Oat (Avena sativa L.) seedlings were grown in nutrient solutions deficient in either calcium, magnesium, or potassium as well as in single-salt solutions containing each of these elements. Strontium was studied using a nutrient solution in which calcium was replaced by strontium.The absence of magnesium or potassium did not affect root elongation in media containing calcium. Solutions containing only calcium nitrate supported some root growth but growth was negligible in all other single-salt solutions. When calcium was replaced by strontium, primary root growth and cell elongation were significantly reduced. However, there were no other readily detectable morphological changes resembling symptoms of calcium deficiency.Root hair elongation was not affected by the exclusion of either magnesium or potassium from nutrient solutions, but calcium deficiency (sodium substitution) resulted in less elongation and in deformation. Of all the single-salt solutions, only calcium nitrate supported normal root hair growth. Although root hairs were not deformed, formation and elongation were considerably reduced in strontium-substituted solutions. Strontium, in mineral nutrient media, may prevent toxic effects of other ions but does not completely replace calcium for root and root hair growth of oats.