Soil degradation issues are assuming increasing importance in Northeast Thailand and are challenging the concept of sustainability of current land management systems. In this study, the impacts of land conversion from natural Dipterocarp forest and wetland to agricultural production on soil chemical properties are compared. Soil samples were collected at 10 cm increments to 1m from a Dipterocarp or wetland forest (forest) and adjacent paddy rice (cultivated) production system along a transect. Since conversion to agriculture production ranging from 40 to over 100 years., the cultivated site has undergone a significant decline in soil pH as a result of reduction in soil organic C leading to loss of exchangeable basic cations; especially Ca, Mg and K; and Al domination of the exchange complex. Consequently, the ability of the paddy soil to retain basic cations has been compromised. A significant component of the extracted cations from the surface soil of the cultivated site is in a form that was non-exchangeable and therefore subject to leaching at the onset of the wet season. The approach adopted in this study to assess soil degradation from a soil chemical perspective has demonstrated the fragility of these sandy soil after continuous rice production. This is evidenced by the degree of degradation as measured using an index that takes into account changes in the surface charge characteristics and basic cation capacity of a soil. It is suggested that the degradation index may assist in quantification of what is commonly referred to as 'soil health'. The long-tern consequences of soil degradation are permanent and bring into question the sustainability of current production practices on this sit. Soils that have a low buffering capacity (ie. low clay and organic matter content) are prone to acidification and cation depletion which has a dramatic effect on the productivity of these paddy soils.