Simplified measurement of soil pH using an agar-contact technique

A method for the indirect measurement of soil-pH is described. This method allows the spatial arrangement of soil and rhizosphere to be conserved. The soil is brought into contact with a layer of agar, containing bromocresol purple. A nylon gauze is placed between soil and agar. For quantitative pH measurements, a micro-electrode is inserted into the agar after three hours of contact between soil and agar.The validity of the method was checked by comparing its results with those obtained by standard procedures. At different pH-levels (pH 5.0 to 7.0) in either a sandy or a clay soil, a high correlation (r²=0.98) was found between the two methods. However, in the case of the clay soil, the agar-pH was significantly lower than the standard-pH. In the sandy soil, in the range pH 5.0 to 6.0, the results of both methods agreed very well. The agar method was used to measure the pH dynamics in the rhizosphere of lucerne seedlings, grown in rhizotrons.