents a number that is on average more than 0 and 15 or less, and(B3) a polyhydric alcohol or a polycarboxylic acid having a molecular weight of 30,000 or less.
