The ability of the root to change the pH of the soil in its immediate vicinity affects the uptake of both beneficial nutrients and phytotoxic metals and thus regulates the rate of introduction of mineral elements into the food chain. This project examines the effect of root growth zones on the pH of the rhizosphere. Standard methods for study of soil pH must be modified to short term, nonequilibrium measurements to analyze effects of growth zones. In a pilot study we are looking at pH in the rhizospheres of growth zones for roots elongating at different rates in response to ambient temperature. We are testing the hypothesis that the pH field can be invariant with growth rate. In our system we used both a pH sensitive dye (Bromocresol purple--transition interval pH 5.2-6.8, pK 6.3) in an agar film and a low immersion pH sensitive electrode (Lazar industries) to characterize pH changes in the rhizosphere due to the influence of root growth zones. Seedlings of Zea mays L. were cultivated in the dark in sterile sand maintained at 26 C or 20 C (supporting elongation rates of respectively 2.75 and 1.39 mm/h). The pH was measured at 2.5 mm increments in agar films that had been in contact with the root-sand system. Preliminary results indicate a pH minimum of 5.38 in the soil next to the apical 2.5 mm at both incubation temperatures. The growth zone acidifies the sand as far as 5 mm from the root surface. This project was supported by grant # 96-37-D from the Kearney Foundation for Soil Science.

Key words: elongation rate, growth zone, pH, rhizosphere, root, Zea mays