MANY tropical soils are poor in inorganic nutrients and rely on the recycling of nutrients from soil organic matter to maintain fertility. In undisturbed rainforests such nutrients are recycled via the litter(1); ’slash-and-burn’agriculture, meanwhile, depends on the mineralization of organic nutrients from the plant remains(2,3) or on (short-lived) inputs from ash(4). This dependence on organic nutrients in tropical soils has the result that tests of soil quality which only give isolated measures of inorganic nutrient status are unreliable(5), and that the effects of fertilization can be inconsistent because of leaching or fixation of inorganic nutrients. Here we attempt to quantify the role of organic matter in sustaining the fertility of soils from three different climate zones. We estimate rates of carbon turnover from ecological measurements and C-14 dating. and determine its relation to the soil carbon and nutrient budgets. We find that agriculture without supplementary fertilization was economical for 65 years on temperate prairie and for sis years in a tropical semi-arid thorn forest. An extremely nutrient-poor Amazonian soil showed no potential for agriculture beyond the three-year lifespan of the forest litter mat, once biological nutrient cycles were interrupted by slash-burning. These observations suggest that quantification of organic-matter cycling may provide an important guide to the agricultural potential of soils.