The present invention relates to (i) a plant growth-improving agent containing a growth-improving component that increases a concentration of an oxoanion in . an area around a plant, the oxoanion being heavier than a sulfate ion and containing four oxygen atoms, (ii) a seed to which such a plant-growth-improving agent has been applied, and (iii) a plant growth-improving method including a cultivating step of growing a plant in the presence of such a growth-improving component.
