The carbon and nitrogen contents of the ploughed layers from Salamanca province cultivated soils have been determined. From these data is established that there are no change in organic matter contents throughout thirty years (1954-1983). The actual levels of soil organic carbon and nitrogen are low or very low; furthermore, there is a very high polymerisation degree of the humic substances, varying to the climatic seasonal changes and to the rapid mineralisation of the polymerised organic fractions.