Sandy soils are susceptible to acidification or alkalization due to their small acid neutralizing capacity (ANC) (Lesturgez et al.,2006). The occurrence of salt-affected soils is well-known problem in poor-drained areas of northeast Thailand, but acidic soils are also widespread in well-drained upland areas (Miura et al., 1992). Soil acidification is a natural process accelerated by agriculture when the downward flux of water is dominant in the soil profiles (Helyar and Porter, 1989). In cropland soils, net mineralization and nitrification of N fertilizer and soil organic N can increase