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 µM 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 µM 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 µM Al. Relative root growth increased from about 30% to >80% at 37 µM 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.