Method for the quantification of acid production by plants in gel

Abstract

Hydrogen (H⁺) and hydroxyl ion (OH^‐) production by the tropical grass, Brachiaria humidicola, is quantified using a method in which the plants are grown in soil then transferred to agar gel for 24 h. The amount of H⁺ and OH^‐produced was calculated from the pH of the melted gel and the gels’ buffer curve. Values were obtained for plants of different ages and with nitrogen (N) supplied in the gel as nitrate (NC₃ ^‐), ammonium (NH₄ ⁺), or ammonium nitrate (NH₄NO₃) and compared with data calculated using the sum of H⁺ changes in differently colored zones of the gel. Daily H⁺ and OH^‐ production increased with plant age and total dry matter for the NH₄ ⁺‐ and NO₃ ^‐‐fed plants, respectively. By integrating the data over time, a value of 0.33 mmol H⁺ plant^‐1 was obtained for the total H⁺ production over 62 d. The proposed method was sufficiently rapid and versatile to allow the comparison between plant species or genotypes, which were grown using a variety of nutrient supplies. This procedure may indicate how acid production affects plant nutrient acquisition and aid the prediction of soil acidification by different plant species or cultivars.