Simple Methods for the Measurement of the Acidity of Forest Soil

KAWANA & AIBA¹⁾ reported on a simple and speedy method to measure the exchangeable acidity and phosphate absorption coefficient of the fresh soil to pave the way for adequate forest fertilization. The purpose of this paper is to examine the adaptability of the simple soil treatment described in the previous paper¹⁾ in preparation for the determination of the pH value of fresh soil and the adaptability of the determination of pH value using pH-test paper.<br> A certain amount of fresh soil was put in a small cup, and 1 N-KCl solution or distilled water was added until it became creamy. After about 3 minutes, the pH value of this treated soil was determined with glass electrode pH-meter or pH-test papers. The pH-test papers used in this test were P. P. (Phenol purple), B. C. G. (Bromcresol green), B. P. B. (Bromphenol blue), C. P. R. (Clorphenol red), and M. R. (Methyl red) (made by TOYO ROSHI in Japan). The usual method of determination of pH value is described in the cited literature²⁾. Results are summarized as follows:<br> Determination of pH (KCI)<br> 1) Using a glass electrode pH-meter, the pH value of fresh soil obtained by the simple soil treatment agreed with the value derived from the usual soil treatment²⁾ within the limits of ±0.2 pH, if only an error of -0.2 pH was corrected (Fig. 1).<br> 2) The pH value determined with some pH-test papers correlated linearly (correlation coefficient _??_1.0) to the pH value determined with a glass electrode pH-meter within the limits of ±0.2 pH (B.C.G.) and =0.1 pH (M.R. and B. P. B.), if only an error was corrected (Fig. 2 and 4). On the other hand, the pH-test paper of P.P. was not practicable (Fig. 3 and 5).<br> 3) The pH value determined with the simple soil treatment and pH-test papers agreed with that of the usual method within the limits of ±0.3 pH (Fig. 7). Determination of pH (H₂O)<br> 4) Differences in pH value of simple soil treatment between distilled water and CO₂free water was 0.05 pH (Fig. 9), and pH value between 60 and 3 minutes after the treatment was 0.05 pH (Fig. 8). The pH value determined with the simple soil treatment and glass electrode pH-meter agreed with the result of the usual method within the limits of ±0.1 pH, if only an error of +0.1 pH was corrected (Fig. 10).<br> 5) It was impossible to measure the pH value with pH-test paper (Fig. 11), for the pH value was not in direct proportion to the pH value with a glass electrode pH-meter.<br> 6) The pH value obtained from the usual soil treatment changed with filtration (Fig. 12) and buffering capacity of water-leachate of fresh soil was very small compared with KCl-leachate (Fig. 13).