Assessment of cation and anion exchange and pH buffering in an Amazonian Ultisol

Knowledge about cation and anion exchange and pH buffering in soils is essential for developing an efficient nutrient management system, especially in tropical soils where nutrient retention capacity may be low. Sorption characteristics and pH buffering of an Amazonian Ultisol were studied in sequential batch experiments. The objective was to gain additional information on the cation and anion exchange and pH buffer properties of this soil by using a coupled equilibrium model. The CEC values in the surface soil and subsoil were below 30 mmolc kg−1, indicating that the soil is highly limited in retaining nutrient cations against leaching. The anion exchange capacity (AEC) of the soils for both depths was small (1–4 mmolc kg−1), indicating insignificant retention of NO3 and its enhanced losses in seepage water following fertilization. The exchange coefficients (Gaines–Thomas formulation) of the surface soil followed the order Al>Ca>K>Mg>Na, whereas the order for the subsoil was Al>K>Na>Mg=Ca, indicating that K transferred by leaching from the surface soil to deeper depths may be retained preferentially in the subsoil. More protons could be buffered in the surface soil than in the subsoil. Addition of protons to the surface soil released equivalent amounts of Mb (Ca, Mg, K, Na) cations because of CEC reduction, and an insignificant amount of Al. However, input of protons to the subsoil released large amounts of Al. The model satisfactorily predicted the sorption values for various elements in both depths, indicating that the main processes are understood, but some deviations between modeled and experimental values were noted for the pH in the surface soil. The results indicated a very poor capacity of the soil for nutrient retention and pH buffering and a high risk of Al toxicity. Management options should substitute ‘slash and burn’ practices (which result in further nutrient losses) with mulch producing technologies and strive for only small additions of fertilizer (to reduce nutrient losses and avoid further soil acidification).