An Improved Method for Non-destructive Measurements of the pH at the Root-Soil Interface (Rhizosphere)

A simple and non-destructive method is described which allows the measurement of the rhizosphere pH of soil-grown plants simultaneously, by optical methods (pH indicator) and by microelectrodes. The surface of the root-soil interface is covered by a prefixed thin sheet ( ~3 mm) of agar mixed with a pH indicator (e.g. bromocresol green). Following observations of the pH changes in the agar, detailed measurements of the pH can be made by pushing antimony microelectrodes through the agar into the rhizosphere. Examples are presented of the pH gradients along the roots of both, 4 year old Norway Spruce trees supplied with NH4-N or NO3-N (pot experiment) and 60 year old Norway Spruce trees grown on acid mineral soil. In long unbranched roots a distinct pH gradient occurs in the rhizosphere with high values in basal and apical zones and low values in the zone 2 - 5 em behind the root apex. The rhizosphere pH can therefore be considerably higher or lower compared to the bulk soil. This method can be used to accurately measure the pH gradient across the rhizosphere, even by distances less than 1 mm from the root surface. These root-induced pH changes could considerably affect microbial activity and solubility of mineral elements in the rhizosphere and thus also mineral element uptake particularly on acid soils.