Manganese (Mn) toxicity can develop in peanuts grown on low pH soils. The objectives of this study were to quantify the impact of soil pH and magnesium (Mg) on the uptake of Mn and the development of Mn toxicity symptoms in peanut plants and to evaluate the use of the Mg:Mn ratio as a diagnostic tool for Mn toxicity in peanuts. Three greenhouse tests were utilized to meet these objectives: a study to determine dolomitic limestone effects, an experiment comparing rate effects of calcium (Ca), Mg, and potassium (K) on Mn toxicity, and a test to separate the effects of pH, Mn, and Mg on Mn toxicity. Soil, leaf, and stem samples were taken for analysis, and toxicity ratings were made. Increasing pH diminished the toxicity rating and leaf Mn concentration and increased the leaf Mg:Mn ratio. The toxicity rating was significantly correlated with both leaf Mn and Mg:Mn ratio, but leaf Mn generally had stronger correlations and was more useful in diagnosis. Magnesium application resulted in a marked reduction in the Mn toxicity rating and leaf and stem Mn concentrations in the second experiment; however, this result was not repeated in the final test. Using Mg to prevent Mn toxicity would require large Mg applications, which could have the serious detrimental effect of interfering with Ca uptake by the peanut fruit. Liming is the more practical method for avoidance of Mn toxicity in peanuts.