Rainfall, surface runoff, and root zone pH values were determined in an effort to explain symptoms of decline in white oaks (Quercus alba L.) growing in, and downslope from, a parking lot. The soil was a Morley silt loam (Typic Hapludalf). Root zones within and downslope from the lot were dominated by alkaline soil reactions. Soil pH values along 15-m transects radiating from a white oak tree in a healthy stand upslope from the lot were predominantly acid beneath the crown, averaging between pH 5.5 and 6.1 to a depth of 90 cm within a 1.5 m radius of the tree trunk. Rainfall pH values varied from 4.3 to 5.9. Rainwater runoff from the parking lot and roadway varied from pH 7.5 to 8.0. Sampled in late summer, foliage of stressed trees in, and downslope from, the parking lot were all deficient in manganese (26-38 mg kg(-1)). It was concluded that Morley soils can be alkalinized by alkaline runoff from surfaces of calcium carbonate-containing structures, thereby reducing the solubility of soil manganese to a degree sufficient to cause deficiency of this element in white oaks.