This invention provides methods of enhancing plant growth or yield, comprising exposing soil to hydrogen gas (H.sub.2), and growing plants in the soil. The H.sub.2 gas employed can be generated by electrolysis of water, produced by H.sub.2 evolving microorganisms, and/or produced by legumes. In accordance with the invention, soil can be exposed to H.sub.2 and plants then grown in it, or H.sub.2 can be applied directly to soil in which plants are already growing. In one embodiment, the invention comprises obtaining an aqueous extract from H.sub.2-treated soil, and applying the extract to soil, seeds, and or plant roots.