The storage and release of nutrients by soil organic matter (SOM) is the primary determinant of soil fertility in low-input agriculture of semiarid NE Brazil. Traditional shifting cultivation systems have utilized the SOM built up during the fallow phase to supply nutrients for a cultivation phase of some 4-6 years. In this paper we analyze the turnover, stabilization and quality of organic matter in land use systems of NE Brazil. This analysis relies on a review of our own and literature data as well as farmers' perceptions recorded in a survey of 240 farms. Components critical for the understanding of SOM balances and transformations are residue inputs under native and agricultural vegetation, rates and controls of SOM mineralization under cultivation, controls on SOM accretion under fallow vegetation, and the quality of SOM with respect to nutrient supply. While all these factors are known in outline, the detail of understanding that would be required for fine tuning management systems to be sustainable under present (and increasing) production pressure is still lacking. In particular quantity and quality of organic matter inputs from different vegetation types, controls on SOM stabilization under different cultivation regimes, and the rates and synchrony with plant demand of nutrient release from mineralizing organic matter need to be investigated in further detail.