A study was conducted in order to quantify root-growth effects of Ca2+ and SO42- (as gypsum) in hydroponic solutions containing root-damaging aluminum on varieties of tall fescue (Festuca arundinacea) seedlings. An incomplete factorial experiment was performed in a growth chamber using one-liter pots containing levels of Al from 0-74 �M with Ca levels from 0-10 mM. Tall fescue seedlings were grown for 7 d, harvested, air-dried, scanned and weighed for treatment comparisons. Total root length as calculated from digital scanning was highly correlated with root mass due to root-diameter uniformity in hydroponic solutions. Slight but statistically significant differences existed between varieties in Al-only solutions at low Al concentrations. All varieties showed less than 15% relative root growth in 37 �M Al and higher concentrations in the absence of Ca. An increase in Ca2+ and SO42- at a given concentration of Al provided a protective effect from Al toxicity, resulting in increased root growth. The greatest relative root response to increased Ca levels occurred at Al levels of 37 and 74 �M Al. Relative root growth increased from about 30% to >80% at 37 �M Al as Ca increased from 2.5 to 10 mM. A simple logistic model adequately described the effects of Al and Ca on root growth and may be useful as a predictor of the quantity of gypsum needed to overcome Al toxicity in acid subsoils. Model goodness of fit using non-linear regression was R² = 0.86, 0.95, and 0.96 for the three varieties.