This method for promoting plant growth treats plants with a compound indicated in formula (1). However, methods for promoting plant growth by treating plants with the compound indicated in (1) below or an agriculturally acceptable salt thereof are excluded. (1) benzo[b]thiophene-3-carboxylic acid. The method set forth in one of the above, the plant being soybeans. The method set forth in one of the above, the plant being cotton. A plant seed resulting from treatment with the compound of claim 1 containing an effective quantity of the compound indicated in formula (1). A composition for promoting plant growth and containing an inert component and the compound indicated in formula (1).
