A water-retaining support for a plant exhibiting a water-retaining ability comparable to that of a polyacrylic acid-type hydrogel without inhibition of root origination or root elongation. The water-retaining support for a plant includes a hydrogel-forming polymer having a calcium ion absorption of less than 50 mg per 1 g of its dry weight and having a water absorption magnification in ion-exchange water (at 25° C.) of 100 or more. When the water-retaining support is used, a plant may be supplied with sufficient water without suffering from a deficiency of calcium ions.