Minespoil Acidity and Rowcrop Productivity

The development of extremely to strongly acid (pH 3.5–5.5) minespoil at a surface coal mine in Illinois resulted in unusually low field corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] grain yields, particularly in years of severe drought stress. Spoil acidity, measured in terms of pH, was evaluated as a factor contributing to or interacting with drought stress. Drought stress was most clearly expressed in corn by increasing numbers of barren plants. Very strongly acid spoil confounded drought stress and resulted in barren plants, even in wet years. In 1980, the driest year, variation in spoil pH accounted for 58% of the variation in both barrenness and corn grain yield without topsoil replacement. Where 20 cm of topsoil had been replaced, variation in spoil pH immediately underlying the topsoil accounted for 40 and 34% of the barrenness and grain yield, respectively, in 1980. Soybean grain yield correlated most strongly ( r = 0.969) with spoil pH in a year of adequate precipitation, suggesting a different physiological response than that for corn. Depressed soybean grain yield occurred, and plants were characterized by potentially phytotoxic leaf concentrations of Al (>70 mg/kg) and Mn (>300 mg/kg), where the pH was less than 5.5. Results indicate that newly reclaimed minespoils having a pH ≤4.5 are unacceptable for row crop production in Illinois. Spoils having a pH between 4.5 and 5.5 should also be avoided or limed.