A field laboratory method to determine total potential and actual acidity in acid sulphate soils

A new quantitative method is presented for the determination, in a field laboratory, of total potential and actual acidity of acid sulphate soils. The method is based on the fast titration of the total acidity of the soil sample with sodium hydroxide. For the determination of the ‘total actual acidity’ a soil sample is suspended in a 1 molar sodium chloride solution and titrated subsequently. The ‘total potential acidity’ is determined by oxidation, with 30% H202, of a soil sample suspended in a 1 molar sodium chloride solution. After the oxidation has been completed, the sample is titrated with NaOH as well.,The total acidity thus determined, gives a quantitative measure of the maximal actual or potential acidity which has to be neutralized or leached to permit the soil pH to rise to 5.5 under actual circumstances (actual acidity) or after the reduced soil is completely aerated (= potential acidity). The method has been tested on some 60 samples of actual, potential and non acid sulphate soils from Vietnam and Indonesia, ranging from clay to peat. The potential acidity determined by way of the proposed method was closely related to the total content of sulphur species. The relation with pyrite was poor because sulphur species other than pyrite were found to be important sources of acidity, especially organic sulphur. pH after oxidation by H202 turned out to be a very poor parameter for the potential acidity. Total potential acidity varied manifold at a certain pH, especially below pH 2.5. Guidelines are given for the application of the method in a field laboratory. Two simple volumetric soil sampling devices are proposed which enable a direct determination of actual and potential acidity per hectare of soil over a certain depth.