Effects of Foliar and Soil Acidity on the Rhizosphere pH of Alfalfa, Corn, and Soybean

The acidity of ambient rainfall and its effect on soil and plants is a growing concern. Glass microelectrodes were constructed and used to investigate the effect of soil pH and foliar application of acid rain on the rhizosphere pH of alfalfa (Medicago sativa L. cv. Arrow), corn (Zea mays L. cv. B73 × MO17), and soybean [Glycine max (L.) Merr. cv. Williams 82]. Plant roots were grown in minirhizotrons containing a reformed sample of Seymour (fine, montmorillonitic, mesic Aquic Argiudoll) silt loam A horizon over a silty clay loam Bt horizon. Low and high pH levels of 4.9 and 6.2 in the A horizon and 4.0 and 5.7 in the Bt horizon were established using dilute sulfuric acid or calcium oxide, respectively. Plants received daily applications of simulated rain, which was either acid (pH 3.1) or nonacid (pH 5.6). After 5, 6, or 15 d of foliar applications to corn, soybean, or alfalfa, respectively, the rhizosphere pH was measured using a glass microelectrode. The pH values for corn and soybean increased with distance from the root while the pH values for alfalfa decreased with distance. As the soil pH increased from 4 to near 6, the difference between the pH at the root surface and the bulk soil increased from 0 to near 1. A trend for lateral root pH values at all distances to be slightly higher than main roots was observed. Increasing the pH of the A horizon had no significant effect on the rhizosphere pH of corn roots growing into the Bt horizon, but significantly increased soybean rhizosphere pH in the Bt horizon. Acid rain applications caused foliar damage, and tended to decrease rhizosphere pH, but few effects were significant. The data emphasize the effect of roots on the heterogeneity of soil pH and show that reliable rhizosphere pH measurements of in situ roots can be made using glass microelectrodes. Contribution from the Missouri Agric. Exp. Stn. Journal series no. 11,184. This research was supported by USDA acid precipitation special grant 87-CRSR-2-3202.