A foliarly applicable plant nutrient composition comprises, in aqueous solution, (a) a first component comprising an agriculturally acceptable source of foliarly absorbable silicon； (b) a second component selected from agriculturally acceptable sources of thiosulfate ions, agents effective to inhibit polymerization of silicic acid or silicate ions, and mixtures thereof； and (c) as a third component, an agriculturally acceptable mixture of compounds selected from the group consisting of organic acids, organic compounds having functional groups capable of reversibly binding or complexing with inorganic anions, and mixtures thereof. The composition is useful for silicon nutrition of a plant and for reducing susceptibility of a plant to fungal or bacterial disease.
